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Unrealized increase in global mean surface air temperature (GMST) may result from the climate system not being in steady state with forcings and/or from cessation of negative aerosol forcing that would result from decreases in emissions. An observation-constrained method is applied to infer the dependence of Earth's climate sensitivity on forcing by anthropogenic aerosols within the uncertainty on that forcing given by the Fifth (2013) Assessment Report of the Intergovernmental Panel on Climate Change. Within these uncertainty ranges the increase in GMST due to temperature lag for future forcings held constant is slight (0.09-0.19K over 20years; 0.12-0.26K over 100years). However, the incremental increase in GMST that would result from a hypothetical abrupt cessation of sources of aerosols could be quite large but is highly uncertain, 0.1-1.3K over 20years. Decrease in CO2 abundance and forcing following abrupt cessation of emissions would offset these increases in GMST over 100years by as little as 0.09K to as much as 0.8K. The uncertainties quantified here greatly limit confidence in projections of change in GMST that would result from any strategy for future reduction of emissions.

Plain Language Summary Earth's surface temperature has increased by more than 1 degrees C in response to anthropogenic greenhouse gases, principally CO2. This warming influence is offset in part by a highly uncertain cooling influence by anthropogenic aerosols. As CO2 is long lived in the atmosphere (centuries), whereas aerosols are short lived (weeks), the reduction of sources of aerosols that would result from reducing fossil fuel combustion would lead to temporary warming before cooling from decreased CO2 became dominant. Within current uncertainties a hypothetical abrupt cessation of anthropogenic sources of CO2 and aerosols could result in minimal increase in global temperature to as much as 1.3 degrees C additional increase over a few decades. These findings have important implications on the consequences of prior and prospective future emissions.