New results reported in two papers in Nature from the CLOUD experiment at CERN imply that the pre-industrial climate may have had brighter and more extensive clouds than previously thought, sharpening our understanding of the impact of human activities on climate. CLOUD (Cosmics Leaving Outdoor Droplets) is designed to understand how aerosol particles form and grow in the atmosphere, and the effect this has on clouds and climate. It comprises a 26 m3 vacuum chamber containing atmospheric particles, into which beams of charged pions are fired from the Proton Synchrotron to mimic the seeding of clouds by galactic cosmic rays.

The increase in aerosols and clouds since pre-industrial times is one of the largest sources of uncertainty in climate change, according to the Intergovernmental Panel on Climate Change. The new CLOUD results show 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 burning fossil fuels – was essential to initiate aerosol particle formation. CLOUD finds that oxidized biogenic vapours dominate particle growth in unpolluted environments, starting just after the first few molecules have stuck together and continuing all the way up to sizes above 50–100 nm, where the particles can seed cloud droplets.

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

CLOUD, which has produced a series of high-impact publications following its first results in 2011, is the first experiment to reach the demanding technological performance and ultralow contaminant levels necessary to be able to measure aerosol nucleation and growth under controlled conditions in the laboratory.