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#### 2 Modeling the Climate System within the Broader SCC Modeling Structure Pages 9-16

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 From page 9...... The SCCCIAMs use the causal chain of mo e odeling steps to project in s ncremental cchanges in cclimate changge and resul lting econom damages; see Figure 2-1. mic FIGURE 2-1 SCC modeling cau chain. Read the entire page → From page 10...... There are several steps in the causal chain for each SCC-IAM that are worth highlighting because they are different across models and have notable implications for the ultimate calculation of an SCC estimate. We discuss these differences in more detail below, but flag them here:  emissions can vary in terms of their coverage and time path;  the reference and perturbed temperature trajectories depend on the way the climate system is modeled within each SCC-IAM; and  there are significant observed differences in the global climate responses across SCC IAMs and the regional temperatures derived by downscaling (i.e., by establishing geographically fine-scale information from changes in aggregate climate conditions) Read the entire page → From page 11...... othera Additive functions for coastal Quadratic function of Power function of Sea Level Rise protection costs, dryland loss, and global sea level rise global sea level rise (SLR) Damage wetland loss, based on an internal Specification cost-benefit rule for optimal (i.e., Damage = αSLR2) Read the entire page → From page 12...... In averaging the results across models and emissions scenarios, all models and all emissions scenarios are given equal weight. Figure 2-2 is an example of the resulting frequency distribution for 2020 SCC estimates as reported in the IWG's 2015 technical support documents.13 The average value of the SCC is shown for each discount rate, using a vertical line, as is the 95th percentile of the frequency distribution of SCC results for the case of a 3 percent discount rate. Read the entire page → From page 13...... Sp pecifically, th average value of the S he v SCC is repor rted, as well as the 1st, 5 5th, 10th, 25t 50th, 75th 90th, 95th and 99th percentiles of the frequen distribut th, h, h, p f ncy tion of SCC estimates Table 2-2 illustrates th for a disc s. his count rate of 3 percent fo each emis f for ssions scena ario (i.e., 15 sets of result s ts) Read the entire page → From page 14...... SCC ESTIMATE E ES In summary, each single estimate of the 150,000 SCC estima for each discount rat n e t ates te depends on the SCC- -IAM used, the socioeco t onomic and e emissions sccenario, a dra from the aw assumed distribution of the ECS, and, for FU , UND and PA AGE, a draw from the dis stributions o of their part ticular uncer rtain parameters. The resulting four official SCC estimates f an emissions C for year are the mean of the 150,000 results for each discoun rate, as we as the 95t percentile for t 0 e nt ell th e the 3 percent discoun rate (see Table 2-3) Read the entire page → From page 15...... . The SCC estimat rise over time becaus in the models, S tes se, future em missions prodduce larger incremental damages as the economy grows and temperature i y d e rises. Read the entire page →

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