Nature 476, 429–433 (2011)

Do cosmic rays and solar activity affect our climate? Researchers have proposed that cosmic rays, which hit Earth in greater quantities when sunspot activity lulls, might help to nucleate cloud particles and thus cool the Earth. A long-anticipated particle physics experiment designed to test this link — CLOUD (Cosmics Leaving Outdoor Droplets) — has now yielded its first results.

The experiment, based at Europe's particle-physics laboratory CERN, near Geneva in Switzerland, uses a particle beam to mimic cosmic rays in an ultraclean steel cloud chamber, where temperature, water vapour and other atmospheric constituents can be carefully controlled. The team aims to quantify the roles of various gases in nucleating the precursors of cloud particles, so that these numbers can be plugged into climate models.

CERN's Jasper Kirkby and colleagues report that sulphuric acid and ammonia vapours can enhance nucleation by up to a factor of ten under the conditions they studied. However, the nucleation rate within CLOUD was, surprisingly, only one tenth to one thousandth of what happens in the real world, so other vapours — perhaps amines — must play a big role. CLOUD will tackle that question next.