Cosmic rays, clouds, and climate

A correlation between a global average of low cloud cover and the flux of c osmic rays incident in the atmosphere has been observed during the last sol ar cycle. The ionising potential of Earth bound cosmic rays are modulated b y the state of the heliosphere, while clouds play an important role in the Earth's radiation budget through trapping outgoing radiation and reflecting incoming radiation. If a physical link between these two features can be e stablished, it would provide a mechanism linking solar activity and Earth's climate. Recent satellite observations have further revealed a correlation between cosmic ray flux and low cloud top temperature. The temperature of a cloud depends on the radiation properties determined by its droplet distr ibution. Low clouds are warm (> 273 K) and therefore consist of liquid wate r droplets. At typical atmospheric supersaturations (similar to1%) a liquid cloud drop will only form in the presence of an aerosol, which acts as a c ondensation site. The droplet distribution of a cloud will then depend on t he number of aerosols activated as cloud condensation nuclei (CCN) and the level of super saturation. Based on observational evidence it is argued tha t a mechanism to explain the cosmic ray-cloud link might be found through t he role of atmospheric ionisation in aerosol production and/or growth. Obse rvations of local aerosol increases in low cloud due to ship exhaust indica te that a small perturbation in atmospheric aerosol can have a major impact on low cloud radiative properties. Thus, a moderate influence on atmospher ic aerosol distributions from cosmic ray ionisation would have a strong inf luence on the Earth's radiation budget. Historical evidence over the past 1 000 years indicates that changes in climate have occurred in accord with va riability in cosmic ray intensities. Such changes are in agreement with the sign of cloud radiative forcing associated with cosmic ray variability as estimated from satellite observations.