New study finds natural aerosols may contribute more uncertainty than previously thought to how climate responds to anthropogenic GHG emission
Natural aerosols, such as emissions from volcanoes or plants, may contribute more uncertainty than previously thought to estimates of how the climate might respond to anthropogenic greenhouse gas emissions, according to a new study by researchers at the University of Leeds (UK) and their colleagues in the US and Australia published in the journal Nature. The finding could further complicate attempts to model climate change.
The team showed that the effect of aerosols on the climate since industrialisation depends strongly on what the atmosphere was like before pollution, when aerosols were produced only from natural emissions.
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.—Carslaw et al.
Aerosols tend to increase the brightness of clouds, which would increase the reflection of solar radiation to space, thereby partially masking the climate-warming effects of greenhouse gas emissions. Firmly establishing the effect of aerosol-induced changes on cloud brightness is an important challenge for climate scientists.
Our results provide a clear path for scientists to reduce the uncertainty in aerosol effects on climate because we have been able to rank the causes for the uncertainty.—Professor Carslaw
The research was funded by the Natural Environment Research Council, the EC Seventh Framework Programme and the National Centre for Atmospheric Science.
K. S. Carslaw, L. A. Lee, C. L. Reddington, K. J. Pringle, A. Rap, P. M. Forster, G. W. Mann, D. V. Spracklen, M. T. Woodhouse, L. A. Regayre & J. R. Pierce (2013) “Large contribution of natural aerosols to uncertainty in indirect forcing” Nature doi: 10.1038/nature12674