Search

Aerosols and their effect on the radiative properties of clouds are one of the largest sources of uncertainty in calculations of the Earth's energy budget. Here the sensitivity of aerosol‐cloud albedo effect forcing to 31 aerosol parameters is quantified. Sensitivities are compared over three periods; 1850–2008, 1978–2008, and 1998–2008. Despite declining global anthropogenic SO2emissions during 1978–2008, a cancelation of regional positive and negative forcings leads to a near‐zero global mean cloud albedo effect forcing. In contrast to existing negative estimates, our results suggest that the aerosol‐cloud albedo effect was likely positive (0.006 to 0.028Wm−2) in the recent decade, making it harder to explain the temperature hiatus as a forced response. Proportional contributions to forcing variance from aerosol processes and natural and anthropogenic emissions are found to be period dependent. To better constrain forcing estimates, the processes that dominate uncertainty on the timescale of interest must be better understood. Forcing sensitivity to aerosol parameters is strongly period dependentUnderstanding near‐future climate is limited if a single period is consideredIn recent decades, parametric uncertainty is smaller than model diversity

The uncertainty in present-day anthropogenic forcing is dominated by uncertainty in the strength of the contribution from aerosol. Much of the uncertainty in the direct aerosol forcing can be attributed to uncertainty in the anthropogenic fraction of aerosol in the present-day atmosphere, due to a lack of historical observations. Here, we present a robust relationship between total present-day aerosol optical depth and the anthropogenic contribution across three multimodel ensembles and a large single-model perturbed parameter ensemble. Using observations of aerosol optical depth, we determine a reduced likely range of the anthropogenic component and hence a reduced uncertainty in the direct forcing of aerosol. Despite the impacts of global warming already being felt around the world, it is still unclear how much of the effect of greenhouse gasses is being offset by the cooling effect of atmospheric aerosol through the scattering of incoming sunlight and the modification of clouds. A large part of the difficulty in determining the effect of aerosol is in understanding the proportion of present-day aerosol that is due to human activity. In this work, we demonstrate a strong relationship between the total amount of aerosol in the present-day atmosphere (something we can measure) and the amount due to human activity (something we cannot). We further show that this allows us to reduce the uncertainty in the cooling effect of aerosols. A strong relationship is demonstrated between total present-day aerosol loading and the anthropogenic contribution, across a variety of modelsObservations of the total present-day aerosol loading are thus used to constrain uncertainty in anthropogenic aerosol and aerosol direct forcingApplying these constraints to one million samples of a 26-parameter perturbed parameter ensemble leads to a clear-sky RFariestimate of -0.69 ± 0.14 Wm-2