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Thematic Guide to Integrated Assessment Modeling



Massachusetts Institute of Technology

Begun in 1991, the Joint Program on the Science and Policy of Global Change at the Massachusetts Institute of Technology (MIT) is a three-stage program with a minimum horizon of 10 years. The first of the three stages, establishing the basic framework of the study, is now under way (MIT 1993).

The primary element in this program is the development of a large integrated model to be constructed principally by reconciling and joining existing component models. The first stage of work involves runs of component models and judgmental integration between modules.

This model is modular, designed to be able to accommodate improvements in each component as understanding advances. The stated goals of the modeling project are that it should be global in scope, encompass all key aspects of the climate-change issue, be capable of evaluating policy choices, be able to support uncertainty analysis, and be adaptable to new insights and improved components.

Emissions are modeled by an extensive modification and update of the GREEN model, a computable general-equilibrium model originally developed at the OECD (Oliveira-Martins et al. 1992). GREEN represents the economies of 12 world trading regions, each with 14 production sectors. The model computes an equilibrium by maximizing the present-value utility of consumption under a myopic-recursive formulation. Production technology is represented in a set of nested input-output models, through which technologies are represented with somewhat greater disaggregation than the 14 basic sectors, permitting some input substitution. Technological change is represented by externally determined changes of some input-output coefficients over time and in specified backstop technologies. Major modifications of GREEN being undertaken include adding emissions of non-CO2 greenhouse gases and aerosols; mapping GREEN's 12 world regions onto latitude bands to provide emissions in the form required for modeling their atmospheric chemistry; and making the model's formulation more dynamic, which represents the most serious challenge of the three. Subsequent planned revisions include improving the emissions modeling by disaggregating the agriculture sector of GREEN and developing a land-use model.

The atmosphere is currently represented by two models, one for chemistry and one for climate dynamics, which will later be merged. The climate model is a two-dimensional zonally averaged model that closely parallels the Goddard Institute of Space Studies (GISS) Global Circulation Model (GCM), and which includes all significant greenhouse gases and 12 types of aerosols. (This model explicitly represents only latitude and altitude, with the average climate results at each latitude and altitude closely matching the results obtained from the three-dimensional GISS GCM.) Resolution is nine vertical layers by 24 latitude bands. The model presently includes a simple ocean and will be extended to include a three-dimensional ocean before being similarly expanded for land, since changes in ocean circulation appear, in recent transient GCM runs, to be the most important driver of regional climate change around doubled-CO2 levels.

The project will represent climate impacts on unmanaged ecosystems using the Terrestrial Ecosystems Model (TEM) of Melillo et al. (1993), a process-based model of ecosystems that estimates how changed climate and doubled CO2 will change net primary production and nitrogen cycling, at one-month time step and half-degree spatial resolution. The model represents 18 major vegetation types parametrically and projects how the net primary productivity (NPP) and nitrogen cycling of each type will be changed by changed climate and CO2 concentration, but does not examine how vegetation types might move as a consequence of these changes.

Planning is still under way to determine how the project will relate impacts in specific market sectors to changes in agricultural productivity and ecosystemic change. Most impacts will be expressed in monetary terms. Where this is not appropriate or feasible, a vector of significant valued impacts will be developed for non-market impacts. Representation of a variety of emission abatement policies is straightforward within the GREEN model component, and policies for adaptation will be reflected in the impacts models.

The integrated model will be computationally demanding. Component models are presently being run on two high-speed workstations. One of these is dedicated to the climate model, which is the most computationally demanding, running on the order of 100 times per year. (In contrast, one run of GREEN takes about 5 minutes on this machine.) All other models run on the second machine. When the integrated model is running, it will be used for uncertainty analysis but the precise form of such analysis is still under discussion. Relatively long run times will likely preclude a full sample-based analysis of parameter uncertainties; rather, the model may be used to examine sensitivity of key outputs to a small number of major discrete alternatives, either in the form of scenarios or in structural representation of particular submodels.

Other activities of the project include semi-annual Policy Forums used to keep project researchers in touch with policy-makers, and a planned series of short notes on aspects of global climate change for lay-people. The project's first report (Jacoby and Prinn 1994) summarizes current scientific understanding of climate change and related processes in non-technical terms, and discusses key current uncertainties and their policy implications.

Further information on this project and its activities can be obtained by contacting the following:

MIT Joint Program on the Science and Policy of Global Change
1 Amherst Street
Bldg. E40-271
Cambridge, MA 02139-4307


The next section is Battelle Pacific Northwest Laboratories.





Parson, E.A. and K. Fisher-Vanden, Searching for Integrated Assessment: A Preliminary Investigation of Methods, Models, and Projects in the Integrated Assessment of Global Climatic Change. Consortium for International Earth Science Information Network (CIESIN). University Center, Mich. 1995.


Suggested Citation

Center for International Earth Science Information Network (CIESIN). 1995. Thematic Guide to Integrated Assessment Modeling of Climate Change [online]. Palisades, NY: CIESIN. Available at [accessed DATE].



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