Trace Gas & Aerosol Cycles in the Earth System (Traces)

Unraveling natural variability and linkages of trace-gases and climate over the last 150 k yrs

Click here to see a summary of the latest Traces workshop in 1999.

The Paleo trace gas "challenge" is based on unraveling the natural variability and linkages of trace-gases and climate over the last 150 ky. Natural atmospheric trace-gas concentrations and climatic change are clearly linked, but the full extent of these linkages have, obviously, yet to be fully understood. In seeking to measure improvements in our understanding, we can test our models against the historic record, recognizing that there are differences between past and future change. An important test, for example, is to explain the co-evolution of climate and atmospheric carbon change over the last 150 kyrs. 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 150 kyrs may shed light on important biogeochemical systems and their coupling with the physical climate system. A preliminary discussion of this problem began at the recent GAIM Science Conference, where it was suggested that it was important to understand the events during the transitions from the last glacial to the present interglacial and from the last interglacial to the last glacial, because of the strong variations of the entire Earth system, as reflected in the biogeochemical parameters.

Paleo-parameters are generally not identical with the parameters measured in monitoring networks and used in model experiments. Consequently, it is critical to understand the relationships between available observational data/measurements and the quantities of interest in global biogeochemical models. Transformation of proxy information into model parameters is normally accomplished through transfer functions. The importance of proxy-information for Global Change science is often underestimated because of the difficulty in converting them into the standard physical and chemical parameters. On the other hand, the proxy information reflects the actual regional and local impact of extreme weather events and of climate change in the past and thus gives indications regarding potential future impacts, in which we are ultimately interested.

A new joint GAIM-PAGES-IGAC effort is planned to focus research on defining the causes of the observed atmospheric carbon changes over the last 150 ky, and in particular the roles of the terrestrial biosphere and oceans in generating these changes. This effort will involve close interaction between the modelling communities of GAIM (e.g., terrestrial biosphere, ocean modelling), PAGES (e.g., physical atmosphere, ice modelling), and IGAC (e.g. trace gases). Emphasis will be placed on the use of time-dependent, coupled climate system models, and in producing models that are capable of simulating all aspects of the observed record of change. GAIM will work with PAGES and IGAC to develop the observational framework needed on the basis of "paleo" data, including records of oceanographic (e.g., physical, biological, and chemical), terrestrial (e.g., vegetation and soils), cryospheric (e.g., ice sheet and sea ice), and atmospheric (e.g., trace-gas) changes. A specific result of the project will be an improved ability to anticipate future natural and anthropogenic trace-gas and climatic changes.

The outline of a strategy to address the major issues in the "challenge" emerged at the IGBP Congress in April, 1996. The strategy is based on two complementary approaches commonly used in Quaternary science- "time slice" and "time series". These approaches are currently pursued separately, both by data specialists and by modellers. In addition, the key representatives from each IGBP Core Project were determined (See "Personnel", below).

The time slice approach is typified by BIOME 6000: data are gathered for a particular period (which must be fairly "thick", e.g. often ± 500 yr) because of dating control problems, the emphasis being on spatial resolution, and on establishing generalized spatial patterns in variables that are very extensively measured. These patterns can then be compared with "snapshot" simulations using full three-dimensional models, with slowly varying boundary conditions (e.g. orbital conditions, ice-sheet extents and heights) specified. The time slice approach was adopted by CLIMAP for sea-surface conditions, extended to the terrestrial realm by COHMAP [COHMAP, 1988; Wright et al., 1993], and is currently being pursued further by TEMPO and PMIP/PMAP. The approach has led to major advances in our understanding of the processes determining the broad outlines of regional to continental scale changes in climate on glacial interglacial time scales.

The time series approach instead focuses on high-resolution temporal studies, often including more esoteric signals, either of global quantities (such as atmospheric CO2 concentration) or of more local signals. Such studies are epitomized by ice-core research, but similar tactics are increasingly being adopted in marine and terrestrial sedimentary investigations. The modelling counterpart is the development of "2 1/2- dimensional" models which include highly simplified representations of the oceanic and atmospheric circulations, in order to introduce the possibility of interactively modelling the dynamics of sluggish parts of the system such as ice sheets and crustal deformation [Gallee, 1992; Peltier and Marshall, 1995]. The time series approach has yielded major advances in the last five years with regard to Dansgaard-Oeschger events, Bond cycles and Heinrich events as well as the recognition of considerable fine structure in the climate of the present interglacial. Time-dependent models also have contributed to our understanding of the factors that may be involved in pacing the glacial-interglacial cycles, and most recently also the nature of the oscillations in the thermohaline circulation under different freshwater flux regimes (glacial vs. interglacial).

We plan to further develop the time slice approach in the next four years, with activities directed toward improving our understanding of the controls on atmospheric composition on long time scales. Some key areas are as follows:

Priorities in the further development of time series investigations include: