Emissions reductions focused on anthropogenic climate-forcing agents with relatively short atmospheric lifetimes, such as methane (CH 4 ) and black carbon, have been suggested as a strategy to reduce the rate of climate change over the next several decades. We find that reductions of methane and black carbon would likely have only a modest impact on near-term global climate warming. Even with maximally feasible reductions phased in from 2015 to 2035, global mean temperatures in 2050 would be reduced by 0.16°C, with a range of 0.04-0.35°C because of uncertainties in carbonaceous aerosol emissions and aerosol forcing per unit of emissions. The high end of this range is only possible if total historical aerosol forcing is relatively small. More realistic emission reductions would likely provide an even smaller climate benefit. We find that the climate benefit from reductions in short-lived forcing agents are smaller than previously estimated. These nearterm climate benefits of targeted reductions in short-lived forcers are not substantially different in magnitude from the benefits from a comprehensive climate policy.M itigation focused on short-lived climate forcers (SLCFs) is a potentially attractive option to reduce the magnitude of anthropogenic climate change, given the potential for a shorterterm influence on climate compared with carbon dioxide mitigation (1-4). These proposals have focused on methane (CH 4 ) and black carbon (BC), which are thought to be the two most important positive forcing agents after carbon dioxide (5, 6).Current forcing from methane, including its effect on tropospheric ozone and stratospheric water vapor, is of the order 0.65 W/m 2 , and the impact of BC was recently assessed to be 1 W/m 2 , although with very high uncertainty (6). A forcing reduction of 0.5 W/m 2 , less than a third of the central estimate for total BC and CH 4 forcing, would result in a long-term temperature decrease of 0.4°C, for a central equilibrium climate sensitivity of 3.0°C per CO 2 doubling. As discussed in SI Appendix, section 2, however, only a portion of this reduction would be realized in the short term.Although this simple calculation points to the potential benefit from SLCF mitigation, this potential is subject to many uncertainties, including the impact of coemitted pollutants and the large uncertainty in both aerosol forcing and carbonaceous aerosol emissions. With respect to BC mitigation, a critical constraint is that there is strong evidence that net aerosol forcing is negative (SI Appendix, section 6) (7-10).In addition, the potential for SLCF-focused policies must be examined within a consistent, evolving context. This is particularly true for the so-called developing world, where, in many regions, incomes are growing, and, as a result, pollution control polices are rapidly changing. In the parlance of scenarios research, this means that potential SLCF polices need to be evaluated against a consistent reference scenario, as described below. Any near-term reductions will take place within a con...
This project examines renewable energy deployment in the United States using a version of the Global Change Assessment Model (GCAM) with a detailed representation of renewables, the GCAM-RE. Electricity generation was modeled in four generation segments and 12-subregions. This level of regional and sector detail allows a more explicit representation of renewable energy generation. Wind, solar thermal power, and central solar PV plants are implemented in explicit resource classes with new intermittency parameterizations appropriate for each technology. A scenario analysis examines a range of assumptions for technology characteristics, climate policy, and long distance transmission. We find that renewable generation levels grow over the century in all scenarios. As expected, renewable generation increases with lower renewable technology costs, more stringent climate policy, and if alternative low-carbon technologies are not available. The availability of long distance transmission lowers policy costs and changes the renewable generation mix.iv
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