Integration of IGBP Science & Earth System Modelling


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 an integrated prognostic biogeochemical model. Such integration will involve coordination with each of the IGBP Core Projects. The integration program will be structured in three segments, 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 segments will be at the IGBP Core Project level, where GAIM will work with Core Project modelling teams to help facilitate inter-subsystem coordination.

Subsystem models
The subsystem integration plan 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 has linked atmosphere GCM's (mainly as drivers) with ocean GCM's containing carbon chemistry and crude biology; b) similarly, the terrestrial carbon models (NPP Efforts) are being driven (partially) by GCM results; and c) we are taking the preliminary steps with the GCM Atmosphere transport codes studies 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 is 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 convene workshops 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 formulation. The parallel development of coupled Earth system models has several advantages. The first 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. This 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.

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, 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 486 class machine (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; whereas, the purpose of the latter is more out-reach and educational.

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. Consequently, we will 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. We plan to initiate this effort with a workshop in early 1988 with the goal of comparing the sensitivity of various models to a suite of input parameters.

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 an integrated Earth system model. 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 to that the necessary parameters will flow through one subsystem model to the next. 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, the Task Force is composed of key scientists from these leading institution world-wide. The composition of the Task Force is determine 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.

This activity ties in closely with the subsystem level problem described above. 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. We will work closely with each of the IGBP Core Project modelling teams to help direct the modelling efforts in a direction which will result in the most efficient coupling possible.