October 10, 2016

expert reaction to new study on climate forcing from aerosols

Publishing in the journal PNAS a group of researchers have analysed mechanisms of formation of atmospheric aerosols and their effect on climate change.

Prof. Piers Forster, Professor of Climate Change at the University of Leeds, said:

“Historically human emissions of SO2 have increased cloud brightness and cooled the Earth offsetting a fraction of greenhouse gas warming.  This paper revises this strength of this cooling effect downwards, although it still remains quite uncertain.

“The 27% revision only relates to the strength of this cooling effect which only has a small role to play in future climate change. It does not mean that the planet will warm 27% less than we thought or that the planet is 27% less sensitive to the effects of CO2 which is by far the dominant factor.

“To fully understand the climate implications of this work we need to include these effects in our climate models but the result may help us rule out some very high warming future scenarios both by making the very highest climate sensitivities less likely and by reducing the warming bounce we might expect from cleaning up SO2 emissions to improve air quality.

“The overall picture is unchanged – manmade global warming remains a dangerous reality.”

Prof. Richard Allan, Professor of Climate Science at the University of Reading, said:

“These new laboratory measurements combined with computer simulations imply that a cooling effect from pollutant particles which offsets some of the greenhouse gas warming over the past few centuries may be slightly smaller than thought. However, the proposed change to the size of this cooling effect is well within the range of possibilities already considered and so it does not alter the conclusion that the continued emissions of greenhouse gases by human activities will lead to substantial and damaging climate change.

“Pollutant particles or ‘aerosols’ have caused clouds to reflect more sunlight, rather like a mirror. The size of this cooling effect is very difficult to pin down and also depends upon how naturally bright clouds were before industrialisation hundreds of years ago.

“In addition to aerosol pollution, plants and trees produce natural aerosol particles that make clouds brighter than they would otherwise be. Combining new laboratory experiments of particle formation with simulations of the atmosphere, scientists have discovered that before pollution affected cloud brightness, they were already quite reflective, more so than previously thought. This is important because if clouds were already quite bright, the addition of pollution particles would not have increased their brightness as much as anticipated.

“If it takes more heating from human activities to explain the observed 1 degree Celsius warming of the planet’s surface since pre-industrial times, this suggests future warming in response to continued increases in greenhouse gas concentrations could be marginally smaller.

“The difficulty with these conclusions is that the researchers have to rely on very indirect methods of assessing how bright the unpolluted clouds were before industrialisation. Also the precise effect of particle pollution on present day cloud brightness is not well known and the proposed update is already well within the range of possibilities already considered.”

‘Reduced anthropogenic aerosol radiative forcing caused by biogenic new particle formation’ by Hamish Gordon et al. published in PNAS on Monday 10 October 2016. 

Declared interests

None declared