Large contribution of natural aerosols to uncertainty in indirect forcing

The effect of anthropogenic aerosols on cloud droplet concentrations and radiative properties is the source of one of the largest uncertainties in the radiative forcing of climate over the industrial period. This uncertainty affects our ability to estimate how sensitive the climate is to greenhouse gas emissions. Here we perform a sensitivity analysis on a global model to quantify the uncertainty in cloud radiative forcing over the industrial period caused by uncertainties in aerosol emissions and processes. Our results show that 45 per cent of the variance of aerosol forcing since about 1750 arises from uncertainties in natural emissions of volcanic sulphur dioxide, marine dimethylsulphide, biogenic volatile organic carbon, biomass burning and sea spray. Only 34 per cent of the variance is associated with anthropogenic emissions. The results point to the importance of understanding pristine pre-industrial-like environments, with natural aerosols only, and suggest that improved measurements and evaluation of simulated aerosols in polluted present-day conditions will not necessarily result in commensurate reductions in the uncertainty of forcing estimates. It has been assumed that a better understanding of the effects of anthropogenic aerosols will greatly reduce the large uncertainties associated with our predictions of the radiative forcing effects of aerosols on climate; however, this study shows that nearly half of the uncertainty in the radiative effect of aerosols on clouds derives from uncertainties in pre-industrial natural aerosols. Natural versus anthropogenic aerosols in climate forcing Firmly establishing the influence of aerosols on cloud albedo — their forcing effect on climate in essence — is one of the greatest challenges of modern climate science. It is often tacitly assumed that the continued high uncertainties are linked mainly to anthropogenic emissions. In other words, if the anthropogenic effects could be better understood, so would the overall effect. Now Ken Carslaw and colleagues present an analysis of 28 parameters representing aerosol and precursor gas emissions and other factors that could influence cloud brightness. They find that only 34 per cent of the variance in aerosol forcing since pre-industrial times (around 1750) is associated with anthropogenic emissions, with 45 per cent of the variance linked to natural emissions of volcanic sulphur dioxide, marine dimethylsulphide and other natural sources. This work casts doubts on