There are many different approaches within the family of integrated models. This diversity is important for a balanced understanding of the issues because different types of models can shed light on different aspects of the same problem. For example, many analyses start with a particular emissions baseline and examine the economic and ecological implications of meeting a given emissions target (e.g., Alcamo, 1994; Edmonds et al., 1997; Morita et al., 1997b; Murty et al., 1997; Yohe, et al., 1998; Jacoby and Wing, 1999; Nordhaus and Boyer, 1999; Tol, 1999a,b; Yohe and Jacobsen, 1999). In such analyses, impacts are first assessed under a so-called "business-as-usual" or reference-case scenario. The analysis is then repeated with a constraint on the future. The change in impacts represents the climate-related benefits of the policy.
Other approaches select a different starting point. For example, Wigley et al. (1996) begin with atmospheric CO2 concentrations and explore a range of stabilization targets. For each target they employ "inverse methods" to determine the implications for global CO2 emissions. Recognizing that a particular concentration target can be achieved through a variety of emission pathways and that impacts may be path-dependent, they identify the implications of the choice of emissions pathway on temperature change and sea-level rise.
Two other approaches"tolerable windows" (Toth et al., 1997; Yohe, 1997; Petschel-Held et al., 1999; Yohe and Toth, 2000) and "safe corridors" (Alcamo et al., 1998)also utilize inverse methods but begin further down the causal chain. Here the focus is on the range of emissions that would keep emission reduction costs and climate change impacts within "acceptable" limits. Working with policymakers, the analysts identify the set of impacts for consideration. Bounds are specified, and the cost of achieving the objective is calculated. If mitigation costs are deemed too costly, policymakers have the opportunity to relax the binding constraint. In this way, the team is able to move iteratively toward an acceptable solution.
Integrated assessment analyses also can be distinguished by their approach to optimization. For example, the focus of the UNFCCC is cost-effectiveness analysis. Article 2 states that the ultimate goal is "stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system." Mitigation cost is more of a consideration in how the target is to be achieved. The Convention states that policies and measures to deal with climate change should be cost-effective to ensure global benefits at the lowest possible cost. Several analysts, beginning with Nordhaus (1991), have identified the least-cost path for achieving a particular concentration target.
Despite the goal of the UNFCCC, several integrated assessment frameworks are designed for benefit-cost analyses. These models identify the emissions pathway that minimizes the sum of mitigation costs and climate change damages. Such policy optimization models have been developed by Nordhaus (1991, 1992, 1994b), Peck and Teisberg (1992, 1994, 1995), Chattopadhyay and Parikh (1993), Parikh and Gokarn (1993), Maddison (1995), Manne et al. (1995), Manne and Richels (1995), Nordhaus and Yang (1996), Yohe (1996), Edmonds et al. (1997), Tol (1997, 1999c,d), and Nordhaus and Boyer (1999).
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