Casting a cloud over climate

They have been a long time coming, but results are finally in from the CLOUD experiment at CERN. CLOUD is a particle physics study designed to test an hypothesis that atmospheric aerosols which play a key role in cloud formation are generated in the air by cosmic rays, which are modulated by natural variations in solar radiation. The idea first explored in the late nineties by my former colleagues in Denmark Henrik Svensmark and Eigil Friis-Christensen is that climate change is driven at least in part by such non-human forces.

The CLOUD experiment results are clear enough, and the BBC science correspondent Pallab Ghosh has done an excellent job in presenting them for the general public. I do wish, though, that mainstream media science journalists would properly cite their sources. Even though Nature is a subscription-access journal, website links would lead interested lay readers to freely viewable abstracts and accompanying discussion articles.

In short, the CLOUD experiment shows that atmospheric aerosols formed when sulphuric acid and ammonia molecules combine through cosmic ray interactions create only a small fraction of the total number of cloud seed particles. Climate models which include the effect of aerosols and clouds will need to be revised in light of the experimental results, but the overall effect of cosmic rays is very small. That, however, has not stopped sceptics from spinning the story as one that backs their science-lite and politically-rich line on anthropogenic climate change.

Now I’ve read through the entire Nature paper and supplementary data, and in their study Jasper Kirkby and his colleagues show clearly that cosmic ray-induced aerosols cannot grow sufficiently large to significantly influence the Earth’s solar radiation budget. The limit seems to be around two nanometres across for the aerosol clusters, and there is simply not enough sulphuric acid in the atmosphere for them to grow to the 100-nanometre size required for significant cloud formation.

When he first embarked on this study, Kirkby thought that cosmic rays would likely account for between half and all of the increase in the Earth’s surface temperature observed over the past century. It may have taken a decade, but Kirkby has proved himself wrong, which is what good scientists have no fear of doing. Some other commentators are not so magnanimous.

Further reading

Kirkby et al., “Role of sulphuric acid, ammonia and galactic cosmic rays in atmospheric aerosol nucleation”, Nature 476, 429 (2011)

Henrik Svensmark & Eigil Friis-Christensen, “Variation of cosmic ray flux and global cloud coverage—a missing link in solar-climate relationships”, J. Atmos. Terr. Phys. 59, 1225 (1997)