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Thematic Guide to Integrated Assessment Modeling
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Future Emissions Paths
Future climate change depends on the future path of world emissions, which any integrated assessment study must somehow project. Methods used to do so largely fall into three classes: external specification of emission scenarios, detailed representation of technologies, and aggregate economic modeling.
External Specification of Scenarios
Many studies specify externally a few representative scenarios of emission time-paths, either as discretely specified separate futures (often four, to discourage the reader from taking the middle of three as a prediction) or as points on a probability distribution of future emissions. Scenarios may be drawn from other studies or from authoritative quasi-governmental sources such as the Intergovernmental Panel on Climate Change (IPCC). It is usual and advantageous for different assessments to use common input scenarios, to standardize their inputs and permit controlled comparison of results. Emission scenarios are most often used when the assessment focuses on other, downstream aspects of the climate issue. While scenarios may reflect causal modeling of emissions done in the study that originated them, this modeling is normally inaccessible when its results are imported as scenarios. Consequently, studies based on emissions scenarios can normally do only limited investigation of policies to change emission paths.
Specification of Technologies That Generate Emissions
The second approach is detailed specification of the technologies that generate emissions, often called "bottom-up" modeling. In this approach, the present and future mix of technologies in each economic sector is described by its costs, inputs, and outputs, including emissions. Descriptions can be at levels of aggregation ranging from broad economic sectors down to individual plants. The advantage of this approach is that it allows precise specification of particular known or projected technical innovations. For example, one can specify that a particular kind of combined-cycle gas turbine for electrical generation will be available in the year 2005, with specified cost, efficiency, and CO2 emissions. But the approach can require specifying time-paths of huge numbers of technical coefficients, risking arbitrariness and spurious precision.
Economic Modeling With Embedded Emissions Coefficients
The third approach is economic modeling with embedded emission coefficients, projecting future emissions as the outcome of specified production relationships, preferences, and aggregate growth. This approach permits the effect of economic policies to be represented, through the dependence of emitting activities on prices and incomes. Detail can range from simple models of the aggregate economy, through aggregate models coupled to more detailed representation of the energy sector, through full dynamic general equilibrium models. The most basic design issues in economic modeling of future emissions include whether agents have foresight and how they make decisions over time (including whether they are able to make labor-leisure and consumption-saving decisions as well as commodity consumption decisions); how capital is structured and vintaged, and whether it is malleable once invested; the degree of disaggregation among world regions and how they interact through flows of capital, people, and goods; and perhaps most fundamental, the extent to which common determinants of behavior are assumed to hold in all world regions, e.g., the basis on which investment decisions are made.
Other Considerations
Mixtures of these three approaches are possible. Some studies, for example, model trends in CO2 emissions but use scenarios for other gases. Others combine an aggregate model of the economy with enough technical detail in certain emission-intensive sectors, such as agriculture or energy production, that specific new technologies can be added to the economy.An important question in projecting emissions, which cuts across these approaches, is which emissions are tracked. Some assessments project only CO2; others project all greenhouse emissions, but express them in a simple CO2-equivalent metric; others specify emissions of each major greenhouse gas separately. It is becoming increasingly clear that detailed atmospheric modeling requires specifying emissions of aerosols as well as greenhouse gases, and that local air-pollutant emissions can also be implicated in determining changes in radiative forcing.
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