Current emissions of anthropogenic greenhouse gases (GHGs) have already committed the planet to an increase in average surface temperature by the end of the century that may be above the critical threshold for tipping elements of the climate system into abrupt change with potentially irreversible and unmanageable consequences. This would mean that the climate system is close to entering if not already within the zone of ''dangerous anthropogenic interference'' (DAI). Scientific and policy literature refers to the need for ''early,'' ''urgent,'' ''rapid,'' and ''fast-action'' mitigation to help avoid DAI and abrupt climate changes. We define ''fast-action'' to include regulatory measures that can begin within 2-3 years, be substantially implemented in 5-10 years, and produce a climate response within decades. We discuss strategies for short-lived non-CO 2 GHGs and particles, where existing agreements can be used to accomplish mitigation objectives. Policy makers can amend the Montreal Protocol to phase down the production and consumption of hydrofluorocarbons (HFCs) with high global warming potential. Other fast-action strategies can reduce emissions of black carbon particles and precursor gases that lead to ozone formation in the lower atmosphere, and increase biosequestration, including through biochar. These and other fastaction strategies may reduce the risk of abrupt climate change in the next few decades by complementing cuts in CO 2 emissions. biosequestration ͉ black carbon ͉ hydrofluorocarbons ͉ tipping points ͉ tropospheric ozone

Abstract. There is growing international interest in mitigating climate change during the early part of this century by reducing emissions of short-lived climate pollutants (SLCPs), in addition to reducing emissions of CO2. The SLCPs include methane (CH4), black carbon aerosols (BC), tropospheric ozone (O3) and hydrofluorocarbons (HFCs). Recent studies have estimated that by mitigating emissions of CH4, BC, and O3 using available technologies, about 0.5 to 0.6 °C warming can be avoided by mid-21st century. Here we show that avoiding production and use of high-GWP (global warming potential) HFCs by using technologically feasible low-GWP substitutes to meet the increasing global demand can avoid as much as another 0.5 °C warming by the end of the century. This combined mitigation of SLCPs would cut the cumulative warming since 2005 by 50% at 2050 and by 60% at 2100 from the CO2-only mitigation scenarios, significantly reducing the rate of warming and lowering the probability of exceeding the 2 °C warming threshold during this century.

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Significance This study clarifies the need for comprehensive CO 2 and non-CO 2 mitigation approaches to address both near-term and long-term warming. Non-CO 2 greenhouse gases (GHGs) are responsible for nearly half of all climate forcing from GHG. However, the importance of non-CO 2 pollutants, in particular short-lived climate pollutants, in climate mitigation has been underrepresented. When historical emissions are partitioned into fossil fuel (FF)- and non-FF-related sources, we find that nearly half of the positive forcing from FF and land-use change sources of CO 2 emissions has been masked by coemission of cooling aerosols. Pairing decarbonization with mitigation measures targeting non-CO 2 pollutants is essential for limiting not only the near-term (next 25 y) warming but also the 2100 warming below 2 °C.