Climate change. What role for short-lived climate pollutants in mitigation policy?

S hort-lived climate pollutants (SLCPs) include methane (CH4), black carbon (BC), tropospheric ozone, and hydrofluorocarbons (HFCs). They are important contributors to anthropogenic climate change, responsible for as much as one-third of the current total greenhouse forcing ( 1). An emerging strategy, which we refer to as hybrid climate mitigation (HCM), emphasizes reducing SLCPs in parallel with longlived carbon dioxide (CO2) so as to achieve climate goals, as well as health and food security benefi ts, associated with some of the SLCPs. Proponents of HCM argue that we should focus substantial effort on reducing SLCPs now, as we wait for suffi cient political will to reduce CO2 emissions ( 2– 4). But others ( 5) worry that any strategy involving SLCPs risks delaying efforts to reduce CO2, the main greenhouse gas most important for long-term warming if emissions continue as projected. We attempt to clarify this emerging HCM strategy. Reducing emissions of SLCPs is an essential component of any comprehensive climate action plan for addressing both nearterm and long-term climate change impacts ( 1, 3). There are real opportunities to reduce emissions of SLCPs without distracting from other mitigation efforts focused on CO2. But the dangers of delaying efforts to reduce CO2 emissions are serious and must be articulated clearly to the policy community. We believe that such a delay can be prevented with appropriate policies, and that both short (decades) and long (century or longer) time scales must be considered. Direct comparisons of the climate infl uence of SLCPs and CO2 require making a judgment about the relative importance of short and long time scales. SLCPs have a powerful impact on climate, but they persist in the atmosphere for only a short time— days to weeks for BC, a decade for CH4, and about 15 years for some HFCs. Thus, immediate reductions in SLCPs will result in relatively immediate climate benefits, as the effects on climate depend largely on the emission rate, or fl ow, of SLCPs to the atmosphere. In contrast, CO2 has a very long atmospheric lifetime; more than 20% will remain for thousands to tens of thousands of years ( 6). Thus, climate effects from CO2 depend on the cumulative emissions, or stock, of CO2 in the atmosphere ( 7). In the next year, monthly mean CO2 concentrations will reach 400 parts per million (ppm); annual mean CO2 concentrations have been rising more than 2 ppm per year because of emissions from fossil fuel use, and this will continue for at least the next several decades because of the dominance of fossil fuels in our world energy system. Because it is the most dominant greenhouse gas, nearcomplete reduction in CO2 emissions is the only way to limit the rise of global temperatures and to avoid the risk of catastrophic impacts. But a partial reduction in CO2 emissions over the next few decades will produce minimal relief from climate impacts until mid-century because of the long time scales of CO2 in the atmosphere and the momentum of climate change due to the CO2 already emitted. One way to diminish climate impacts in the next few decades is to also reduce emissions of What Role for Short-Lived Climate Pollutants in Mitigation Policy? CLIMATE CHANGE