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ECONOMIC REPORT OF THE PRESIDENT

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these resulted in insignificant changes to the overall estimates released<br />

in May 2013. The IWG also sought independent expert advice from the<br />

National Academies of Sciences, Engineering, and Medicine (NAS) to<br />

inform future updates of the SC-CO2 estimates. In August 2016, the<br />

IWG updated its technical support document to incorporate January<br />

2016 feedback from the NAS by enhancing the presentation and discussion<br />

of quantified uncertainty around the current SC-CO2 estimates.<br />

The NAS Committee recommended against a near-term update of the<br />

estimates. Also in August 2016, the IWG issued new estimates of the<br />

social costs of two additional GHGs, methane (CH 4<br />

) and nitrous oxide<br />

(N 2<br />

O), applying the same methodology as that used to estimate the<br />

SC-CO2 (IWG 2016a).<br />

To estimate the SC-CO2, SC-CH4, and SC-N2O, three integrated<br />

assessment models (IAMs) are employed. IAMs couple models of<br />

atmospheric gas cycles and climate systems with aggregate models of the<br />

global economy and human behavior to represent the impacts of GHG<br />

emissions on the climate and human welfare. Within IAMs, the equations<br />

that represent the influence of emissions on the climate are based<br />

on scientific assessments, while the equations that map climate impacts<br />

to human welfare (“damage functions”) are based on economic research<br />

evaluating the effects of climate on various market and non-market<br />

sectors, including its effects on sea level rise, agricultural productivity,<br />

human health, energy-system costs, and coastal resources. Estimating the<br />

social cost of emissions for a given GHG at the margin involves perturbing<br />

the emissions of that gas in a given year and forecasting the increase<br />

in monetized climate damages relative to the baseline. These incremental<br />

damages are then discounted back to the perturbation year to represent<br />

the marginal social cost of emissions of the specific GHG in that year.<br />

The estimates of the cost of emissions released in a given year represent<br />

the present value of the additional damages that occur from those<br />

emissions between the year in which they are emitted and the year 2300.<br />

The choice of discount rate over such a long time horizon implicates<br />

philosophical and ethical perspectives about tradeoffs in consumption<br />

across generations, and debates about the appropriate discount rate in<br />

climate change analysis persist (Goulder and Williams 2012; Arrow, et<br />

al. 2013; Arrow, et al. 2014). Thus, the IWG presents the SC-CO2 under<br />

three alternative discount rate scenarios, and, given the potential for<br />

lower-probability, but higher-impact outcomes from climate change, a<br />

fourth value is presented to represent the estimated marginal damages<br />

associated with these “tail” outcomes (IWG 2015, IWG 2016b). All four<br />

current estimates of the SC-CO2, from 2010 to 2050, are below.<br />

Sources: IWG (2013, 2015, 2016a, 2016b), Goulder and Williams<br />

(2012), Arrow et al (2013, 2014).<br />

440 | Chapter 7

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