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.

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Figure 1: The global distribution of annual mean aerosol first indirect forcing and associated uncertainty.
Figure 2: Magnitude and sources of uncertainty in global mean aerosol first indirect forcing.
Figure 3: Schematic explaining the importance of natural emissions for forcing uncertainty.


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This research has received funding from the Natural Environment Research Council AEROS project (project number NE/G006172/1) and GASSP project (project number NE/J024252/1), the EC Seventh Framework Programme under grant agreement FP7-ENV-2010-265148 (Integrated Project PEGASOS), and the National Centre for Atmospheric Science. K.S.C. and P.M.F. are currently Royal Society Wolfson Merit Award holders.

Author information




K.S.C. wrote the manuscript. L.A.L. did the statistical analysis. C.L.R., K.J.P. and G.W.M. performed the aerosol modelling. M.T.W., L.A.R. and K.J.P. prepared the emissions. K.S.C., L.A.L. and C.L.R. did the data interpretation. A.R. and P.M.F. did the forcing calculations. All authors contributed to the editing of the manuscript.

Corresponding author

Correspondence to K. S. Carslaw.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Validation of the global annual mean forcing emulator.

The error bars show the emulator 95% uncertainty range around the mean prediction. The 1:1 line is shown.

Extended Data Table 1 Emissions of aerosols and precursor gases used in the 1750–2000 simulations
Extended Data Table 2 Emissions of aerosols and precursor gases used in the 1850–2000, 1900–2000 and 1850–1980 simulations.
Extended Data Table 3 Parameters and their maximum ranges used in the model simulations.
Extended Data Table 4 Results for the different periods.

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Carslaw, K., Lee, L., Reddington, C. et al. Large contribution of natural aerosols to uncertainty in indirect forcing. Nature 503, 67–71 (2013).

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