CERN experiment points to a cloudier pre-industrial climate VNR

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 change5. 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, 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. ANIMATIONS (abstract) 5:11 - 5:20 The CLOUD experiment at CERN consists of a large chamber that allows to study the formation of cloud seeds under precisely controlled conditions. The effect of cosmic rays on the formation of tiny cloud condensation nuclei is studied using particle beams from CERN’s accelerators. 5:21 - 5:35 There is a strong link between the cloud cover and the climate. Clouds reflect sunlight and have a strong net cooling effect. The absence of clouds would result in a huge extra heat load on Earth. 5:36-5:47 Cloud formation starts from aerosol particles in the atmosphere, which become the seeds of cloud droplets. 5:48 - 6:08 Before this result was known, it was generally assumed that the presence of sulphuric acid in the atmosphere was essential to form seed particles for clouds. Since most sulphuric acid in the atmosphere comes from human activities, such as burning fossil fuel, it was assumed that the pre-industrial climate was much less cloudy than today. 6:09 - 6:20 The CLOUD experiment has shown that natural processes, namely vapours emitted by trees, are an abundant source of seed particles for clouds. 6:21 - 6:34 The presence of these tree vapours produces clouds without man-made pollution. Therefore, the cloudiness in pre-industrial times was more similar to the present situation than previously thought. 6:35 - 6:43 The CLOUD experiment has shown that these natural (biogenic) vapours form cloud seeds much more rapidly in the presence of ions produced by cosmic rays. 6:44 - 7:03 The CLOUD experiment has also shown that these natural (biogenic) vapours dominate the growth of seed particles in unpolluted environments, from the first few molecules all the way up to sizes near 100 nanometres where the particles seed cloud droplets. 7:04 - 7:13 The experiment has measured the growth rate of the seed particles and has shown that their growth accelerates with their size, starting from the size of a few molecules up to a size of 100 nanometres.