Has the IPCC got it wrong over aerosols?

Aerosols help cool the atmosphere by encouraging cloud droplets to form icy particles which reflect sunlight, but new research suggests they could also have the opposite effect and warm the atmosphere too.

Richard Lindzen, Professor of Meteorology at MIT , and his former postdoctoral researcher Youn-Sang Choi analysed cloud formation and dust aerosol data collected by CALIPSO – NASA’s Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation satellite – from June 2006 to May 2007.

They discovered that there were 20% fewer super-cooled cloud particles – a mixture of water and ice that reflect more sunlight that ice alone – in regions that had dust aerosols, which the researcher suggest could warm the atmosphere in those areas.

“Current climate models generally over-predict current warming and assume that the excessive warming is cancelled by aerosols,” Lindzen and Choi said, “Our research offers a potentially important example of where the secondary effect is to warm, thus reducing the ability of aerosols to compensate for excessive warming in current models.”

The researchers believe the decrease in super-cooled particles happens because aerosols travel to a layer of the atmosphere where the temperature is about -20°C and kill super-cooled cloud droplets by causing them to form ice. This means clouds reflect less sunlight, which could have a warming effect on the climate.

“The IPCC assumed that all the secondary effects of aerosols would be to increase reflectivity, so it has left out a very important factor that could lead to the opposite effect,” Lindzen said.

The IPCC – the Intergovernmental Panel on Climate Change – will review the research as part of the peer-reviewed work in preparation for the Fifth Assessment Report about climate change due in 2013.

Aerosols can be both natural – microscopic particles like dust blown from desert winds as studied in this research – and anthropogenic activity, like liquid droplets from fuel combustion.