PLAN

 1998-2002


 

 Introduction

The GAIM Task Force was formed as an overarching framework activity of the IGBP for coordinating and stimulating different multi-disciplinary research components that can be synthesized to formulate an integrated view of the Earth System. The Goal of GAIM is to advance the study of the coupled dynamics of the Earth system using as tools both data and models. The challenge to GAIM is to initiate activities that will lead to the rapid development and application of a suite of Global Prognostic Biogeochemical Models. These global biogeochemical models would subsequently be linked, partly through hydrological coupling, to General Circulation Models (GCMs). These, together with socioeconomic observations and models which would address future anthropogenic perturbations, would lead to robust prognostic models of the Earth system. In order to make progress toward its goal, the Task Force must:

Much of the progress to date in modelling specific components within the global biogeochemical subsystem sets the context for modelling activities within the various IGBP Core Projects. GAIM recognizes, supports, and will benefit from these efforts. GAIM activity is by definition cross-cutting; therefore, the activities of GAIM intersect fundamentally with all the Core Projects as well as with the World Climate Research Program (WCRP) and International Human Dimensions Programme (IHDP).

In its initial stages, GAIM concentrated on certain key issues concerning the Carbon Cycle and aspects of the coupling between terrestrial ecosystem and climate. This start-up phase aimed to develop and test programmatic techniques and model development for tractable cross-cutting problems which could then be used as smaller-scale examples of the larger Earth System issues which GAIM must ultimately face. The theme of the carbon cycle was selected because of the relative maturity of research in the relevant individual disciplines. With the success of several model development projects and model intercomparison activities, GAIM is now poised, as the basis of this GAIM PLAN, to extend its analysis to broader issues which will be encountered in global biogeochemical models. GAIM will move now to its role as integrator of IGBP science. It will focus on Tasks which cut across the realms of the Core Projects. In addition, GAIM will examine theoretical and practical modelling techniques which will enable the effective coupling of subsystem models and the development and evaluation of Earth System Models. A few examples of these foci were articulated at a meeting of the GAIM Task Force in Barcelona in April, 1997 and are described below. The scientific plan for the next phase of GAIM represents the next step toward its ultimate goal, yet it is recognized that the necessary theoretical, technological and data resources are not yet available. The broadly cross-cutting issues and projects described in this GAIM PLAN are aimed at providing some of these resources, and are part of a continuous re-evaluation of GAIM science, implementation and organizational structure. Each of the activities in the GAIM PLAN are directed toward integration of IGBP scientific results.

 

Integration of IGBP Science

As Earth subsystem models develop to a more robust level, GAIM is preparing to enter an integrative phase. This will involve the establishment of techniques for coupling and integration of biogeochemical subsystem models in preparation for the construction of integrated prognostic biogeochemical models. Such integration will involve coordination with each of the IGBP Core Projects. The integration program will have three aspects, each contributing to the overall objective of developing the modelling capacity which will ensure the achievement of GAIM's goals. The first will be at the subsystem level, where GAIM activities will be designed to bring developing subsystem models into boundary compatibility. This will be done through modelling workshops involving intercomparisons of like subsystem models, and intercomparisons involving coupling between adjacent subsystem models (which must match boundary conditions and fluxes). The second segment will be at the system level, where simple Earth system models are compared to highlight differences in coupling techniques, inter-element fluxes, and sensitivity studies to reveal the differences between models of the relative importance of individual system parameters. The third segment will be at the IGBP Core Project level, where GAIM will work with Core Project modelling teams to help facilitate inter-subsystem coordination. In addition, GAIM will act as a connection between IGBP and its sister organizations, WCRP and IHDP. GAIM will organize workshops with inter-programme modelling teams to facilitate such connections.

Subsystem models

Subsystem integration involves four key linkages: land-atmosphere, ocean-atmosphere, atmospheric physics with atmospheric chemistry, and land-ocean. These subsystems have different space and time scales (which themselves depend upon what is being tracked) and are often quite stiff as linked systems and therefore difficult in perturbation experiments. There are also greatly differing degrees of parameterization with little understanding as to effects. Important linkage experiments, however, can NOW be done: a) the ocean carbon model inter-comparison project (OCMIP) has linked atmosphere GCM's (mainly as drivers) with ocean GCM's containing carbon chemistry and crude biology; b) the terrestrial carbon models (NPP Efforts) are being driven partially by GCM results; and c) GCM Atmosphere transport codes are being explored for tackling the chemistry connection.

Subsystem models are being developed at present with a variety of structures and emphases. While each model is taken to represent the processes within a biogeochemical subsystem, the analytical and numerical formulations are widely disparate, and often lead to significant differences in model results. A fundamental issue is the development of subsystem models in such a way that the boundary conditions and fluxes for each will be compatible with each of the others. This compatibility is defined in terms of the ability of each model to provide the necessary input to define the boundary conditions needed to most efficiently run the others. For example, the boundary between the terrestrial and marine biogeochemical systems involves physical and chemical conditions and fluxes which are so complex that no single subsystem model presently accounts for all. Thus, matching boundary fluxes at the boundary would be impossible unless carefully coordinated during the model development phase.

As each of the subsystems becomes better understood and models converge on realistic values of output parameters, it will be timely to couple compatible models to form a more complete Earth system model. While it is not necessary to assume (and not possible to mandate) that all models of a particular subsystem will have identical input/output parameterizations, it is essential the each model be coupled only to other subsystem models with compatible parameterizations. Thus we envision the emergence of a suite of coupled models, each with consistent coupling and interactions between model components, but each based on a different style of process formulation. The parallel development of coupled Earth system models has several advantages. The most important is that because no single model (even an integrated Earth system model based on compatibly coupled subsystem models) accounts for all processes and interactions in the Earth system, each model will necessarily result in slight differences in inter-component fluxes and sensitivities. This will set the stage for Earth system model intercomparison which will highlight the relative importance of the various processes, interactions, and feedbacks between subsystems modelled by each of the integrated models. Such intercomparisons should ultimately lead to modified integrated models which correctly account for the interactions to which the Earth system is most sensitive, while becoming unburdened from those to which it is demonstrably insensitive.

Models of the various aspects of the Earth System are each associated with some uncertainties (errors). The uncertainties arise from 1) incomplete theoretical or mechanistic understanding of Earth System processes, 2) inaccurate formulation of processes within the modelled subsystem (e.g. ocean circulation and trace gas transport), and 3) inaccurate or simplistic boundary conditions. The first two issues are being addressed through various model development and intercomparison projects (e.g. NPP, OCMIP). The latter source of uncertainty is a more complex problem and bears on subsystem model coupling. Because gas, water, and energy exchange between subsystems (e.g. ocean, atmosphere) determines their respective modelled boundary conditions, model coupling can lead to more accurate modelled fluxes across the boundary. However, coupling introduces its own uncertainties and sources of error. It is thus necessary to consider the effect of model coupling to TOTAL uncertainty. The obvious goal is to develop Earth System models in which this total uncertainty is reduced to levels less than the individual uncertainties of the subsystem models with prescribed boundary conditions.

System level

It is the ultimate mission of the GAIM Task Force to promote the development of integrated models of the Earth's biogeochemical system for eventual linking to the physical climate system studied through WCRP as well as the societal systems studied through IHDP. Simple models of the Earth system already exist, but they are not sufficiently robust to incorporate the detailed subsystem models being developed throughout the IGBP. It will nevertheless be instructive to examine such simple holistic models because some features which emerge may help identify and thus forestall potential problems in developing more comprehensive models on the basis of subsystem model coupling. Important insights can be gained from existing simple models of the Earth system, so we will build on these simple models in two ways:

1. An organized simple but total Earth System Model approach that raises difficult system dynamic issues (chaos, feedbacks, parameterization sensitivities, policy effects, etc.), and

2. An effort to collect and document existing models of key features of the Earth System (e.g. Carbon Cycle) that could run on a PC (or run over the WWW).

The purpose of the former is to highlight key scientific issues that may be lost in the large model efforts while the purpose of the latter is out-reach and education.

In order to assess the validity of Earth system models, it is critical to understand the sensitivity of the system to each of the input data. Heuristic and mathematical models are becoming developed to a point now where it is appropriate to consider model sensitivity. It will be necessary to conduct model sensitivity analyses of dynamic vegetation models, ocean carbon cycle models, GCMs, and hydrologic models as well as for simple Earth system models with respect to the various input climate and ecological data.

In the next several years, GAIM will begin to address some of the more theoretical issues involved in complex model development, coupling and evaluation. These issues are not necessarily limited to Earth System applications, nor are they being addressed primarily by Earth scientists. GAIM will explore the existing theory and focus its efforts on development of applications for Earth System Modelling. Examples of issues to be addressed in this aspect of GAIM include schemes for quantifying and comparing coupled and uncoupled model result uncertainties, determining the minimum necessary resolution for model validation data, inverse methods for applying model validation data, quantifying system responses to subsystem-level and system-level perturbations, and sensitivity studies of coupled systems.

IGBP Core Project Integration

Each of the IGBP Core Projects is developing models of the appropriate biogeochemical subsystems. Once these are completed, it will be GAIM's task to promote the coupling of the various subsystem models and the development of integrated Earth system models. Model coupling will require advance planning so that it will be possible to most effectively match boundary conditions and fluxes. Thus, input and output data sets will need to be assessed and standardized, model temporal and spatial resolutions will have to be matched or scaled where necessary, and common numerical protocols will need to be defined so that the necessary parameters will flow through one subsystem model to the next. GAIM will work closely with the Core Projects to ensure that subsystem model linkages can be made smoothly.

The development of Earth System Models is a complex problem, to which the extensive resources of various institutions will be applied. The GAIM Task Force will not compete with these efforts, but rather will encourage and complement the efforts, as the Task Force is composed of key scientists from these leading institutions world-wide. The composition of the Task Force is determined by the scientific issue being addressed, and will continue to evolve in response to the development of new Earth System Models. As such, GAIM will provide a means for planning and coordination between these various institutional efforts.

The IGBP Core Projects are organized in such a fashion to encourage interactions and collaborations between scientists specializing in each of the Earth's biogeochemical subsystems. As such, the framework is already in place for organization of collaborative and intercomparison activities which should lead most effectively toward meaningfully coupled models. GAIM will need to work closely with each of the IGBP Core Project modelling teams to help steer the modelling efforts in a direction which will result in the most efficient coupling possible.

Scientific Questions

The new GAIM PLAN is based on a set of fundamental questions. (These questions encompass the scope of the original "6 key research questions" indicated in IGBP Report 28, 1994.) These questions are for the most part too general and broad to answer directly. They are based on a set of basic observations which must be explained if we are to understand key aspects of the Earth System. The process of seeking answers to the Fundamental Questions should fill the gaps in our understanding of the connections in the "Bretherton Diagram" as well as provide the framework for constructing reliable global prognostic biogeochemical models, GAIM's ultimate goal. As such, the Fundamental Questions address the outstanding gaps in our understanding of the linkages between the various part of the Earth System as represented by the Core Projects. The fundamental questions are each composed of a set of sub-questions which are at a more tractable scope given existing scientific research programs. While even the sub-questions are too broad to be answered by the results of individual research projects, the coordinated collaboration of research groups throughout IGBP (and beyond) can effectively address them. The narrower scope of the sub-questions will provide results in the nearer-term, making related research more readily fundable in the still somewhat disciplinary funding climate of national agencies worldwide. While inroads are being made to ensure adequate funding of ambitious interdisciplinary research such as that with which GAIM has been charged, there are a number of important issues which can be addressed now in the more "traditional" framework.

The four Fundamental Questions and their sub-questions will be addressed by GAIM research directly in some cases, and by Core Projects in others. Individual research projects will generally encompass only part of each question. The results of investigations of all sub-questions together will be used to address each of the Fundamental Questions. For the next several years, this will be accomplished through workshops on selected topics which will promote the synthesis and integration of IGBP scientific results in order to address the Fundamental Questions and ultimately lead to the development of a suite of global prognostic biogeochemical models. For some subquestions, a specific research plan has been formulated and preliminary work is already underway. In contrast, some other questions should be regarded as a "call to arms" for GAIM and the rest of IGBP in conjunction with the international global change research community.

The sub-questions within the Fundamental Questions follow a general pattern including a foundation based on "paleo" (1A, 2A...), then issues related to understanding ongoing system-level processes (1B, 2B...), and finally questions aimed toward developing prognostic capabilities (1C, 2C...). Each of these sub-questions will require the integration of a number of distinct research projects. Some of these projects will be conducted by GAIM, while others will be done by the Core Projects and other programs (e.g. WCRP, IHDP). Each project will be designed to address a specific sub-question, and will require development of theory, models, and datasets throughout IGBP Core Projects and framework activities. GAIM will take the lead in coordinating project results by convening the appropriate intercomparisons, syntheses, etc. This will entail a much greater level of interaction between GAIM and the Core Projects at all levels of activity than there has been in the past. In part, the ability to address this level of issues has been made possible by scientific advances within the Core Projects over the last several years, and the successful integration of IGBP science will depend on greater interaction with and between the Core Projects.

  


 

Overarching Vision

In seeking to develop a prognostic capability for the Earth’s biogeochemical system that could be linked with the physical-climate system, we pose a set of fundamental questions about the interplay between biogeochemistry and climate. Each of these questions represents a plane of knowledge that cuts across the Earth system; moreover, each of these questions is motivated by observations and they are posed as a search for coherent explanations of observations of global environmental change. In large part, the IGBP Core Projects in various combinations are already working toward the answers to some aspects of these questions, and the GAIM PLAN is designed to work closely with the Core Projects on the corresponding aspects of these issues. The various sub-issues of each Fundamental Question represent a bridge between the answers we seek in order to develop prognostic biogeochemical models and the results which are available from specific disciplinary research projects.

In the last several years, there has been some progress in the development of simple Earth System models. While these models include all subsystems (atmospheric and ocean circulation, terrestrial ecosystems, trace gas exchanges, the hydrologic cycle, etc.), they treat them and their interaction in a very rudimentary manner. Consequently, they cannot be considered robust for prognostic applications. However, some things can be learned from even the most elementary models from the standpoint of subsystem coupling and sensitivity. Thus it is necessary to more fully develop the key issues described in this GAIM PLAN in order to make real progress toward robust Earth System models.

 


 

Fundamental Observables and Forcings

The Fundamental Questions are based on a set of fundamental observations. We seek coherent explanations for these observations. While the observations listed below only partially represent the already observed changes in the Earth System, a coherent explanation should also account for additional observations and future data sets.

 

 
Climate system: CO2 - O2
OBSERVATIONAL RECORD
ADDITIONAL OBSERVATIONS TO BE EXPLAINED
FORCINGS

(changes in 1-way; 2-way)

Keeling (D) 1957 + DCO2 seasonal cycle
10% change in NDVI
temperature/rainfall
Keeling (D) 1957 + DCO2 seasonal cycle
ocean color as biotic index, pCO2 and S'CO2
seasonal change (spring)
Flask network CO2
N deposition
SST
O2/N2 (last 5 years)
-
UVB
Natural 13C, 14C, 18O
-
sealevel
Bomb 14C
-
radiation/cloud
280-200-280 ice core (150 ka)
-
land use
Mauna Loa record
-
energy CO2 - N

  Climate system: CH4  
OBSERVATIONAL RECORD
ADDITIONAL OBSERVATIONS
FORCINGS
Ice core 1700-
13CH4, other isotopes
climate (T,P, water vapor)
atmospheric record - historical
wetlands
wetlands
100 year firn/snow record
recent time/space CH4
rice, agriculture
long-term a) Holocene CH4 record; b) glacial
 -
energy

 
Climate system: S
 
OBSERVATIONAL RECORD
ADDITIONAL OBSERVATIONS
FORCINGS
Greenland & other ice cores
 -
industrial emissions
DMS distributions
Aerosols, etc.
coal, oil, etc.
 -
 -
natural cycles (DMS, etc.)

 
Vegetation-Climate interaction
 
OBSERVATIONAL RECORD
ADDITIONAL OBSERVATIONS
FORCINGS
distribution of vegetation- Current, Past
Soil moisture
Atmospheric trace gas composition- CO2, etc.
NDVI: patterns of seasonal activity
 -
Changes in hydrology
Time series of phenology
 -
 -

 
Land cover changes
 
OBSERVATIONAL RECORD
ADDITIONAL OBSERVATIONS
FORCINGS
Land cover
N deposition
economics
DSoil carbon
changes in erosion & sedimentation
population & demographics
 -
 -
international social programs
 -
 -
climate variability

 


Fundamental Questions

1. What controls the partitioning of the major biogeochemical elements in the Earth System?

What are the patterns and processes by which C, N, P, S, Fe and other biologically important elements are partitioned among the major active reservoirs (vegetation and soils, atmosphere, continental water, coastal zone, open ocean)?

1A. What changes in elemental partitioning were associated with sea-level changes and other factors during glacial-interglacial cycles and how did these changes interact with marine and terrestrial productivity?

1B. What processes control horizontal transport of biogeochemically active species (CO2, CH4, P, S, N, etc.) above, at, and below the Earth's surface?

(What is the stoichiometry of riverine fluxes today and how has this changed due to human activity? - Continental Aquatic Systems)

(What is the effect of horizontal atmospheric transport of trace gases on global atmospheric composition? - Transcom)

(What is the role of ocean circulation in redistributing CO2 and other trace gases, and how does this affect ocean-atmosphere gas exchange? - OCMIP)

1C. How will changing climate and land use alter the couplings between biogeochemical cycles of different elements?

2. How do changes in ecosystems interact with the physical climate system?

What processes determine how climate change affects marine and terrestrial ecosystems, and what are the potential climate feedbacks due to these processes?

2A. What have been the impacts of climate changes on marine and terrestrial ecosystems during the past 200 ka, and what have been the feedback effects on the physical atmosphere/ocean system?

2B. What is the role of ecosystem level processes (growth, competition, disturbance, mortality, decomposition, soil organic matter dynamics, migration) on the broad-scale structure of the biosphere? To what extent may plant population processes accelerate or delay climate- or CO2-related changes in the distribution of vegetation?

2C. How will future natural and anthropogenic changes in ecosystems and their interactions with climate affect the Earth System? In particular, what are the likely consequences of future land-use changes for the climate of the next 200 years?

3. How do changes in the radiatively and chemically active gas composition of the atmosphere interact with the physical climate system?

What controls atmospheric composition and what feedbacks exist between trace gases and terrestrial/marine sources and sinks?

3A. What have been the causes and consequences of natural atmospheric composition changes during the past 200 ka? (Paleo Trace Gas and Aerosol Initiative)

3B. What controls the sources and sinks of CO2, CH4, N2O, NOx, NMHC and CO in the biosphere and how are changes in climate likely to impact on the atmospheric concentrations of these gases? How can we explain the observed variability and trends of atmospheric aerosols, CO2, CH4, N2O and tropospheric O3 during recent decades?

3C. How would we expect future climate changes to interact with atmospheric trace gas composition, and what will be the consequences for scenarios covering the next 200 years?

4. Given our understanding of the couplings among physical and biogeochemical aspects of the Earth system, what will be the nature of its future interactions with human activities?

How will the nature of anthropogenic influences on the Earth System change in response to global change and how will the perception of environmental impacts alter human activities?

4A. Can rapid climate change events, like those that have happened during ice ages and deglaciations, also be triggered by human alterations of the Earth system?

4B. What are the likely relative magnitudes of the climatic effects of different anthropogenic drivers of global change, e.g. land use changes versus fossil fuel burning?

4C. How will an increased understanding of anthropogenic alterations of the Earth System affect future land use and emissions policy? What socio-economic factors will modulate enactment of and adherence to such policy, and to what extent do existing socio-economic conditions constrain future policy design, magnitude, implementation and time frame?


1. What controls the partitioning of the major biogeochemcial elements in the Earth System?

What are the patterns and processes by which C, N, P, S, Si, Fe and other biologically important elements are partitioned among the major active reservoirs (vegetation and soils, atmosphere, continental water & coastal zone, open ocean)?

Understanding the biogeochemical cycles of the major nutrients, micronutrients and carbon is fundamental to the scientific understanding and ultimately management of the earth system. Under preindustrial and preagricultural conditions we assume that these cycles were in a quasi-equilibrium, although exhibiting variability on all time scales (from seasonal to glacial-interglacial). The advent of human domination of terrestrial systems has changed this largely stationary situation and now major trends are occurring, driven by human activities. These trends (superimposed on natural variability) include the transfer of carbon from 'inactive' fossil reservoirs to the active land-atmosphere-ocean system, the transfer of carbon-and-nutrient containing sediment from soils to depositional sites on land, in aquatic and continental shelf systems. In addition to rearrangements of organic matter between long-term reservoirs (soils and sediment, fossil fuels and wood), human activity affects the cycling of elements through very active cycles. Thus, human activity has affected the cycling of short-lived compounds such as nitric oxide, methane and carbon monoxide through the atmosphere. Changes to the cycles of reactive species does not have an immediate and direct effect on the size of biogeochemical reservoirs but it does affect air quality and the ability of the atmosphere to break down industrial pollutants (hydrocarbons, halons, xenobiotics).

Thus, biogeochemistry is a fundamental earth science discipline, and changes to the exchange of materials between land, atmosphere and oceans, modulated by the biota, ultimately control water and air quality and modify climate. An integrated approach is required because of the multiple feedbacks between the cycles and the climate system. In addition, because the biogeochemical cycles function on multiple time scales, with important effects of decadal to centennial time scale processes, they pose a major methodological research challenge. The importance of long-term dynamics makes paleostudies and modeling central tools. The existence of 'fast' processes such as photochemistry links the field to fundamental physical chemistry and atmospheric chemistry. The character of the field of biogeochemistry makes it one of the focal points of GAIM and IGBP.

 

1A. What changes in elemental partitioning were associated with sea-level changes and other factors during glacial-interglacial cycles and how did these changes interact with marine and terrestrial productivity?

Sea level has varied throughout geologic time due to various changes in the relationship between the volume of the Earth's ocean basins and the volume of the Earth's ocean water [Sahagian, 1991 #497]. On timescales of less than a million years, basin volume can be considered invariant, so that only ocean water volume changes are significant. These can be caused by water flux between the ocean and continental ice sheets/glaciers, thermal expansion due to changing sea surface temperatures (SSTs), and water flux from various continental reservoirs such as aquifers, forests and dammed impoundments. For the timescale of glacial-interglacial cycles, the anthropogenic factors do not play a role, but fluxes from ice variations and thermal expansion can account for the large sea level variations inferred from proxy records.

Times of higher sea level are normally associated with times of warmer global climate. The link between sea level and climate has both direct and indirect components. The direct component comes from the role of variations in the Earth surface area covered by water. This is in part because water has a much lower albedo than land (even forest is higher), and the radiation balance is thus controlled by sea level (marine inundation extent). In addition, water mixes the temperature over great vertical distances and thus serves to moderate seasonal variations in atmospheric temperature. For instance, during times of high sea level in the geologic past, when large continental areas were flooded, seasonal variations were much less than those of the present day. The indirect link between sea level and climate involves the carbon cycle. There is a strong correlation between atmospheric CO2 concentration and sea level (Fig. 1).

The correlation between sea level, temperature and CO2 is suggestive of a causal link. There is a positive feedback between the three due to the temperature-dependence of CO2 solubility in sea water. As temperature increases, CO2 exsolves to the atmosphere, increasing the greenhouse gas concentration and thus temperature. Warmer atmospheric temperature increases SST and also melts ice, both leading to higher sea level. (Higher sea level also has the direct albedo effect mentioned above.) This positive feedback loop may in part be responsible for the large observed variations in Figure 1.

On the basis of marine 13C records, we infer that the terrestrial biosphere stored less carbon at the Last Glacial Maximum (LGM) than it has in the Holocene. This is in spite of the fact that the non-glaciated land area was about the same globally (ice area was offset by larger areas of exposed continental shelves). We have a far from complete understanding of the interplay between the dynamics of these changes with atmospheric CO2 and the role played by the terrestrial biosphere during the period of deglaciation.

Figure 1: Correlation of sea level, temperature and CO2 variations over the last 160 ka [Tooley, 1993 #3349]. Temperature was derived from oxygen isotopic data from the Huon Peninsula in New Guinea. While there is a strong positive correlation, the cause and effect relationships and feedbacks between them are not well understood.

 

The partitioning of carbon between the ocean, atmosphere and terrestrial ecosystems is thus related to sea level variations. It will be necessary to investigate the links between ocean-atmosphere CO2 flux and the effects of changing atmospheric CO2 concentrations on terrestrial ecosystems. While some inroads regarding atmosphere-ecosystem interactions have already been made by efforts led by GCTE as well as GAIM and WCRP, and ocean-atmosphere fluxes by JGOFS, PAGES, GAIM and WCRP, an integrated understanding of the link and quantification of the feedbacks is yet to be obtained. This will involve coordination between the various IGBP Core Projects as well as WCRP.

Data needs: Pollen records, ice cores, marine sediments, corals.

 

1B. What processes control horizontal transport of biogeochemcially active species (CO2, CH4, P, S, N, etc.) above, at and below the Earth's surface?

(What are the fluxes and stoichiometry of riverine biogeochemical species today and how has this changed due to human activity? - Continental Aquatic Systems)

(What is the effect of atmospheric transport of trace gases on global atmospheric composition? - Transcom)

(What is the role of ocean circulation in redistributing CO2 and other trace gases, and how does this affect ocean-atmosphere gas exchange? - OCMIP)

 

While vertical exchange and transport has been addressed in many projects throughout IGBP, horizontal transport studies have been more limited. In part, this is because horizontal transport involves movement from the realm of one Core Project to that of another. In addition, in introduces another dimension to the models and includes heterogeneities unlike those found in the vertical. However, some significant progress has already been made in developing the horizontal component of transport codes. While there are many examples of necessary directions for improved modelling and observations, three discrete projects are briefly described here. These may serve as example for additional horizontal transport studies.

Horizontal Biogeochemical Transport in Continental Aquatic Systems

An ultimate goal of the IGBP is to understand the Earth system at a level which makes it possible to construct prognostic biogeochemical models for coupling with physical climate and socio-economic models. While there have been considerable efforts initiated by IGBP and other organizations to investigate the vertical exchange and fluxes of biogeochemical species, relatively little attention has been paid to horizontal fluxes across terrestrial systems. These horizontal fluxes may exert a profound influence on the balance of nutrients and thus biota between upland, coastal, and marine ecosystems. The magnitude of the effect of these fluxes relative to vertical fluxes will provide an important constraint on future distributions of nutrients and other biogeochemically active species, enabling IGBP to develop more accurate prognostic models of the Earth system.

Continental Aquatic Systems are defined as all surface and subsurface water involved in the hydrologic cycle on the continents. This includes lakes, rivers, wetlands, soil moisture, and ground water from the point where precipitation reaches the Earth's surface until it reaches the sea in full marine conditions, or until it reaches some other final base level. While the "upstream" boundary of this realm is easily visualized, the "downstream" boundary is a broad zone variable in space and time wherein river water interacts with the ocean until reaching fully marine characteristics with respect to chemistry and ecology. Emphasis is placed on water, sediment, carbon, nitrogen, phosphorus, silicon and micro nutrients.

Continental Aquatic Systems play a critical role in transporting, storing and cycling nutrients in the Earth system. Their importance is highlighted by our emerging understanding of present day climate systems, our growing knowledge of the spatial and temporal patterns of natural variability, the predicted consequences of likely future climate change and above all the implications of population growth and current socio-economic projections for the coming decades. Water flowing on or below the surface of the world’s land masses is one of the main links in many biogeochemical cycles of crucial importance to the functioning of the biosphere as well as the water resource base for human populations.

River systems are linked to regional and continental-scale hydrology through interactions between soil water, evapotranspiration, and runoff in terrestrial ecosystems. As such, river systems, and more generally the water cycle itself, serve as a control on the translocation of constituents over vast distances from the continental landmass to the world's oceans and to the atmosphere. The system serves, in part, to transfer nutrients to marine biological systems and hence potentially affects oceanic productivity. With particular reference to both nutrients and sediment, (DOC), landscape disturbance greatly increases the rate of loss from the terrestrial biosphere and the consequences can be global in scope. This redistribution is important to both donor (landscape) and recipient (aquatic) ecosystems, and we need to develop tools which can quantify these phenomena and hence be able to contribute to the problem of determining the effects of changing climate, land-use and other aspects global environmental change on the redistribution of water and essential nutrients. The issue has in part been addressed in IGBP Report 39 "Modelling the Transport and the Transformation of Terrestrial Materials to Freshwater and Coastal Ecosystems" [Vorosmarty, 1997 #5410].

Continental Aquatic Systems have not been fully incorporated into our growing understanding of the Earth's biogeochemical systems because we lack the answers to some basic questions regarding the role of continental water. These basic questions are as follows:

•What are the present-day stocks, concentrations, and flux fields of fresh water-borne nutrients (from atmosphere through terrestrial ecosystems and societal systems) to the ocean?

•What is the partitioning of water and nutrient stocks and fluxes due to the influence of natural variability versus human perturbations? To what extent have humans affected global nutrient fluxes?

•How do biogeochemical processes affect the flux of nutrients through Continental Aquatic Systems?

•Which aspects of fresh water-related biogeochemical processes and fluxes are most sensitive to projected future changes in nutrient supply and water stocks/fluxes?

•To what extent do changes in global continental aquatic nutrient fluxes affect ecosystem function?

•To what extent do changes in global continental aquatic nutrient fluxes affect water resource utility and sustainability?

•To what extent do changes in global continental aquatic nutrient fluxes affect the Earth system?

The main anthropogenic inputs to continental aquatic systems are from agriculture, industrial activity, and sewage treatment plants. There are three main issues which bear on the anthropogenic effects on biogeochemical river fluxes:

1. The effect of feedbacks with changes in drainage basins on biological cycles and human society;

2. The controls of biological, physical and chemical processes (natural and anthropogenic) on fluxes of sediment, water, micronutrients, nitrogen, carbon, and phosphorous in the catchment cascade; and

3. The chemical attributes and quantities of river-borne fluxes to the ocean of sediment, water, micronutrients, nitrogen, carbon, and phosphorus.

The flux of dissolved organic carbon to the ocean has increased as a result of land use, while the flux of metals has generally increased, but by an amount that varies substantially. Human use of freshwater resources and land has also significantly increased the flux of metals, sediment, and nutrients both to the oceans and within drainage basins, especially during the period of intensive agriculture. Fluxes have doubled of Si, P, and N, total organic carbon, and sediment to the ocean. Dense human populations in Asia produce perhaps as much as 50% of the total flux to the oceans of NO3-N.

The principal issue is to model and understand how specific terrestrially-derived materials are transformed, delivered to, and mobilized along the full cascade of landscape-fluvial systems. The drainage basin serves as an essential organizing principle in this discussion. Adequate consideration must be given to interactions within the river-riparian complex, the role of wetlands, and terrestrial ecosystem dynamics. The downstream boundary is equally complex: a) nutrient and sediment trapping and recycling occurs in the estuarine and near coastal environment, and b) the actual delivery of material to the open ocean appears quite variable and our knowledge is limited by an adequate database.

From a modeling perspective, there are several aspects that need to be addressed. First there is the cycling of water between the land and the atmosphere which can produce a "residual" or runoff. This water and the associated chemical load form the basis of rivers and the recharge of aquifers. This topic is the focus of water transport models which are tied to the coupled dynamics of terrestrial ecosystem and the land-water cycle. These models transforms complex patterns of generated runoff into horizontal transport through the drainage basin (Fig. 2).

 

 

 

 

Figure 2: Major drainage basins surrounding the North Atlantic (from Vorosmarty et al., 1998)

This flow of water contains a variety of biogeochemical compounds (from point and non-point sources) and the models must treat the internal processing within riverine systems. Thus, in addition to the transport of water and the associated chemical constituents, there will be the dynamics of the within river biogeochemical processes that act on the biogeochemical constituents. Any global perspective on surface hydrology must explicitly recognize the impact of human intervention in the water cycle, not only through climate and land-use change, but through the operation of impoundments, inter-basin transfers, and consumptive use.

Coupling of models between the nearshore and drainage basins will be necessary to provide a complete analysis of the interaction of coastal zone and terrestrial ecosystems. Such coupling may require coastal physical oceanographic models linked to biogeochemical process simulations of regional land-coastal margin ecosystems. It is also necessary to model a series of material transformations along the entire continuum of fluvial systems from the points of terrestrial mobilization to delivery and processing in the coastal zone and through to the open ocean (Fig. 3). Multiple component models would be required including terrestrial ecosystem modeling, river continuum, and nutrient cycling.

Model results can be constrained using a database of observed discharge and constituent fluxes at key locations within the drainage basins analyzed. A major initial modelling effort could be to mobilize and transport carbon and nitrogen from the terrestrial landscape into fluvial ecosystems in drainage basins that include both natural and disturbed ecosystems. Efforts such as this will increase our understanding of current and future patterns of landscape impoverishment as well as eutrophication of inland waters. At the continental scale, the model outputs could be linked to complementary studies of coastal ocean productivity.

 

 

 

Figure 3: IGBP linkages

Solid lines indicate programmatic boxes within IGBP. Dashed lines correspond to italicized portions of the Continental Aquatic System. Ocean is included (in outline) because it is the ultimate destination for river-borne water and nutrients.

The cross-cutting issues involved in continental water bear on many of the ongoing programs with IGBP, and responsibility is placed with BAHC to spearhead the effort. The project will require a coordinated effort by several program elements, with communication by all. The path of water and water-borne nutrients conceptually leads through the IGBP as sketched in Figure 3. Starting in the atmosphere (IGAC, WCRP), rain enters the terrestrial biosphere (GCTE), where it is altered by human activities (LUCC, START, IHDP) before entering ground water and rivers (BAHC) on its way to the coast (LOICZ) to finally mix with marine systems (GLOBEC, JGOFS). Insights can be gained from past records of continental aquatic systems (PAGES). The various necessary data would be coordinated by IGBP-DIS, while GAIM would handle model development and coupling between the various boxes in Figure 6, as well as incorporation into emerging Earth system models.

Data needs: river database including runoff, groundwater and nutrient fluxes, dams & reservoirs, population and agriculture statistics

 

TransCom

A key component in the projection of future global change is the ability to predict future concentrations of atmospheric greenhouse gases such as carbon dioxide (CO2) and methane (CH4). Unfortunately, the current state of the science cannot completely account for the growth rate and interannual variations of atmospheric CO2 and CH4 with confidence, so accurate prediction of future concentrations is difficult. One of the objectives of GAIM is to develop coupled ecosystem-atmosphere models that describe time evolution of trace gases with changing climate and changes in anthropogenic forcing. Such coupled models must include an atmospheric module which adequately describes the chemical transformations with the atmosphere, and biospheric modules which describe the emissions from different ecosystems as well as how the emissions react to climate changes. The models must be based on process-level understanding of trace gas exchanges and transformations, but can be constrained by trace gas concentrations measured by the global observing network. This is possible only with a quantitative understanding of transport processes between sources, sites of chemical activity, and observation positions. As such, the atmosphere can act as an "integrator" of biogeochemcial processes. This is particularly valuable because of the large spatial and temporal heterogenieties inherent in surface process and fluxes. The interpretation of atmospheric data using transport models can provide an independent "integral constraint" for the upscaling techniques using local data, process models and remote sensing.

Only about half of the anthropogenic CO2 remains in the atmosphere, and the fate of the other half is not completely understood. Both the ocean and terrestrial biosphere currently act as significant sinks for anthropogenic CO2, but their relative contributions are a matter of intense debate [Houghton, 1995 #5411]. The terrestrial net sink is very difficult to measure directly, even at a single location, because it results from a small imbalance between large natural uptake and efflux by photosynthesis and ecosystem respiration, neither of which can be accurately measured at large spatial scales. Until the mechanisms involved in the terrestrial uptake are more clearly elucidated, predicting the future behavior of such a sink (and therefore the atmospheric concentration) will be very difficult. A significant step toward this end was taken in the recent GCTE synthesis [Walker, 1997 #5419].

The spatial and temporal distribution of atmospheric trace gas concentrations contains a great deal of information about the distribution of sources and sinks at the surface [Conway, 1994 #4075; Francey, 1995 #4098; Keeling, 1995 #4080]. This information is key to the overall effort to understand ecosystem-atmosphere interactions because (1) the concentration field provides validation data for the testing of coupled ecosystem-atmosphere models (a "bottom-up" approach to the problem); and (2) careful analysis of the changing distribution of trace gases can yield estimates of surface fluxes on the largest spatial scales (a "top-down" or "inverse" approach). Direct observation of trace gas concentrations through flask sampling and aircraft campaigns provides the data for these calculations, but calculation of surface emissions and uptake requires a detailed understanding of the atmospheric transport and chemical transformation that occur prior to samples being collected. This requires a numerical simulation model of scalar tracer transport by the atmosphere, which may be driven by analyzed winds or from meteorological principles, and may include gas transport, reactive chemistry, or both. The "top-down" or "inversion" approach has long been used to study sources and sinks of atmospheric CO2 [Ciais, 1995 #4040; Enting, 1989 #4099; Enting, 1991 #4100; Enting, 1995 #4101; Fung, 1983 #5147; Heimann, 1989 #4102; Tans, 1989 #4107; Tans, 1990 #4108]. It has also been used to study atmospheric CH4 [Fung, 1991 #4891], chlorofluorocarbons [Hartley, 1993 #5341; Prather, 1987 #5340], and many other trace gases, both reactive and inert.

As high time-resolution global data on additional species become available (d13C and d18O of atmospheric CO2 and atmospheric O2/N2 ratio), the use of synthesis inversion techniques with atmospheric tracer transport models will result in much more reliable estimates of the changing global carbon budget of the atmosphere. Improvements in the quality and quantity of the observational data and in the mathematical formalism associated with the inversion calculation have brought us to the point where one of the greatest sources of uncertainty now lies in the transport models themselves.

Atmospheric trace gas concentration is affected both by chemical and physical processes. Some trace gases such as methane are chemically reactive in the atmosphere, being lost to oxidation. They are also physically transported so that its atmospheric distribution is not directly related to its ground sources. Both mechanisms must be quantified in order to understand global atmospheric trace gas distribution, but atmospheric trace gas transport codes have been highly variable in their results, and in need of reconciliation. In order to effectively diagnose transport codes, they must be first compared in their prediction of passive trace gases, so that the physical effects can be separated from the chemical. Consequently, we have begun by considering the simpler case of chemically non-reactive CO2, and as a first step, we examine some passive tracers which have no sinks so that we can most effectively compare model results and thus promote model refinement. We will later treat reactive species such as methane separately, in preparation for ultimately incorporating these into the demonstrably realistic transport codes developed in association with the transport component of the project.

An important source of uncertainty in these calculations is the simulated transport itself, which varies among the many transport models used by the community. It is necessary to conduct a series of 3-dimensional tracer model intercomparison experiments with leading transport codes which are intended to (1) quantify the degree of uncertainty in current carbon budget estimates that results from uncertainty in model transport; (2) identify the specific sources of uncertainties in the models; and (3) identify key areas to focus future transport model development and improvements in the global observing system that will reduce the uncertainty in carbon budget inversion calculations.

The primary goal of Transcom is to improve our ability to deduce the Carbon budget of the Earth's surface from atmospheric observations. To accomplish this it will be necessary to quantify and diagnose the uncertainty in inversion calculations of the global carbon budget that result from errors in the simulated transport. The specific objectives of TransCom are (1) quantify the degree of uncertainty in current carbon budget estimates that results from uncertainty in model transport; (2) identify the specific sources of uncertainties in the models; and (3) identify key areas to focus future transport model development and improvements in the global observing system that will reduce the uncertainty in carbon budget inversion calculations. Results of an initial intercomparison of simulations of fossil fuel CO2 and the influence of seasonal vegetation were reported by [Rayner, 1995 #4106] and by [Law, 1996 #4105]. A subsequent phase involving calibration simulations of sulfur hexafluoride (SF6) has been conducted as well [Denning et al., in Review]. These preliminary results are summarized in GAIM Report #4 "Atmospheric Tracer Transport Model Intercomparison Project (TransCom)." Our initial intercomparison of global transport models used in the CO2 inversion problem revealed that inversion estimates of some carbon budget components may currently be uncertain by about a factor of two due to transport alone [Law, 1996 #4105; Rayner, 1995 #4106]. Because these models also form the dynamical core of many models of reactive chemical species, this problem is also of serious concern to those in the global atmospheric chemistry community.

Data needs: CO2, CH4 distribution, tower data.

 

OCMIP

One millennium from now, long after all fossil fuel supplies will have been exhausted, the ocean will have absorbed and retained about 7 out of every 8 molecules of anthropogenic CO2 ever emitted to the atmosphere. Confidence in this prediction comes from the agreement between related simulations in global ocean, carbon-cycle models, from the simplest (which divides the ocean into a few boxes) to the most complex (which contains many thousands of grid cells). Less confidence can be given to any one model's prediction about more immediate changes, e.g., over the next couple of centuries. Also less agreement exists concerning today's regional uptake patterns, as well as those during other times. Models differ substantially even without consideration of potential changes in ocean circulation and possible shifts in the composition of oceanic plankton, much less the large uncertainty in future emissions of anthropogenic CO2. Marine carbon cycle models have provided important constraints on the large-scale patterns of the marine uptake of anthropogenic CO2 [Siegenthaler, 1993 #4030; Maier-Reimer, 1996 #5007; Sarmiento, 1992 #4034], new production [Najjar, 1992 #4905; Bacastow, 1991 #4931], and remineralization [Shaffer, 1996 #5602; Lefevre, 1996 #5601], three important fluxes that have largely been elusive to direct observation over large spatial scales.

Consequently, the challenge to oceanographers is to synthesize the available data sets and incorporate into models that can be used for simulation of the partitioning of CO2 between the atmosphere and the ocean. GAIM is responding to that challenge in the Ocean Carbon-Cycle Model Intercomparison Project (OCMIP), a coordinated effort of evaluation and intercomparison of 3-D global marine carbon cycle models. The long range objective is to improve man's understanding of the ocean's carbon cycle and the crucial role of its major control -- ocean circulation. Such critical information from ocean carbon-cycle models will be necessary to help governments and international organizations make informed decisions concerning future increases in atmospheric CO2 and climate change. This understanding is also crucial to providing a firm base for proper evaluation of proposed geo-engineering solutions (e.g., ocean disposal of CO2 as a means to help limit rising atmospheric CO2).

Evaluation of marine carbon cycle models using observations of carbon-system and related parameters is necessary in order to establish the reliability of using such models for future prediction. A highly simplified "perturbation" approach, in which the natural carbon cycle and its attendant biological complexity are ignored, is feasible if ocean circulation and biogeochemistry can be assumed as invariant. [Siegenthaler, 1993 #4030]. However, there are many indications that the earth’s climate and ocean circulation are indeed changing and may change dramatically in the future (refs; [Manabe, 1994 #4999]. In that case, the potential for complex marine biogeochemical feedbacks is large [Sarmiento, 1996 #5000], and we are therefore behooved to develop the capability to model those aspects of the natural marine carbon cycle that are relevant to the air-sea partitioning of carbon dioxide. There are important data sets being compiled such as those by the Joint Global Ocean Flux Study (JGOFS), the next generation of satellite ocean color products, and a number of existing seasonal, global scale syntheses of nutrients [Conkright, 1995 #5008], dissolved oxygen [Levitus, 1994 #4913; Najjar, 1997 #5002], surface carbon dioxide [Takahashi, 1997 #5003] and chlorophyll [Yoder, 1993 #5004; Banse, 1994 #5006]. These data present an unprecedented opportunity for the evaluation of models of the natural marine carbon cycle.

Preliminary results of ocean carbon cycle model intercomparisons have identified differences between simulations of both natural and anthropogenic CO2 in various 3-D models, as well as differences between measured and simulated C-14 (for both natural and bomb components, separately) as a means to validate the model circulation fields which drive each of the four carbon-cycle models.

In order to understand the role of the ocean in the global carbon cycle, it will be necessary to improve 3-D marine carbon cycle models for their use in predicting the future partitioning of carbon dioxide between the atmosphere and the ocean, and better quantify important but poorly-known quantities that are relevant to the marine carbon cycle (e.g. new production, aphotic zone remineralization and anthropogenic CO2 fluxes and inventories). Since this will require an understanding of the natural marine carbon cycle and its anthropogenic perturbation, both of which are profoundly influenced by ocean circulation, it will necessary to first evaluate and intercompare the ability of 3-D global models to simulate the natural marine carbon cycle, the uptake of anthropogenic CO2, and response to perturbations.

Since preindustrial times, the rise in atmospheric CO2 has caused the change in the air-to-sea CO2 flux to be positive everywhere. This perturbation to the natural system, termed anthropogenic CO2, is difficult to measure in the ocean. In preliminary studies, four models have been used to estimate oceanic uptake of anthropogenic CO2. Although models agree to within 20% for global uptake of anthropogenic CO2 during the 1980's, regional uptake can differ by much more. Ocean uptake is highest in the high latitudes and at the equator (Fig. 4), i.e., in zones where deep waters uncontaminated with anthropogenic CO2 communicate readily with the surface (via upwelling and convection). The Southern Ocean dominates as the major sink also because of its large surface area. Models differ by more than 100% in their predictions of how much anthropogenic CO2 is absorbed in the Southern Ocean. Additionally, the predicted position of maximum uptake in the same region differs between models by nearly 20%.

 

Figure 4: Global ocean, zonal mean air-to-sea flux of anthropogenic CO2 given in Pg C yr-1 degree-1 as calculated by four OCMIP models for 1990. The oceanic uptake per band of latitude is most important in the Southern Ocean, but even there predictions vary considerably between models, both in the total absorbed and in the position of maximum absorption.

 

Simulations differ most in the Southern Ocean for natural CO2, as well. OCMIP simulates the natural carbon cycle in order to properly validate ocean carbon-cycle models with ocean CO2 measurements as well as to better constrain our understanding of the partitioning of CO2 between the terrestrial biosphere and the ocean. Uncertainties in the latter are perhaps best exemplified by discrepancies between predictions from ocean models vs. those from atmospheric models which use observed distributions of CO2 and C-13 in the atmosphere to back out present-day carbon fluxes to and from the atmosphere [Tans, 1990 #4108; Ciais, 1995 #4040].

For the natural carbon cycle, it is necessary to run separate runs simulations to distinguish the effects of two major processes which along with ocean circulation control the distribution of natural CO2. The first relates to the temperature-dependent solubility of CO2. The cold waters which fill the deep ocean from the high latitudes are rich in CO2. Secondly, ocean biota act to reduce surface ocean CO2 through the combined action of planktonic uptake, rapid transport to depth of resulting particulate organic carbon, and subsequent bacterial degradation. We denote these two processes as the solubility and biological pumps, respectively [Volk, 1988 #5009].

We take the natural carbon cycle to be the state of the ocean-atmosphere carbon system prior to significant anthropogenic influence on the global carbon budget, usually considered to be the millennia leading up to the 19th century, when atmospheric CO2 concentrations were "steady" at about 280 matm. The natural marine carbon cycle plays an important role in the partitioning of carbon dioxide between the atmosphere and the ocean through two important processes: the solubility pump and the biological pump [Volk, 1988 #5009], both of which act to create a global mean increase of dissolved inorganic carbon (DIC) with depth, and therefore to maintain atmospheric CO2 at a level considerably lower by about a factor of three [Najjar, 1992 #4905] than it would otherwise be.

The solubility pump maintains a vertical DIC gradient due to the fact that cold waters, which originate in high latitudes and fill up the deep ocean, can hold more DIC than warm waters at equilibrium with a fixed atmospheric pCO2, a result of the higher solubility and dissociation of CO2 (into carbonate and bicarbonate ions) in cold water. The vertical DIC gradient depends not only on the vertical temperature gradient but also the degree to which surface waters equilibrate with the atmosphere before sinking. Unlike most gases, the equilibration of surface water CO2 with the atmosphere takes about one year, and therefore the strength of the solubility pump may depend critically on the kinetics of air-sea gas exchange.

The biological pump consists of two separate pumps: that of organic matter and calcium carbonate. The organic matter pump affects the DIC distribution through the photosynthetic formation of organic carbon in surface waters and the sinking and subsequent remineralization of this organic matter deeper in the water column. The carbonate pump affects the DIC distribution though the biogenic precipitation of calcium carbonate in surface waters and the subsequent sinking and dissolution of this material deeper in the water column. These two pumps also affect the alkalinity of seawater through the nitrate and dissolved calcium distributions.

The prominent importance of the marine carbon cycle in global carbon cycle make it essential to accurately model ocean circulation and its role in carbon uptake, release, and transport. As further models develop and are compared on the basis of their performance regarding natural and anthropogenic CO2 and various tracers, GAIM will work closely with the Core Projects, particularly JGOFS, to ensure that the models are compatible for coupling with atmospheric and terrestrial models toward the goal of integrated Earth System models.

Data needs: marine CO2, ocean color, circulation (horizontal & vertical), bomb 14C.

 

1C. How will changing climate and land use alter the couplings between biogeochemical cycles of different elements?

Human land use has transformed most of the Earth’s surface (i.e., its land cover) during the last 200 years with prominent influence on several subsystems of the total Earth system. The terrestrial biosphere itself is fundamentally modified by the activities of land clearing for agriculture and other purposes (industrialization, urbanization), as well as by forest and rangeland management which is taking place throughout the world although with widely differing intensities. The atmosphere is affected by these changes both through an altered energy balance over a significant part of the land surface (i.e., the more intensively managed areas) where physical properties such as albedo and roughness are modified, and through changed fluxes of H2O, CO2, CH4 and other trace gases between soils/vegetation and the troposphere [Mosier, 1991 #4049; Solomon, 1993 #4764; Woodwell, 1983 #4885]. Finally, in many coastal zones, the oceans are receiving greatly altered fluxes of carbon, nutrients and inorganic sediments from rivers due to altered land use. Aside from climate change, these changes together represent the most critical component of anthropogenic global change [Turner, 1995 #4886].

Land use changes have profound effects on the biogeochemistry of carbon, radiatively and photochemically active gases, and aerosol production (dust and biomass burning). Land use changes also affect hydrology and erosion, and by changing surface albedo and energy exchange, can have direct effects on climate. People often create highly heterogeneous landscapes which form mosaics that may encompass activities with highly divergent effects on ecological processes. The spatial arrangement of landscapes can affect exchanges of water and associated solutes and particulates in freshwater and coastal margin areas, with land cover at the land-water margins having substantial effects on water chemistry. The arrangement of landscapes also affects biological diversity, pests and pathogens, invasibility and extinctions.

The amount of forcing land use changes exert on the total Earth system is currently unknown. Sensitivity studies with altered land cover distributions in general circulation models have shown that unrealistically drastic changes, such as total deforestation of all tropical or boreal forests, may lead to feedbacks in atmospheric circulation, resulting in inadequate climatic conditions to support the vegetation which occurred prior to the perturbation [Claussen, 1996 #4887; Kutzbach, 1996 #4882]. As purely sensitivity studies of the atmospheric circulation, however, these global experiments do not attempt to mimic the land use changes which have actually occurred–they only indicate that such feedbacks indeed may be critical for the stability of the overall system. Regional climate simulations, on the other hand, have shown that at the continental scale, important teleconnections may exist through which tropical forest clearing may cause a change in climate conditions in much less disturbed areas [Salati, 1986 #4888].

A fundamental research issue from an Earth System modeling point of view, is the degree of sensitivity of the feedbacks between terrestrial biosphere, atmosphere and oceans, to the processes involved in land use change, particularly with respect to the required scale resolutions in time and space. It is possible that the required accuracy of observed or simulated data sets of changing land cover could be at a relatively coarse level. If so, then investigation of the past dynamics of such interactions, (e.g. for the last 200 years) may provide insights and should be explored in tandem with the development of predictive models that can be extrapolated into the future.

In terrestrial systems, the allocation of plant productivity among different plant parts is of fundamental importance. Although theory and experiments suggest that allocation should respond to climate and other environmental changes, very few observations exist for model validation. Litterfall, litter chemistry and decomposition rates, soil respiration, and nutrient availability are key measurements at ecosystem scale for accurate interpretation of observed changes in fluxes and storage. The lack of validation data extends to marine systems as well with regard to phytoplankton and trophic levels. Export of particulate and dissolved organic matter below the euphotic zone is a primary variable. Measurements of NPP, respiration and allocation, coupled with net CO2 fluxes determined from eddy covariance will be necessary for validation of modelled marine ecosystem changes.

Today, a consistent data base on land use change over the last 200 years at a global scale does not exist. However, little progress can be made on predicting future land use change, nor on its implications for the coupled Earth System, without testing models with observations from the past. The period of the last 200 years is particularly important for this purpose because in that time not only did the global population increase 10-fold, but population density in several regions of the world increased by more than 100 times, and agriculture replaced natural vegetation rapidly, changing the face of large parts of North America, China, India, and elsewhere. In order to study the causes and effects of land use change, it will be important to have parallel efforts to compile data regarding population changes as well as climate, soil carbon and land productivity at the same temporal and spatial scale.

Starting in 1980, global land cover data sets began to be available from satellites both at low resolution (1-4 km) and, for specific regions, at high resolution (10-30 meters). Such observations will continue to be made in the future, complemented with radar observations of land the land surface as well. These data have been classified into 17 classes including several classes of land cover defined by land use. It will be important to build a land use data base from historical and other sources up to 1980, and to ensure consistency between both types of observations. These data will make it possible to establish a time series of land cover change so that terrestrial biospheric trace gas sources and sinks can be related to future observed variations in atmospheric composition and temperature. This will lead to a better assessment of anthropogenic drivers of climate changes in the future.

Data needs: 200 year land cover/use data

 


2. How do changes in ecosystems interact with the physical climate system?

What processes determine how climate change affects marine and terrestrial ecosystems, and what are the potential climate feedbacks due to these processes?

The physical climate system is comprised of atmospheric, oceanic, and land (and ice) components. Their changes affect marine and terrestrial ecosystems, in general through controls on temperatures, solar radiation, moisture, and other environmental factors, and through nutrient addition and removal by transport. Over land, nutrients are largely added by atmospheric transport, accompanied by wet and dry removal processes. Removal is largely accomplished through runoff but in some cases, e.g. ammonium ions and N2O, the atmosphere also removes nutrients. Similarly, nutrients are removed and added to the oceanic euphotic zones through climate related transport processes, including turbulent mixing, large scale circulations and upwelling, and gravitational settling. The large changes in the nutrient availibilities during El Niño is a good example of climate coupling to oceanic ecosystems.

Ecosystems are strongly controlled by land and oceanic temperatures, at a cellular level through the temperature dependencies of enzymatic processes, and cellular sensitivities to temperatures outside the organisms' range of acclimation. Temperatures in turn depend on solar and thermal radiation and surface sensible and latent heat fluxes. These have both atmospheric and especially over land ecosystem controls. The atmospheric controls largely depend on the atmospheric hydrological cycle in providing clouds, precipitation, and surface moisture fields. On the ground, soil moisture is needed to maintain the evapotranspiration of vegetation and hence allow the uptake of CO2 from the air, and N and P from the soil. High water tables can reduce the oxidation state of the soil, switching biological carbon and nitrogen oxidation processes into other pathways such as methanogenesis.

Ecosystems in turn feedback on climate through their controls on surface temperatures and exchanges of water, CO2, and other climatalogically important gases. Temperatures are particularly affected by ecosystem controls of albedo and surface roughness. Shading of snow surfaces by forests, especially in the springtime, has a major effect on high latitude temperatures relative to the effect of poleward tundra or equatorward grasslands. Tropical forest cool relative to pastures even though they absorb more energy through lower abedos. Their cooling is affected by greater roughness, hence more turbulent mixing, and greater evapotranspiration, especially during the dry season, where soil layers reached by roots become dry.

The carbon cycle is dominated by the exchange of CO2 between the land, atmosphere and ocean. As a greenhouse gas, CO2 affects climate by altering the atmospheric energy balance. Exchanges between the atmosphere and other reservoirs determine how atmospheric CO2 concentration varies with climate change or in response to perturbations such as fossil fuel combustion or land use. Hence the globally averaged CO2 concentration constitutes a direct link between the physical climate system and the terrestrial and marine biogeochemical cycles. There are also other geochemical cycles that may contribute to a direct link between biology and climate. Two such examples are the effect of DMS production by some species of marine phytoplankton and the fertilizing effect of atmospheric iron deposition on marine primary productivity [Bates, 1997 #5208; Hegg, 1991 #5209; Platt, 1997 #5207].

Marine ecosystem models often simulate the coupled dynamics of carbon and nutrients, but differ from terrestrial ecosystem models in that the focus is much more on trophic interactions. The understanding of processes that determine abundance, fluctuations, and production of marine animals must necessarily involve coupled physical-biological models, linking performance of the individual organism to local physical processes, and linking both the biology and local physics to basin-scale changes in global climate. Modelling is expected to play an important role at several levels. The explicit incorporation of physical variables and processes in biological population models should lead to significant strides forward. Appropriately constructed models of both physical and biological processes should guide the choice of field experiments and observations, while result of those field exercises should feed back interactively into the models.

2A. What have been the impacts of climate changes on marine and terrestrial ecosystems during the past 200 ka, and what have been the feedback effects on the physical atmosphere/ocean system?

Sedimentary archives recording the past composition of biotic assemblages on land and in the ocean are providing an ever-increasing body of knowledge about the nature of climatically forced variations in ecosystem structure on decadal/centennial to multimillenial time scales. Important examples of such data sources are pollen assemblages in peat and lake sediments, plant macrofossil assemblages in middens, fossil vertebrate and beetle remains in a variety of terrestrial deposits, and microfossil assemblages of planktonic and benthonic marine organisms in deep-sea sediments. Some of these data sources enable us to make inferences regarding climate (for example, assemblages of planktonic foraminifera in deep-sea cores are commonly interpreted in terms of sea-surface temperatures). Other data from the same archives give additional paleoclimatic information, e.g. alkenone ratios in marine sediments (a geochemical proxy for sea-surface temperatures) and a variety of biological and geomorphic indicators of changing lake levels (a proxy for lake and catchment water balance).

Although the role of natural climate change on glacial-interglacial time scales in forcing changes in ecosystems has long been understood, it has only more recently been appreciated that such large-scale changes in the distribution of terrestrial ecosystem types (biomes) could have major feedback effects on the physical climate system. Such feedbacks, acting through the modification of land-atmosphere exchanges by vegetation properties such as surface albedo and conductance to water vapor are known as biogeophysical feedbacks. They are distinct from biogeochemical feedbacks (involving the exchange of CO2 and other trace gases). Biogeophysical feedbacks act rapidly and directly on the atmosphere, whereas biogeochemical feedbacks act indirectly through modifying the atmospheric burden of radiatively active gases and aerosols. However, large-scale structural changes in ecosystems occur naturally at slower rates than modifications to CO2 or trace gas exchange. Thus, in order to investigate the working of biogeophysical feedbacks under natural conditions, it is necessary to describe and model conditions farther back in the past than it is provided by direct observational records.

It will ultimately be necessary to quantify the role of biogeophysical feedbacks in determining the change in mean climate and biome distribution between various times in the geologic past and the present. Preliminary sensitivity experiments with AGCMs where the land surface has been artificially changed to match the known distribution of biomes of the past (e.g. 6000 yr BP) have shown that both the modelled monsoon expansion and the high-latitude warming are greatly amplified by the change in biome distribution [Foley, 1994 #4881; Kutzbach, 1996 #4882]. AGCM simulations that do not allow for these land-surface changes greatly underestimate both climate changes- the qualitative nature of the changes is generally simulated, but the magnitudes fall well short of those indicated by the validation data such as those synthesized by the BOIME 6000 project.

Similar feedbacks may have been involved at other times, too. The climate during the last glacial-maximum may have been considerably modified by the great expansion of tropical grasslands [Crowley, 1997 #4883], itself a consequence of cold ocean conditions (generating less precipitation) and possibly also of low CO2 (favoring C4 over C3-type photosynthesis in the tropics). The start of the last glaciation may also have been aided by the effects of biogeophysical feedbacks: low northern-hemisphere summer insulation led to cold summers in the north, a retreat of forest, and an increase in spring albedo that set off a "chain reaction" helping to start snow accumulation in the areas where ice sheets were later to develop. On the other hand, experiments with current DGVMs have shown that many vegetation types exhibit considerable "inertia" (on time scales of a century or more) in their response to scenarios of future climate change. In some cases this inertia simply buffers the system against rapid change but in other cases where tolerance levels of existing vegetation (e.g. with respect to drought) are exceeded, there can be an opposite effect such that the existing vegetation is not immediately replaced by a climatically adapted type but rather by a generalist or opportunist assemblage. The situation is further complicated by land use changes which can serve as a trigger. The potential importance of such effects is not well known, yet is controversial because these two types of ecosystem response have radically different implications for the feedback of climate-induced vegetation changes on both the physical climate system and the global carbon cycle. These issues all point to the potentially large role of vegetation changes in the climate system, prompting the development of asynchronously coupled AGCM/biome models (as used e.g. by de Noblet et al. 1996) and ultimately, GCMs that include a dynamical biosphere component.

Although these various modelling studies focused on land-atmosphere interactions, there is ample evidence that the actual past climate change involved the ocean as well. In the model of Foley et al. (1994), sea-ice albedo feedback was crucial in amplifying the effect of a relatively small change in total forest area around the Arctic. In northern Africa during the Holocene, coupled model experiments have generally been unable to simulate the full northward migration of the monsoon as shown by the paleodata and there reasons to suspect that changed North Atlantic SST patterns might have played a synergistic role. Indded, recent model experiments with a simplified coupled ocean-atmosphere-vegetation model have successfully predicted both the general features of the mid-Holocene climate and the observed abrupt collapse of the northern African monsoon a little over 5000 years ago. Glacial initiation is certainly not limited to orbital forcing and atmosphere-land interactions; major changes in the thermohaline circulation also occurred towards the end of the last interglacial and presumably contributed to the eventual global cooling. Full understanding of such atmosphere-biosphere-ocean interactions may also require consideration of how vegetation properties influence freshwater fluxes, an important boundary condition for ocean circulation.

These case studies all indicate the necessity of modelling atmospheric, oceanic and biospheric dynamics in a single framework. They cast some doubt on current models of the physical atmosphere-ocean system, as used to predict possible future climate changes, due to the omission of biogeophysical (and hydrological?) feedbacks associated with changes in terrestrial ecosystems. In particular, current models omit a potential mechanism for amplifying warming in high latitudes. Paleodata can provide benchmarks for the evaluation of more closely coupled earth system models, when forced by known boundary conditions in the past. Yet the task of predicting the future is more complex still, because the land surface no longer evolves under the control of the physical environment alone. Instead, it is greatly (and increasingly) modified by land use changes, including deforestation, which themselves would be expected to alter climate through biogeophysical mechanisms. This fundamental question thus relates to developing a functional understanding of the role of the biosphere as an interactive component of the physical climate system. A subsequent section deals in part with how these same biogeophysical processes may act as an additional, so far neglected component of how human activities may be modifying the global climate.

Data needs: Pollen records, ice cores, marine sediments, lake records, corals

2B. What is the role of ecosystem level processes (growth, competition, disturbance, mortality, decomposition, soil organic matter dynamics, migration) on the broad-scale structure of the biosphere? To what extent may plant population processes accelerate or delay climate- or CO2-related changes in the distribution of vegetation?

The importance of climate-vegetation interactions in governing Earth's surface energy and moisture fluxes has been recognized for many years with regard to the tropics [Dickinson, 1988 #5236; Henderson-Sellers, 1984 #3672] and more recently for boreal forests [Bonan, 1992 #5594]. Plant metabolic processes move carbon, nutrients and water through plants and soil on rapid as well as intermediate time scales. This cycling affects the energy balance and provides key controls on biogenic emissions. Some of the carbon fixed by photosynthesis is incorporated into plant tissue and is thus delayed from returning to the atmosphere until it is oxidized by fire or decomposition. The structure of terrestrial ecosystems, which responds on longer time scales, is the integration of the intermediate time scale processes, and responds to climate changes with alterations of species composition as well as migration of biomes. The carbon loop is ultimately closed back to the climate system, since it is the structure of ecosystems that controls the terrestrial boundary conditions for carbon and water exchange, surface roughness, albedo, and latent heat exchange. It is possible to treat the hierarchy of time scales by using a nested approach. For example, the metabolic activities of terrestrial plants associated with growth and maintenance constitute the fastest interactions (seconds to days) and determine latent heat, energy water and CO2 exchange through gross photosynthesis and respiration. Intermediate processes (days to weeks) include the development of leaf area, soil water balance, trace gas exchange, and decomposition of organic soil materials. Longer term (annual) time steps encompass net primary productivity, ecosystem production, and long-term changes in carbon and nutrient pools in plants and soils. At the longest time scales (decades to millennia), biome distributions respond to changes in climate and atmospheric chemistry.

Vegetation density and net primary productivity react to changes in atmospheric composition as well as climate (temperature, precipitation) [Myneni, 1997 #4819]. The exercise of predicting changes in vegetation as a result of various global change scenarios should pave the way for accurate ecosystem-atmosphere interaction modelling capabilities within GAIM and IGBP in general. Recent research has shown that the distribution of vegetation has a significant influence on climate, particularly over the continents.

In climate models, changing vegetation corresponding to land use change modifies the climate significantly (because of changes to albedo, roughness and the Bowen ratio). The models indicate that expansion of boreal forests in response to atmospheric greenhouse warming would enhance warming through reduced albedo, particularly in times of snow cover. This feedback between ecosystem migration and climate is one example of the acceleration of global change by ecosystem level processes. Exploration of the details of these processes requires further development of Dynamic Global Vegetation Models (DGVMs). These models treat successional dynamics as well as ecosystem redistribution. For example, following the abandonment of agricultural land, fluxes and pools of C, N, and P in secondary vegetation often do not attain the same levels as found in the "undisturbed" natural vegetation. The recovery of natural vegetation in abandoned areas depends on the intensity and duration of agricultural activity and the amount of soil organic matter at the time of abandonment. To simulate the biogeochemistry of secondary vegetation, models must capture patterns of plant growth during secondary succession. These patterns depend substantially on the status of nutrient pools inherited from the previous stage. Changes in hydrology are also involved because plants that experience water stress will alter the allocation of carbon toward the roots. Presses such as reproduction, establishment and light competition are included in DGVMs and interact with C, N and H2O cycles. Disturbance regimes such as fire and land use are also incorporated. These forcing functions may themselves be altered in response to ecosystem changes exhibited by the terrestrial system.

In paleoexperiments using climate models, the simulation of reconstructed environments appears to require correct specification of land cover. On the basis of models, we can thus infer how particular ecosystem types partition the surface energy budget. Two main lines of research follow from this observation. First is the coupling of DGVMs to physical climate systems to allow for reciprocal influences as climate and vegetation change over time. Second is a concerted effort to test models of land-atmosphere coupling against observations. While this is difficult using traditional predicted-observed relationships (because of the spatial continuity of the atmosphere making spatial comparisons only weakly appropriate, and because of the lack of ability to replicate). Replications in time using data assimilation and forecast-validation cycles may be a new avenue for testing land-atmosphere coupling, as they have been for other aspects of atmospheric models.

The coupling of ecological models with climate models needs to be considerably developed before providing robust results. However, some preliminary interpretations are possible based on present models. The most fundamental is that climate-vegetation interactions at the ecosystem level can substantially alter the sensitivity of climate models. In addition, correlations have been found between CO2 concentrations and stomatal parameters; C4 plants may have proliferated in the Miocene due to H2O and CO2 stress. If these stresses are reduced in response to changes in atmospheric chemistry and precipitation patterns, there may be fundamental consequences in terrestrial vegetation and thus to the relationship between climate and ecosystems. In addition, several lines of evidence (carbon isotopes, pollen records, ecosystem model results) point to a significant role of CO2 concentration in influencing the changes in vegetation distribution between the last glacial maximum and the Holocene. AGCM experiments have indicated that the effect of changing CO2 on stomatal conductance alone (and possible further compensatory effects on leaf area index) could have significant and complex regional effects on climate. These issues must be resolved before we can hope to understand the effects of anthropogenically-induced changes in atmospheric chemistry and climate.

Data needs: H2O CO2 heat momentum fluxes, leaf area, ET, 200 year land cover/use data

2C. How will future natural and anthropogenic changes in ecosystems and their interactions with climate affect the Earth System? In particular, what are the likely consequences of future land-use changes for the climate of the next 200 years?

It is well known that even though the continental surface represents only one third of the planet and that not all of this is vegetated, the area of foliage exceeds the total planetary surface. Anthropogenic alterations in land cover can thus have a profound effect on the exchange of energy, water and nutrients between the soil, terrestiral ecosystems, and the atmosphere.

Land plants are critical conduits of energy, water and trace gas exchanges. Despite these facts, global climate models have only recently begun to incorporate sophisticated parameterizations of the land/vegetation surface. Most coupled ocean-atmosphere GCMs use a "bucket" model for continental processes. The simple 15cm "bucket" model is a much poorer representation of land/vegetation processes than many of the SVATS (Soil-Vegetation-atmosphere transfer schemes) now available.

Critical gaps between models have resulted from the different scientific and technical issues confronting climate modellers and the ecosystem modellers. The former group focus on expediency in computational process and adequacy in delivering energy, water and momentum exchanges and budgets. The latter group has developed its models from two different, and complementary directions: 1) energy, carbon and other trace element exchanges and budgets, and 2) vegetation dynamics and relations to climate. In addition, there are significant differences in the time and space scales of these model types: "land-surface scheme" - minutes to days; "biogeochemical" - days to years; and "biome model" - years to millennia. Some of these are included in Dynamic Global Vegetation Models (DGVMs), and are even now beginning to be addressed in atmospheric GCMs.

The credibility at large scales of current models remains essentially unknown because of a chronic lack of validation data. The data void is accentuated by the diversity of quantities being predicted: energy; carbon and nitrogen; and ecosystem type distributions. In the absence of adequate observational information, many of the inputs for one model type are derived from predictions of other models which masquerade as "data". There is an urgent need to assemble and document data (observed and derived from data assimilation) of direct use for evaluating vegetation/biome models.

There is also an urgent need for a well organized suite of model intercomparisons. GAIM is moving into a position to establish a nested series of model intercomparison projects aimed at establishing the importance of land/vegetation for climate simulations. Each level of the nested set should be defined in terms of the best available data for both input and evaluation so that model performance becomes the primary discriminant. Nesting is crucial because much richer data sets are available at the site and watershed level, thus enabling testing many of the internal dynamics. However, testing overall performance across large regional to global environmental gradients is a sine qua non for credibility at large scales. Thus, evaluation at multiple scales is a key part of our strategy.

Human land use is expected to continue to intensify in the future, likely at accelerated rates in large areas, due to

1) agricultural responses to a growing food demand;

2) changes in forest exploitation (clearing, wood production);

3) afforestation measures attempting to sequester carbon that otherwise would reside longer in the atmosphere; and

4) introduction of additional exotic species into disturbed and undisturbed ecosystems.

Predictive models of the Earth System need to account for past and present extent and intensity of human land modification, and the possible changes OF these in the future. To predict future changes in land use and land cover at the global scale is an unprecedented challenge in global change research, since human decision making at the local scale is one of the most important drivers. Projections therefore carry uncertainties of a different kind, as compared to physical or biological models, reflecting socio-economic constraints as well as non-monetary influences on land use systems. This represents one of the most critical links between IGBP and IHDP, and is a focus of Fundamental Question #4.

Data needs: 200 year land cover/use data, GPPDI, 100 year climate data

 


3. How do changes in the radiatively active gas composition of the atmosphere interact with the biosphere and physical climate system?

What controls atmospheric composition and what feedbacks exist between trace gases and terrestrial/marine sources and sinks?

Terrestrial and marine ecosystems are a primary source/sink of atmospheric trace gases. However, because ecosystems interact with both climate and atmospheric CO2, changes in CO2 concentrations and their associated climatic effects may have a significant influence on trace gas exchange. Methane for example is a product of anaerobic respiration in wetlands. The rate at which it is produced depends on temperature and inundation such that increases in temperature and precipitation would lead to greater methane emissions. Drying leads to reduction in methane production, but it has a further effect of allowing oxidation (or even fire), thus producing CO2. The large areas of high latitude peat may be vulnerable to a shift from reducing to oxidizing conditions, potentially leading to a massive discharge of carbon into the atmosphere. Methane production in wetlands is also linked to atmospheric CO2 concentration, and methanotrophs in dry strata of wetlands can act as methane sinks.

To predict the variation of CO2 as a function of the environment, both in an "off-line mode", i.e. with specified environmental parameters (such as temperature, precipitation), or as component in a coupled earth system model will require a description of 1) the oceanic carbon system including the carbonate chemistry, nutrient cycles, marine biota and sedimentation processes as embedded in the physical marine environment; 2) the carbon system on land including living vegetation and soils and its coupling to both the nutrient cycles of nitrogen and phosphorus, and to the exchange fluxes of water, energy and momentum; and 3) the atmospheric chemistry of the nutrients or trace elements that affect terrestrial and marine primary production.

Changes in the radiative forcing of the climate system by variations of CO2 are documented through observations from air bubbles in ice cores and direct measurements [Bruno, 1997 #5210; Leuenberger, 1992 #5211] over at least four different timescales:

1. Parallel to the glacial-interglacial cycles, significant CO2 concentration variations occurred which reflect concomitant changes in altered distributions of ecosystems on land and changes in ocean circulation, chemistry and marine biology.

2. Following the last ice age, Holocene atmospheric CO2 first underwent a slow increase but was otherwise characterized by relatively constant CO2 levels until the start of the industrial era.

3. During the industrial era, fossil fuel CO2 emissions and changes in land use induced an almost exponential CO2 increase.

4. Beginning in 1958, direct observations document with increasing detail the temporal and spatial patterns of the industrial era CO2 increase.

There are a few particularly conspicuous and intriguing features in these records that require an in-depth assessment by comprehensive models of the global carbon and nutrient cycles. Atmospheric CO2 concentration fell by almost one third during glacial times as compared to interglacial periods and was accompanied by some changes in oceanic nutrient distribution and primary productivity [Mortlock, 1991 #5213; Murray, 1993 #5212]. In addition there has been a substantial increase observed in the amplitude of the seasonal cycle of atmospheric CO2 concentration over the time of direct measurements (more than 15% over 35 years) [Dianovklokov, 1989 #5215; Keeling, 1996 #5058; Randerson, 1997 #5214]. There have also been interannual variations of the atmospheric CO2 growth rate which are believed to reflect transient fluctuations induced by climate fluctuations in the terrestrial and oceanic carbon systems [Braswell, 1997 #5113]; Kindermann, 1996 #4541]. Finally, excess CO2 from anthropogenic emissions (emissions from fossil fuel burning and changes in land use) has been partitioned between the atmosphere, ocean and terrestrial biosphere over the industrial era in ways that must ultimately reflect biogeochemical flux mechanisms.

A host of additional observations exist which allow an assessment of the relative roles of the various components of the earth system that affect changes in atmospheric CO2, and which provide constraints on comprehensive models of the global carbon and nutrient cycles. These include:

• the high resolution spatial and temporal distribution of atmospheric CO2 and some other atmospheric components as observed by the global monitoring networks

• measurements of the air-sea difference in partial pressure in CO2 indicating air sea carbon exchanges

• stable isotopes of carbon and oxygen in atmospheric CO2 (13C, 18O),

• atmospheric oxygen/nitrogen ratio

• the distribution and evolution of natural and bomb radiocarbon in the different reservoirs

• time series of forest inventory data and dendrochronology

• remote sensing data from satellites: the "greenness" of the terrestrial biosphere and the color of the ocean

• time series of phenology

• observations of the distribution and changes of oceanic chemical tracers that are linked to the oceanic carbon system (dissolved inorganic carbon, alkalinity, nutrients, dissolved organic carbon, particulate matter, etc.) from ocean surveys and time series stations (e.g. GEOSECS, WOCE, JGOFS)

• observations on the distribution and changes of atmospheric chemical tracers that can link key biological and physical processes (e.g. iron aerosols, nitrate, DMS)

• eddy flux correlation measurements at selected sites

• aircraft measurements documenting regional net carbon fluxes

All of the above observations must be reconciled with the forcing factors which control the relationship between ecosystems and climate. The key drivers are fossil CO2 and other trace gas emissions, land use and nitrogen deposition. In accounting for the observations in terms of the quantitative history of these drivers, insights will be gained regarding the interaction between climate and ecosystems.

There is some paleo-evidence of a link between global climate and methane concentrations. Since the last glacial maximum (LGM), temperatures and atmospheric methane concentrations have risen substantially. This is thought to be a result of increases in terrestrial ecosystem methane production due to increase in temperature as well as increase in tropical and boreal wetlands areal extents. A more complete understanding of the relationship between methane sources and climate will be necessary before the role of methane can be reliably included in prognostic biogeochemical models.

The terrestrial nitrous oxide budget is largely controlled by soil microbial activity, particularly in tropical regions. However, the relative production of N2O and N2 during denitrification is controlled by the concentrations of oxygen (O2), carbon and nitrate (NO3) in the soil. As such, it can be strongly influenced by agriculture, which may play an increasingly important role as additional land is converted for food production in the future. In systems with high nitrate and low carbon contents, N2O is the dominant product, so we may expect its production to increase in the future. The primary source or CO is fires (both natural and anthropogenic), so the projection of changes in CO will depend on future land use and economic scenarios. Terrestrial ecosystems produce Non-Methane HydroCarbons (NMHCs), so the changes in biome respiration, distribution and decomposition will control changes in NMHC sources. The quantification of these various interactions is necessary for prognostic modelling and will require additional understanding of both the biogeochemical aspects of the system as well as the projected areas and types of agricultural conversion expected in the next two centuries.

The magnitudes of marine sources of N2O and CH4 are controversial. There is little conceptual understanding and few observational constraints, although the general assumption is that marine input is less important than terrestrial. This will need to be quantified in the course of investigating the fluxes between ecosystems and the atmosphere.

3A. What have been the causes and consequences of natural atmospheric composition changes during the past 200 ka? (Paleo Trace Gas and Aerosol Initiative)

The Paleo-Trace Gas and Aerosol Initiative is based on unraveling the natural variability and linkages of trace-gases, aerosols, and climate over the last 200 ka. Natural atmospheric trace-gas concentrations and climatic change are clearly linked, but the full extent of these linkages have yet to be fully understood. In seeking to measure improvements in our understanding, we can test our models against the historical record, recognizing that there are differences between past and future change. An important test is to explain the co-evolution of inferred climate and atmospheric carbon change over the last 200 ka (Fig. 1). Data from ice-cores demonstrate that atmospheric CO2 and CH4 concentrations changed dramatically, and at times abruptly, over this period of interglacial-glacial-interglacial climatic change. These changes included millennial-scale changes, as well as significant decade- to century-scale change in atmospheric carbon and climate. However, the exact climate system mechanisms behind these observed changes have not been elucidated. In particular, an understanding of the mechanisms of the slow decrease followed by rapid increase in CO2 concentrations in the last 130 ka may shed light on important biogeochemical systems and their coupling with the physical climate system.

This initiative is designed to further our understanding of the natural regulation of atmospheric concentrations of CO2 and other radiatively active atmospheric constituents. Such understanding is still very partial, yet essential in order to understand the likely future consequences of fossil fuel burning, industrial emissions, and land use changes, including the feedback effects due to the influence of atmospheric and climate changes on terrestrial and marine processes.

Key issues include the following:

• Explaining the atmospheric composition at the last glacial maximum, when concentrations of major measured greenhouse gases (CO2, CH4, N2O) were exceptionally low while concentrations of both soluble and insoluble mineral dust over the land, ocean and ice sheets were extraordinarily high.

• Development and validation of a suite of trace gas source models and their coupling to AGCMs and atmospheric chemistry/transport models to predict atmospheric composition and its latitudinal gradients under changed climatic boundary conditions.

• Clarifying the sources and transport of mineral dust from the terrestrial surface and its possible implications (a) for radiative forcing in the atmosphere and (b) for marine primary production, with implications for glacial-interglacial changes in climate and atmospheric CO2, respectively.

• Documenting and explaining the time-course of changes in CO2 versus CH4 during periods of rapid climate change, including the last deglaciation and early Holocene.

• Understanding the changes in CH4 during the Holocene, and the origins of the changing latitudinal gradient in terms of changes in the distribution and activity of tropical versus boreal wetlands.

• Analysis of the global-scale controls on the oxidative capacity of the atmosphere over a wide range of boundary conditions.

In addressing this challenge it will be necessary to integrate models and data from across the entire spectrum of global change research. Components requiring integration include: coupled atmosphere-ocean and atmosphere-biosphere models, trace-gas source models (including inverse models) for marine and terrestrial ecosystems, atmospheric chemistry/transport models, ocean and terrestrial carbon models, linkages between the C and N cycles in marine and terrestrial ecosystems, paleoatmospheric measurements (including C isotopes) in ice-cores, terrestrial and marine paleorecords (including C isotopes), and possibly many others.

At present, there are several fundamental gaps in measurements and understanding which impede progress in determining the causes and consequences of changes in atmospheric trace gases. These include, among others,

• A predictive model for the distribution, growth/decay and functionality of wetlands, based on water balance, topography and surface hydrology [Sahagian, 1998 #5434];

• A comprehensive global carbon cycle model including geochemical components (weathering, flux of DOC and DIC down rivers, ocean-sediment interactions affecting total ocean alkalinity...) as well as biogeochemical components (coupled global cycles of N and C);

• Global-scale, process-based modelling of terrestrial biogenic fluxes of CH4, N2O, NMHC, NOx and CO and their responses to changes in climate and NPP (including effects of CO2 which may provide coupling between CO2 changes and other trace gas fluxes);

• Global synthesis of eolian dust flux estimates from ice cores, marine sediments and terrestrial loess deposits;

• High-precision ice-core measurements of N2O, CO, and 13CO2; if possible, ice-core measurements of 13CH4, CH3D etc.;

• Accurate determination of the paleolatitudinal gradient of CH4 with the help of data from tropical montane ice cores;

• Artifact-free ice-core CO2 measurements and determination of CO2 at fine time resolution during deglaciation and through Dansgaard-Oeschger events;

• Detailed quantification of the present distribution of clathrates (continental slope and permafrost regions) and their inclusion in global models;

• Fine time-resolution CH4 measurements across deglaciation to allow evaluation of the "clathrate-CH4 pulse" hypothesis;

• Synthesis of paleodata describing wetland distribution on the continents and continental shelves from LGM to present;

• Simultaneous flux measurements of several trace gases to help validate multicomponent trace-gas flux models;

The issues involved in the Paleo Trace Gas and Aerosol Initiative will require input from and collaboration between GAIM and various Core Projects (BAHC, DIS, GCTE, LOICZ, JGOFS, PAGES) with links to WCRP (CLIVAR). The work mandates the development of Earth System models that include atmospheric chemistry, trace-gas sources and a comprehensive global carbon cycle in addition to atmospheric, ocean circulation, surface hydrology and terrestrial ecosystem components. Such models will be presented with a powerful challenge which should provide motivation for their development.

Data needs: Pollen records, ice cores, marine sediments, lake records, high precision isotopic measurements, dust flux estimates, past wetlands distributions, charcoal flux records corals.

 

3B. What controls the sources of CO2, CH4, N2O, NOx, NMHC and CO in the biosphere and how are changes in climate likely to impact on the atmospheric concentrations of these gases? How can we explain the observed variability and trends of atmospheric aerosols, CO2, CH4, N2O and tropospheric O3 during recent decades?

Atmospheric aerosols play an important role in atmospheric chemistry as well as in the global radiation budget [Andreae, 1995 #5193]. Aerosol particles can be produced by two distinct mechanisms: direct injection into the atmosphere (e.g., soil dust, sea salt) resulting in so-called "primary" aerosols, or production of "secondary" aerosols by the conversion of gaseous precursors into liquid or solid particles. Primary aerosols dominate the "coarse" size fraction (diameter >1 µm), while secondary particles constitute most of the "fine" fraction (diameter <1 µm).

Aerosols interact with the Earth’s radiation budget directly by scattering and absorbing radiation, and indirectly by modifying the extent and radiative properties of clouds. These effects depend on the size spectrum of the aerosol particles and their chemical composition (which determines their optical characteristics). Both of these properties vary greatly in both space and time because aerosols have many different sources and short atmospheric lifetimes (hours to perhaps a month in the troposphere; months to a year or two in the stratosphere). This makes assessment of the climatic effects much more difficult than that of the long-lived and relatively evenly distributed greenhouse gases CO2, CH4, and N2O. The types of aerosols that contribute most to direct radiative forcing are sulfates from natural and anthropogenic sources, soil dust from wind erosion mainly in arid and semi-arid regions, and organic aerosols from oxidation of hydrocarbons.

Sulfate aerosols have one important anthropogenic source and one important natural source. The anthropogenic source is oxidation of sulfur dioxide (SO2) from fossil fuel combustion. Fossil fuel burning now accounts for about two-thirds of sulfur emissions to the atmosphere globally and this proportion is projected to increase as energy demands increase, particularly in Asia. The natural source of sulfate aerosols is oxidation of dimethylsulfide (DMS) produced by marine phytoplankton. The resulting aerosols can affect the population of cloud condensation nuclei (CCN) and thereby change the albedo of marine stratiform clouds. Several years ago a negative feedback loop was hypothesized whereby if phytoplankton productivity were to increase, more DMS would be produced leading to more sulfate aerosols and CCN, higher cloud albedo, and, hence, less light penetrating to the ocean surface which would slow photosynthesis and productivity. Proof of this hypothesis remains elusive. Recent studies suggest that sea salt aerosols may contribute more significantly to CCN populations than has been thought. They may also have a substantial direct radiative forcing effect in remote ocean areas. The production of sea salt aerosols depends exponentially on wind speed. Thus, climatic change in wind fields over the oceans would have a disproportionately large effect on sea salt production.

Soil dust is mobilized by strong winds, mainly in the great desert regions of North Africa, central Asia, and, to a lesser extent, Australia. Substantial amounts are transported great distances. For example, Asian dust is regularly detected at Hawaii in the central North Pacific and Saharan dust in consistently observed at Barbados in the western Atlantic. The size of the desert regions and the intensity of the winds over them influence the amounts of dust mobilized and the altitudes to which it is lofted. Desertification of large regions of the Sahel, tropical forest conversion, overgrazing, and inappropriate agricultural practices have probably increased dust injection to the atmosphere. Analysis of mineral aerosols in ice cores shows elevated levels of soil dust during the ice ages, when large continental shelf areas were open to erosion by strong winds in the Southern Hemisphere. Such an enhanced aerosol burden would contribute to intensified negative radiative forcing during an ice age.

Terrestrial ecosystems, especially those in the tropics, represent a very large source of organic matter to the atmosphere. They emit terpenes and other gaseous hydrocarbons which are oxidized photochemically in the atmosphere. The relatively low volatility products condense into organic aerosols. Forest vegetation also sheds organic particles in the form of waxy leaf cuticles and ejects droplets (guttation) which also leads to aerosol formation. This aerosol source must be expected to decline as tropical forests are reduced since grasslands and degraded lands which replace the forests appear to produce little biogenic aerosol. Anthropogenic hydrocarbons can presumably also be oxidized to organic aerosols. However, their emissions are much smaller than those of the natural compounds and their conversion efficiency to aerosols is probably quite low. Nevertheless, organic aerosols were recently found to make a major contribution to direct radiative forcing over the U.S. East Coast where sulfate aerosols would be expected to dominate. While measurements from a single campaign limited in time and space are by no means definitive, they do raise the possibility that organic aerosols are much more important in radiative forcing than has been thought. Depending on industrialization schemes in the developing world, industrial emissions could increase drastically in the future. Determining the relative contributions of marine, terrestrial and anthropogenic contributions and the effects of land use changes, nitrogen deposition and other factors will necessarily involve a large number of cross-cutting interdisciplinary activities across IGBP, WCRP and IHDP.

While the atmospheric abundances of CO2 and CH4 are increasing steadily, their rates of growth in the past decades have shown interannual variations as large as a factor of two [Keeling, 1995 #4080]. Although these changes in growth rates are not significant radiatively, their variations contain crucial information about the sensitivity of their exchange processes to climate and other perturbations. It is necessary to identify the agents for interannual variations and to quantify their sensitivities to climate perturbations in order to predict future abundances of these greenhouse gases. Ability to model the variations of the CO2 and CH4 growth rates in the past decades means the quantification of the sensitivity of the system response to seasonal and perturbations about the present-day climate. This is a prerequisite to future projections and to the formulation of stabilization strategies.

In the past decade, observations of the carbon system have expanded from concentration measurements at a few locations to now include measurements of isotopes and other atmospheric signatures (e.g. O2/N2 ratios) of biosphere-atmosphere exchange processes, terrestrial and oceanic productivity from satellites, and oceanic carbon parameters from repeat oceanic transects (WOCE), among others. IGBP-DIS has sponsored the compilation of many global datasets of vegetation, soils and oceans which underpin global models of the terrestrial and marine biosphere. In addition, there is new understanding gained from field campaigns that focus on processes (e.g. GCTE, JGOFS). GAIM has sponsored critical assessment of the capabilities of global models of the system modules in key model intercomparison projects. Current activities with which GAIM is involved that bear on the problem include the Ocean Carbon-Cycle Intercomparison Project (OCMIP), the Global Wetlands functional parameterization activity, the coupled atmosphere-land-ocean carbon system project, and to some extent others as well.

Simultaneously, under the auspices of WCRP, there has been marked progress in our understanding the El-Nino/Southern Oscillation and other aspects of seasonal to interannual climate variations, and in the compilation of earth system observations. We are now poised to proceed to the next logical phase and improve the biogeochemical models and integrate them with physical climate models with temporal resolution sufficient to highlight atmospheric composition variations at inter-decadal timescales.

Data needs: tower data, 1 km land cover, GPPDI.

3C. How would we expect future climate changes to interact with atmospheric trace gas composition, and what will be the consequences for scenarios covering the next 200 years?

The trace gas composition of the atmosphere is controlled by natural sources (e.g. wetlands, soil activity, air-sea interactions, etc.) as well as anthropogenic sources (e.g. rice production, cattle raising, fossil fuel combustion, biomass burning, agricultural fertilization, industrial emissions, etc.). Some trace gases are conservative in the atmosphere (e.g. CO2), but others (e.g. methane) are reactive at rates which may themselves be climate-dependent. While sources are the subject of question 3B, the fate of trace gases under conditions of climate change is not yet clearly understood, and must be explored before coupled terrestrial-marine-atmospheric prognostic biogeochemical models can be established.

Under conditions of atmospheric warming, sea surface temperatures (SSTs) would be expected to increase, profoundly affecting the solubility of CO2 in the mixed layer of the ocean. This leads to one of the strongest positive feedback mechanisms relevant to global climate. As the ocean uptake of CO2 is reduced, the increased greenhouse effect causes additional warming which further reduces uptake. There are interactions between this mechanism and global ecosystems as well, and there are several other important positive feedbacks between climate and trace gases. Atmospheric CO2 and N deposition both stimulate primary productivity and thus sources of all biogenic trace gases. Nitrogen deposition also stimulates N turnover and production of NOx and N2O. Increased concentrations of reduced trace gases increase the residence time in the atmosphere for all species, including radiatively active gases which then serve to increase temperature and release additional trace gases. Atmospheric warming leading to sea surface temperature increase would reduce the ocean's CO2 uptake, thus removing this important sink in the carbon budget, and leading to further warming.

The discussion above is based on a future carbon cycle which operates as it does today. This is by no means guaranteed or even likely given the profound changes occurring and expected in atmospheric composition. Climate change could well lead to substantial changes in ocean circulation, which would immediately affect the way the anthropogenic CO2 is exchanged between the atmosphere and the oceans [Sarmiento, 1996 #5000]. As one example, deep water formation in the North Atlantic ocean appears to have been turned on and off a number of times during our current ice age epoch [Broecker, 1997 #5592].

A second fundamental uncertainty concerns the operation of the biological pump. Atmospheric CO2 seeks chemical equilibrium with surface waters, not with the deep oceans. Photosynthesis by organisms in the photic zone keep the CO2 concentration of the surface waters substantially lower than the deep. Without photosynthesis in the oceans, and assuming no other surface changes due to organisms (such as calcification), the atmospheric CO2 concentration would be between 900 and 1000 ppm. If, on the other hand, photosynthesis were to proceed everywhere until all nutrients were fully depleted in all surface waters, atmospheric CO2 would be between 110 and 140 ppm. This illustrates the power of the biological pump. The actual pre-industrial CO2 concentration was 280 ppm. Surface nutrients are fully depleted in warm and temperate waters but not at high latitudes. In those areas there is less sunlight, biological processes are slower because of low temperatures, there is less time because surface water is mixed back into deeper layers more quickly, and the availability of micronutrients such as iron could be a factor [DeBaar, 1995 #5593]. The middle two factors are intimately connected with circulation, but all four factors are subject to climate change (the first through cloud cover, the last through deposition of dust).

Similarly for the continents, until we can identify and understand the current land sink of CO2, we cannot predict how much longer it will continue, or whether it will turn into a source instead. Unlike in the oceans, carbon storage on land is very much subject to direct human intervention. The recent Kyoto climate change negotiations included the possibility of harnessing photosynthesis on land to offset some of the fossil fuel emissions. Results with terrestrial biosphere models, including preliminary results with DGVMs, suggest that global warming may overall progressively reduce the strength of the terrestrial carbon sink, thus making CO2 stabilization more difficult. More fundamentally, continuing deforestation may affect regions where carbon is being sequestered today. The Kyoto potocol focuses attention on the need to quantify terrestrial sources and sinks as they operate today, as well as projecting how these sources and sinks may chnage as a consequence of changing land use and climate. At present, however, there is little agreement (either among models, or among different inverse calculations based on atmospheric 13C or O2 measurements) as to the geographic, distribution of the present-day terrestrial sink. Progress on this front requires not only continued refinement of the models but also that this proceeds in conjunction with the develoopment of more explicit verification techniques including, for example,

• the provision of more atmospheric measurement stations over the continents;

• the development of 24C-based approaches to estimating the turnover time of carbon in terrestiral systems; and

• greater attention to the validation of madels with the increasing wealth of direct measurements of CO2 and 13C fluxes bewteen ecosystms and the biosphere.

Both marine and terrestrial ecosystems respond to changes in CO2 concentration in the ocean and atmosphere, respectively. There has been some debate regarding the importance of CO2 fertilization and this will have to be resolved before the feedback between climate and CO2 and be quantified. In addition, the ocean is a source of atmospheric methane, although there is some doubt as to the magnitude of this source and its associated climate feedbacks [Bates, 1996 #4743]. Non-methane hydrocarbons (NMHCs) are produced by natural process in the terrestrial biosphere, in addition to anthropogenic sources. Natural sources are at least twice as strong as anthropogenic, but are very temperature-dependent [Lashof, 1989 #4734]. Consequently, we could expect to see a significant increase in NMHC concentrations in the atmosphere in response to global atmospheric warming.

The various interactions between climate and atmospheric trace gas composition will require consideration of a much broader range of possible scenarios than has been associated with the IPCC process. The models used to date have been oversimplified such that important qualitative results stemming from the feedbacks and interactions discussed above may have been overlooked.

Data needs: Atmospheric CO2 & trace gas records, 200 year land use, 200 year emissions data


4. Given our understanding of the couplings among physical and biogeochemical aspects of the Earth system, what will be the nature of its future interactions with human activities?

How will the nature of anthropogenic influences on the Earth System change in response to global change and how will the perception of environmental impacts alter human activities?

The most important product of Global Change research is a clear understanding of the relationships between climate, ecosystems, and the anthropogenic drivers of global changes such as land use and emissions. As IGBP matures and enters a synthesis phase, some of these relationships are becoming reliably quantified to the point of providing a basis for human activities and policies at personal, national, regional and even global scales. Examples of such relationships are found in CFC's with Ozone, various environmental pollutants with human health, and El Niño with agriculture and storm hazards. As human activities are ever more tightly linked with global change and clear connections are made between adverse environmental changes and anthropogenic drivers, aggregate human activities may be themselves changed in response to the realization or even perception [McDaniels, 1996 #4873] of their regional or global consequences. As such, in constructing prognostic biogeochemical models, it will be essential to incorporate the changes in human activities. In addition, there are changes in human activities which may be caused by the dissemination of the results of the models- as people learn of specific consequences of some types of land use and emissions, they may alter their activities so that model prediction do not come to pass. This feedback loop will lead to nonlinearities which will present a major challenge to the modelling community in the coming years. However, it is expected that the bulk of human decisions will not be made in response to model predictions, but rather to a large number of economic, social, adn political pressures. In order to address the problem of the interaction of human activities with the Earth system, we will focus on three subquestions, each of which is directed toward an important aspect of future anthropogenic drivers. While there are many additional questions and uncertainties regarding the future, these should provide a good starting point- If we can answer these, then we will be in a better position to address additional issues.

Study of the human dimensions of global environmental change encompasses the analysis of the anthropogenic causes of global environmental transformations, the consequences of such changes for societies and economies, and the ways in which people and institutions respond to the changes. It also involves the broader social, political, and economic processes and institutions that frame human interactions with the environment and influence human behavior and decisions. The impacts of global change on societies are expected to increase greatly in the next century. For example, much of the global warming caused by past human activities has not yet occurred. Likewise, the ecological consequences of deforestation and other land use changes are not manifest immediately, but could take decades to centuries before biome shifts can respond to the environmental changes caused by the feedback between climate and ecosystems.

Another major focus is the estimation of the social and economic consequences of anticipated global changes. Such estimates require integration of information regarding environmental changes with information on the social parameters that determine the impact of those changes such as demand for affect natural resources, vulnerability of geographical regions and social groups to particular environmental changes, and the potential for adaptive response.

The social and policy aspects of environmental changes have been investigated for many years, but formal linkages between socioeconomic research and global changes have only been made recently, and are becoming strengthened as IHDP develops its programs. This fundamental question (#4) represents a strong link between the modelling efforts of IGBP and those of IHDP.

4A. Can rapid climate change events, like those that have happened during ice ages and deglaciations, also be triggered by human alterations of the Earth system?

Rapid climate changes have been inferred to have occurred throughout the last few hundred thousand years on the basis of ice and sediment cores. The Younger Dryas was the most important rapid climate change event to occur during the last deglaciation of the North Atlantic region. The Younger Dryas ended abruptly 11,640 years ago as indicated by a rise in high latitude temperatures of 7ûC and a twofold increase in snow accumulation rates. The transitions into the Preboreal, the Preboreal/Younger Dryas transition, and the Younger Dryas/Holocene transition were all remarkably rapid, each occurring over a few decades and possibly even as little as a single decade.

Isotopic temperature records reveal 23 interstadial (Dansgaard/Oeschger) events, first recognized in the GRIP record and verified in the GISP2 record, between 110,000 and 15,000 years ago. These millennial-scale events represent large climate deviations- several degrees of temperature, twofold changes in snow accumulation, order-of-magnitude changes in dust and sea salt loading, and swings in atmospheric methane concentration of 100 ppb by volume. In view of these measures, the events are interpreted to have been of regional to global scale. These events are also recognized in the marine sediment record in regions critical to global ocean circulation.

Heinrich events (during glacial intervals) are short periods of time during which great numbers of icebergs are released to the ocean, carrying with them both fresh water and sediment to be delivered to the surface and bottom of the ocean, respectively. The influx of fresh water to the north Atlantic may disrupt deep-water formation, thus reducing the stability of the thermohaline circulation pattern and precipitating changes between alternative circulation modes [Broecker, 1994 #3894; Watts, 1990 #4680]. Heinrich events are related to Dansgaard-Oeschger cycles which are characterized by rapid increases in temperature, and atmospheric methane concentration (and decreases in e.g. atmospheric dust) followed by slower relaxation. Heinrich events appear to occur during the coldest part of a series of Dansgaard-Oeschger cycles [Bond, 1993 #4625], after which strong warming brings on a new set of Dansgaard-Oeschger cycles. Atmospheric CO2 appears to track only the lower of these processes, i.e. the series of D-O cycles but not the cycles themselves [Stauffer, 1998 #5591]. The impact of these changes was not confined to the N. Atlantic but appears to be very widespread as shown by the demonstrations of correlative cycling (e.g. Mediterranean cycles and Asian loess).

The relationship between the iceberg discharges (Heinrich events) and global climate depends on the inferred cause of the Heinrich events. If they are caused by glacial instabilities in a "binge and purge" scenario [MacAyeal, 1993 #4633; MacAyeal, 1993 #4632], then the influx of fresh water would be considered the driver for global climate change to which other glaciers appear to have responded. Alternatively, if externally-driven climate change causes the Heinrich events, then the same factors which control climate and atmospheric composition would have additional observable effects such as terrestrial ecosystem structure and hydrology. The implications of this, both conceptually and observationally, have not been examined in sufficient detail to distinguish between the two hypotheses, nor to explore the ramifications of each. However, there is some evidence for ice response to CO2 variations [Lindstrom, 1989 #4635]. Further, there is some lead isotopic evidence from Heinrich layers suggesting a Canadian source of glaciogenic sediments which would argue for the externally-driven scenario [Broecker, 1994 #3894]. In addition, instabilities in the coupled atmosphere-ocean circulation system may have periodicities on the order of millennia, and may drive or amplify Dansgaard-Oeschger cycles [Sakai, 1997 #4681]. Lastly, there is evidence of a connection between events in Greenland and Antarctica with a delay in Greenland of about 1-2 kyr [Blunier, 1998 #5747]. Some support for the involvement of "internal" periodicities comes from the fact that quasi-periodic abrupt climate changes persist through the present interglacial, albeit less dramatically, as observed in Holocene records from ocean sediments, lake-level changes and peat profiles.

Additional investigations involving a great number of proxy indicators of past climate change from all parts of the world have reinforced the initial findings, and have highlighted the vulnerability of the Earth's climate system to natural variability. Consequently, these findings have changed our view of the climate system and fundamentally undermined the notion that we have a relatively stable climate system, unperturbable by human activities. One of the most significant global effects of human society is the increase in atmospheric CO2 and other greenhouse gases. If rapid climate events are driven by external factors, then it may be possible to artificially impose the necessary conditions to trigger an alteration in the mode of ocean circulation and thus global climate. While the implications of this regarding effects of greenhouse gas emissions are profound, our understanding of the system is far from sufficient for predictions. Consequently, the following must be better documented and understood than at present:

• the provenance of Heinrich layers

• switching of the ocean circulation pattern

• the exchange of carbon with changing marine conditions

• the various interactions between ice, climate, marine, and terrestrial carbon cycles.

This will require integration of several aspects of IGBP research including PAGES, JGOFS, IGAC, BAHC, GCTE, and LOICZ, in addition to WCRP efforts such as CLIVAR.

Data needs: Pollen records, ice cores, marine sediments, corals.

4B. What are the likely relative magnitudes of the climate effects of different anthropogenic drivers of global change, e.g. land use versus fossil fuel burning?

While natural variability caused by orbital influences and terrestrial feedbacks has been an important (and until recently the sole) driver of climate changes, anthropogenic effects are playing an ever increasingly important role as a result of emissions and land use changes. However, the relative magnitudes of the effects of these are not well constrained. Unlike natural variability, the anthropogenic drivers have changed on time scales which are short relative to the response time of various parts of the Earth system. Consequently, the majority of climatic effects of these drivers will not be felt until decades to centuries in the future. This makes it essential to have the capability to model the effects of anthropogenic drivers so that projections can be made for the effects of past and present as well as future human activities.

Emissions from industry, fossil fuel burning, fires, livestock, agricultural wetlands such as rice paddies, etc. directly alter the chemical composition of the atmosphere. The climatic effects of these act through the atmosphere, both on a physical basis through radiative forcing and a biological basis through the influence of atmospheric composition on ecosystem productivity and distribution. Estimates of future emissions of greenhouse gases are highly sensitive to assumptions about future economic, technological and social changes, particularly with respect to autonomous rates of decarbonization and improvement in the energy-efficiency of technology and the likelihood of further large-scale economic transformations, as well as the stability of preferences. The utilization of energy and materials is multiply determined- they are not simple functions of population and economic activity, but depend on complex interactions of these factors and others. As such, future emissions of greenhouse gases will be the outcome of pressures from increasing affluence and population, as well as countervailing trends that reduce the amount of energy and materials used per unit of economic activity, and the rate of emissions per unit of energy and materials used.

Land use changes have a direct influence on albedo as well as on the exchange of water, carbon and nutrients between the land surface and the atmosphere. However, the magnitude of these effects is not well known, and thus the global change modelling community is unable to include the effect of land use changes quantitatively in global models. The situation is compounded by the lack of prognostic socio-economic models which are needed estimate the future alterations of the land surface in response to growing agricultural requirements. Nevertheless, recent studies such as that reported in the GCTE synthesis indicate that land use changes will be the dominant driver or global climate change on the basis of modifying the structure and function of terrestrial ecosystems. As socio-economic models develop, it will be possible to use projections of future emissions and land use changes in prognostic biogeochemical models, and eventually to couple the socio-economic, biogeochemical and physical models to form integrated Earth System Models.

While both emissions and land use have profound effects on climate, it is not readily apparent which will be the dominant driver of future climate change. Further, it is clear that they are not entirely independent in as much as biomass burning involves both land use and emissions. As global carbon models mature and are coupled to socioeconomic models, it will begin to be possible to predict the relative importance of land use and emissions.

Data needs: 200 year land cover/use data, 200 year emissions data

 

4C. How will an increased understanding of anthropogenic alterations of the Earth System affect future land use and emissions policy? What socio-economic factors will modulate enactment of and adherence to such policy, and to what extent do existing socio-economic conditions constrain future policy design, magnitude, implementation and time frame?

As we further develop our understanding of the causes and consequences of global change, it becomes possible to proactively alter the anthropogenic drivers to minimize environmental impacts. Thus it is difficult to conceptually separate the impacts of global change from response to it- in many cases, responses immediately modify the causes and thus impacts.

The growing international concern regarding the adverse effects of various aspects of global change has already begun to prompt guidelines and protocols (e.g. Montreal, Kyoto). This is being done with only limited quantitative information regarding anthropogenic perturbations and responses of the Earth System. Policy is enacted for a variety of reasons, most of which are not spurred by global environmental concerns, and many of which are at odds with the minimization of emissions and land use changes. Examples include birthrates and tax and other incentives for large families in some countries, Settlement of Amazonia, and Dam-building limits based on protection of local populations rather than global coastal communities (being flooded due to sea level rise). However, broad-based international policies are also being enacted which address global carbon budgets (e.g. Kyoto protocol), but these are general such that the vaguenesses may allow the intent to be counteracted by local governments. As model results become more reliable and detailed both in temporal and spatial scales of biogeochemical processes and consequences of anthropogenic drivers, it will be possible to enact more specific and thus less economically burdensome policies. This should result in effective reduction in detrimental changes in atmospheric composition and land cover, and the minimization of economic impacts will make it more likely that such policies will be enacted and adhered to.

The difficulties involved in enacting global emissions and land use policies given the great international heterogeneity in economic structure, social traditions, and political priorities are highlighted by the successes and failures at recent "climate conventions" such as in Rio and Kyoto. A basic issue for environmental management is that of designing incentive compatible institutions. Such institutions would be capable of internalizing, within individual households, private firms, and public organizations, the costs of the negative effects of human activities (externalities) when they are not penalized by the market. These externalities are a major source of environmental stress. Even if accurate measurement techniques are developed for the costs of such externalities, the question remains of how to design institutions to avoid generating these negative effects. We are now in the position of coping with the consequences of emissions and land use changes after the fact, rather than designing institutions which would minimize these consequences. It is clear that we are presently unable to identify specific policy instruments which would induce the broad international policy community to act in the interest of internationally agreed-upon goals. This lacking complicates the prediction of future land use and emissions levels and highlights the importance of robust modelling of socioeconomic, climate, and biogeochemical systems.

Data needs: economic trends at various scales through and since industrial revolution, educational levels, social systems, 200 yr land use, 200 yr emissions by country/region.

 


References

Andreae, M., Climatic effects of changing atmospheric aerosol levels, in Future Climates of the World: A Modelling Perspective, A. Henderson-Sellers, pp. 347-398, Elsevier, Amsterdam, 1995.

Bacastow, R., and G.R. Stegen, Estimating the Potential for CO2 sequestrian in the ocean using a carbon cycle model, presented adn 91i in Honolulu, 1991.

Banse, K., and D.C. English, Seasonality of Coastal Zone Color Scanner Phytoplankton Pigment in the Offshore Oceans, J. of Geophysical Research--Oceans, 99, 7323-7345, 1994.

Bates, T., K. Kelly, J. Johnson, and R. Gammon, A reevaluation of the open ocean source of methane to the atmosphere, Journal of Geophysical Research, 101, 6953-6961, 1996.

Bates, T.S., and P.K. Quinn, Dimethylsulfide (DMS) in the equatorial Pacific Ocean (1982 to 1996): Evidence of a climate feedback?, Geophysical Research Letters, 24, 861-864, 1997.

Bonan, G., D. Pollard, and S. Thompson, Effects of boreal forest vegetation on global climate, Nature, 359, 46-49, 1992.

Bond, G., W. Broecker, S. Johnson, J. McManus, L. Labeyrie, J. Jouzel, and G. Bonani, Correlations between climate records from North Atlantic sediments and Greenland ice, Nature, 365, 143-147, 1993.

Braswell, B., D. Schimel, E. Linder, and B.M. III, The Response of Global Terrestrial Ecosystems to Interannual Temperature Variability, Science, 278, 870-872, 1997.

Broecker, W., Thermohaline circulation, the Achilles heel of our climate system: Will mad-made CO2 upset teh current balance?, Science, 278, 1582-1588, 1997.

Broecker, W.S., Massive iceberg discharges as triggers for global climate change, , 372, 421-424, 1994.

Bruno, M., and F. Joos, Terrestrial carbon storage during the past 200 years: A Monte Carlo analysis of CO2 data from ice core and atmospheric measurements, Global Biogeochemical Cycles, 11, 111-124, 1997.

Ciais, P., P. Tans, J. White, M. Trolier, D. Schimel, and others, Partitioning of Ocean and Land Uptake of CO2 as Inferred by Delta-C-13 Measurements from the NOAA Climate Monitoring and Diagnostics Laboratory Global Air Sampling Network, J. Geophys. Res., 100, 5051-5070, 1995.

Claussen, M., Variability of global biome patterns as a function of initial and boundary conditions in a climate model, Clim. Dyn., 12, 371-379, 1996.

Conkright, M.D., and E.K. Asem, Global Ionosperic Effects of the October 1989 Geomagnetic Storm, Journal of Geophysical research-Space Physics, 99, 6201-6218, 1995.

Conway, T.J., P. Tans, L. Waterman, K. Thoning, D. Buanerkitzis, K. Masarie, and N. Zhang, Evidence for interannual variability of the carbon cycle from the NOAA/CMDL global air sampling network, J. Geophys. Res., 99D, 22831-22855, 1994.

Crowley, T., and S. Baum, Effect of vegetation in ice age climate model simulation, J. Geophys. Res., 102, 16463-16480, 1997.

DeBaar, H., J. deJong, D. Baker, B. Loscher, C. Veth, U. Bathmann, and V. Smetacek, Importance of iron for plankton blooms and carbon dioxide drawdown in the Southern Ocean, Nature, 373, 412-415, 1995.

Dianovklokov, V.I., and L.N. Uarganov, Spectroscopic Measurements of Atmospheric Carbon Monoxide and Methane. 2. Seasonal Variations and Long-Term Trends, Journal of Atmospheric Chemistry, 8, 153-164, 1989.

Dickinson, R.E., and A. Henderson-Sellers, Modelling tropical deforestation: a study of GCM land-surface parameterizations, Quart. J. Roy. Meteor. Soc., 114(B), 439-462, 1988.

Enting, I., and J. Mansbridge, Seasonal sources and sinks of atmospheric CO2. Direct inversion of filtered data, Tellus, 39B, 318-325, 1989.

Enting, I., and J. Mansbridge, Latitudinal distribution of sources and sinks of CO2: Results of an inversion study, Tellus, 43B, 156-170, 1991.

Enting, I., C. Trudinger, and R. Francey, A synthesis inversion of the concentration and delta 13C of atmospheric CO2, Tellus, 47B, 35-52, 1995.

Foley, J., J. Kutzbach, and M. Coeslevis, Feedbacks between climate and Boreal forests during the Holocene epoch, Nature, 371, 52-54, 1994.

Francey, R.J., P. Tans, C. Allison, I. Enting, J. White, and M. Trolier, Changes in oceanic and terrestrial carbon uptake since 1982, Nature, 373, 326-330, 1995.

Fung, I., J. John, J. Lerner, E. Matthews, and others, 3-Dimensional model synthesis of the global methane cycle, J. Geophys. Res., 96, 13033-13065, 1991.

Fung, I., K. Prentice, E. Mathews, J. Lerner, and G. Russell, Three-dimensional tracer model study of atmospheric CO2: Response to seasonal exchanges with the terrestrial biosphere., J. Geophys. Res., 88, 1281-1294, 1983.

Hartley, D., and R. Prinn, Feasibility of determining surface emissions of trace gases using an inverse method in a three-dimensional chemical transport model, J. Geophys. Res., 98, 5183-5197, 1993.

Hegg, D.A., L.F. Radke, and P.V. Hobbs, Measurements of Aitken Nuclei and Cloud Condensation Nuclei in the Marine Atmosphere and Their Relation to the DMS-Cloud-Climate Hypothesis, Journal of Geophysical Research-Atmospheres, 96, 18727-18733, 1991.

Heimann, M., and C. Keeling, A three-dimensional model of atmospheric CO2 transport based on observed winds: 2. Model description and simulated tracer experiments, in Aspects of Climate Variability in the Pacific and Western Americas, D.H. Peterson, pp. 237-275, 305-363, Amer. Geophys. U., Washington D.C., 1989.

Henderson-Sellers, A., and V. Gornitz, Possible climatic impacts of land cover transformations, with particular emphasis on tropical deforestation, Climatic Change, 6, 231-257, 1984.

Houghton, J.T., L.G.M. Filho, B.A. Callandar, N. Harris, A. Kattenberg, and K. Maskell, Climate Cahnge 1995: Contribution of Working Group 1 to the Second Assessment Report of the Intergovernmental Panel on Climate Change., Climate Change 1995: The Science of Climate Change, 572 pp., Cambridge University Press, new York, 1995.

Keeling, C., T. Whorf, M. Wahlen, and J.V.D. Plicht, Interannual extremes in the rate of rise of atmospheric carbon dioxide since 1980, Nature, 375, 666-670, 1995.

Kutzbach, J., G. Bonan, J. Foley, and S. Harrison, Vegetation and soil feedbacks on the response of the African monsoon to the orbital forcing in the early to middle Holocene, Nature, 381, 503-505, 1996.

Lashof, D., The Dynamic Greenhouse: Feedback Processes That May Influence Future Concentrations Of Atmospheric Trace Gases And Climatic Change, Climatic Change, 14, 213-242, 1989.

Law, R., P. Rayner, A. Denning, D. Erickson, I.Y. Fung, M. Heimann, S. Piper, M. Ramonet, S. TAguchi, J. Taylor, C. Trudinger, and I. Watterson, Variations in modeled atmospheric transport of carbon dioxide and the consequences for CO2 inversions, Global Biogeochem. Cycles, 10, 783-796, 1996.

Leuenberger, M., U. Siegenthaler, and C.C. Langway, Carbon Isotope Composition of Atmospheric CO2 During the Last Ice Age from an Antarctic Ice Core, Nature, 357, 488-490, 1992.

Levitus, S., and T.P. Boyer, World Ocean Atlas 1994, NOAA Atlas NESDID 4, NODC, , Washington, D.C., 1994.

Lindstrom, D., and D. MacAyeal, Scandinavian, Siberian, and Arctic Ocean glaciation - Effect of Holocene atmospheric CO2 variations, Science, 245, 628-631, 1989.

MacAyeal, D., Binge/purge oscillations of the Laurentide ice sheet as a cause of the north Atlantic Heinrich events, Paleoceanography, 8, 775-584, 1993a.

MacAyeal, D., A low-order model of the Heinrich event cycle, Peleoceanography, 8, 767-773, 1993b.

Maier-Reimer, E., U. Mikolajewicz, and A. Winguth, Future Ocean Uptake of CO2--Interaction Between Ocean Circulation and Biology, Climate Dynamics, 12, 711-721, 1996.

Manabe, S., and R.J. Stouffer, Multiple-Century Response of a Coupled Ocean-Atmosphere Model to an Increase of Atmospheric Carbon Dioxide, Journal of Climate, 7, 5-23, 1994.

McDaniels, T., L. Axelrod, and P. Slovic, Perceived ecological risks of global change, Global Envir. Change, 6, 159-171, 1996.

Mortlock, R.A., C.D. Charles, P.N. Froelich, M.A. Zibello, and e. al, Evidence for Lower Productivity in the Antarctic Ocean during the Last Glaciation, Nature, 351, 220-223, 1991.

Mosier, A., D. Schimel, D. Valentine, K. Bronson, and others, Methane and Nitrous Oxide Fluxes in Native, Fertilized and Cultivated Grasslands, Nature, 350, 330-332, 1991.

Murray, R.W., M. Leinen, and A.R. Isern, Biogenic Flux of A1 to Sediment in the Central Equatorial Pacific Ocean-Evidence for Increased Productivity during Glacial Periods, Paleoceanography, 8, 651-670, 1993.

Myneni, R.B., C.D. Keeling, C.J. Tucker, G. Asrar, and e. al., Increased plant growth in the northern high latitudes from 1981 to 1991, Nature, 386, 698-702, 1997.

Najjar, R.G., and R.F. Keeling, Analysis of the mean annual cycle of the dissolved oxygen anomaly in the World Ocean, Journal of Marine Research, 55, 117-151, 1997.

Najjar, R.G., J.L. Sarmiento, and J.R. Toggweiler, Downward transport and fate of organic matter in the ocean: simulations with a general ocean circulation model, Global Biogeochemical Cycles, 6, 45-76, 1992.

Platt, U., and G. LeBras, Influence of DMS on the O-x-NOy partitioning and the NOx distribution in the marine background atmosphere, Geophysical Research Letters, 24, 1935-1938, 1997.

Prather, M., M. McElroy, S. Wofsy, G. Russel, and D. Rind, Chemisty of the global troposphere: Fluorocarbons as tracers of air motion, J. Geophys. Res., 92, 186, 1987.

Randerson, J.T., M.V. Thompson, T.J. Conway, I.Y. Fung, and e. al, The contribution of terrestrial sources and sinks to trends in the seasonal cycle of atmospheric carbon dioxide., Global Biogeochemical Cycles, 11, 535-560, 1997.

Rayner, P., and R. Law, A comparison of modelled responses to prescribed CO2 sources, CSIRO Division of Atmospheric Research, 1995.

Sahagian, D.L., Global Wetland Distribution and Functional Characterization: Trace Gases and the Hydrologic Cycle, IGBP, Stockholm, 1998.

Sahagian, D.L., and A.B. Watts, Introduction to the special volume on measurement, causes and consequences of long term sea level changes, J. Geophys. Res., 96, 6585-6590, 1991.

Sakai, K., and W.R. Peltier, Dansgaard-Oeschger Oscillations in a coupled Atmosphere- Ocean Climate Model, Journal of Climate, May 1997, 949-970, 1997.

Salati, E., Amazon: forest and hydrological cycle, In: C. Rosenzweig & R. E. Dickinson (Ed.) Climate-vegetation interactions, 110-112, 1986.

Sarmiento, J., J. Orr, and U. Siegenthaler, A Perturbation Simulation of CO2 Uptake in an Ocean General Circulation Model, J. Geophys. Res., 97, 3621-3645, 1992.

Sarmiento, J.L., and C. LeQuere, Oceanic Carbon Dioxide Uptake in a Model of Centruy-Scale Global Warming, Science, 274, 1346-1350, 1996.

Siegenthaler, U., and J. Sarmiento, Atmospheric Carbon Dioxide and the Ocean, Nature, 365, 119-125, 1993.

Solomon, A., I.C. Prentice, R. Leemans, and W.P. Cramer, The Interaction of Climate and Land use in Future Terrestrial Carbon Storage and Release, Water, Air, and Soil Pollution, 70, 545-614, 1993.

Stauffer, B., T. Blunier, A. Dallenbach, A. Indermuhle, and others, Atmospheric CO2 concentration and millenial-scale climate change during the last glacial period, Nature, 392, 59-62, 1998.

Takahashi, T., R.A. Feely, R.F. Weiss, R.H. Wanninkhof, and e. al, Global air-sea flux of CO2: An estimate based on measurements of sea-air pCO(2) difference, Proceedings of the National Academy of Sciences of the United States of America, 94, 8292-8299, 1997.

Tans, P., T. Conway, and T. Nakazawa, Latitudinal distribution of the sources and sinks of atmospheric carbon dioxide derived from surface observations and an atmospheric transport model, J. Geophys. Res., 94, 5151-5172, 1989.

Tans, P., I. Fung, and T. Takahashi, Observational constraints on the global atmospheric CO2 budget, Science, 247, 1431-1438, 1990.

Tooley, M.J., Long term changes in eustatic sea level, in Climate and sealevel change: Observations, projections, and implications, E.B. R. Warrick, T. Wigley, pp. 81-107, Cambridge University Press, Cambridge, 1993.

Turner, B.L., D.L. Skole, S. Anderson, G. Fischer, L. Fresco, and R. Leemans, Land Use and Land-Cover Change: Science/Research Plan, IGBP, 35, 1995.

Volk, T., Multi-property modeling of ocean basin carbon fluxes, nasa contractor report, 1988.

Vorosmarty, C.J., R. Wasson, and J. Richey, Modelling the Transport and Transformation of Terrestrial Materials to Freshwater and Coastal Ecosystems, IGBP, Stockholm, 1997.

Walker, B., and W. Steffen, A Synthesis of GCTE and Related Research, IGBP, Stockholm, 1997.

Watts, R., and M. Morantine, Rapid Climatic Change and the Deep Ocean, Climatic Change, 16, 83-97, 1990.

Woodwell, G.M., J.E. Hobbie, R.A. Houghton, J.M. Melillo, B.I. J. M. Moore, B.J. Peterson, and G.R. Shaver, Global deforestation: contribution to atmospheric carbon dioxide, Science, 222, 1081-1086, 1983.

Yoder, J.A., C.R. McClain, G.C. Feldman, and W.E. Esaias, Annual Cycles of Phytoplankton Chlorophyll Concentrations in the Global Ocean--A Satellite View, Global Biogeochemical Cycles, 7, 181-193, 1993.