CERN experiment points to a cloudier pre-industrial climate

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 change4. 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 form