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CERN experiment points to a cloudier pre-industrial climate

26 May 2016
The CLOUD experiment in October 2013
The CLOUD experiment in October 2013

Geneva, 26 May 2016. In two papers published today in the journal Nature, new results from the CLOUD experiment at CERN imply the baseline pristine pre-industrial climate may have been cloudier than presently thought. CLOUD shows that organic vapours emitted by trees produce abundant aerosol particles in the atmosphere in the absence of sulphuric acid. Previously it was thought that sulphuric acid – which largely arises from fossil fuels – was essential to initiate aerosol particle formation. CLOUD finds that these so-called biogenic vapours are also key to the growth of the newly-formed particles up to sizes where they can seed clouds.

“These results are the most important so far by the CLOUD experiment at CERN,”
said CLOUD spokesperson, Jasper Kirkby. “When the nucleation and growth of pure biogenic aerosol particles is included in climate models, it should sharpen our understanding of the impact of human activities on clouds and climate.”

The Intergovernmental Panel on Climate Change (IPCC) considers that the increase in aerosols and clouds since pre-industrial times represents one of the largest sources of uncertainty in climate change. CLOUD is designed to understand how new aerosol particles form and grow in the atmosphere, and their effect on clouds and climate.

CLOUD also finds that ions from galactic cosmic rays strongly enhance the production rate of pure biogenic particles – by a factor 10-100 compared with particles without ions. This suggests that cosmic rays may have played a more important role in aerosol and cloud formation in pre-industrial times than in today’s polluted atmosphere.

A paper published simultaneously in Science (Bianchi, F., et al. Science, doi 10.1126/ science.aad5456 (link is external), 2016) describes an observation of pure organic nucleation at the Jungfraujoch observatory by the same mechanism reported by CLOUD. The measurements did not involve CLOUD directly but most of the authors are also members of the CLOUD collaboration.

“The observation of pure organic nucleation at the Jungfraujoch is very satisfying,”
said Kirkby. “It confirms that the same process discovered by CLOUD in the laboratory also takes place in the atmosphere.”