Near-term climate mitigation by short-lived forcers

Abstract

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...

Transient climate and ambient health impacts due to national solid fuel cookstove emissions

“…1A shows these contributions to the net transient temperature response. SLCP impacts dominate the response for the first half of the century, whereas the GHG impacts, particularly CO2, become increasingly important by 2100, consistent with previous studies of other types of mitigation (29)(30)(31). Cooling caused by removal of the absorptive species (BC, CO2, and CH4) outweighs the warming from removal of reflective aerosols (OC and sulfate, the latter from coal), with BC contributing the most to the surface temperature impact.…”

Integrating Global Climate Change Mitigation Goals with Other Sustainability Objectives: A Synthesis

“…A key point here is that many of the air pollutants also impact radiative forcing as they form aerosols or act as precursors of aerosols or GHGs. There is, however, great uncertainty in the estimates (38,63,(72)(73)(74)(75). Studies focusing on the cobenefits of air pollution policies for mitigation show that they can potentially reduce net radiative forcing and midterm temperature change by up to 0.2…”

“…This can only occur, however, under somewhat debatable assumptions, such as limited or no improvements in the control of air pollutants that cool Earth (e.g., SO 2 , NO x ) (58,69,75). Current science indicates that such climate benefits decrease with increasing mitigation efforts and, more generally, depend greatly on which air pollutants are reduced and to what extent.…”

Multi-model evaluation of short-lived pollutant distributions over east Asia during summer 2008

“…Due to their relatively short lifetimes (e.g. aerosol lifetime in the troposphere is about 1 week, Pruppacher and Klett, 1997), the impact of SLP (as well as ozone precursor) emission reductions on near-term reductions in the rate of climate warming has been examined in several recent studies (Ramanathan and Carmichael, 2008;Jackson, 2009;Penner et al, 2010;Shoemaker et al, 2013;Smith and Mizrahi, 2013;Rogelj et al, 2014).…”