Cosmic Rays, Clouds and Climate
Svensmark, H., Bondo, T. and Svensmark, J. 2009. Cosmic ray decreases affect atmospheric aerosols and clouds. Geophysical Research Letters 36: 10.1029/2009GL038429.
Based on cloud liquid water content data obtained over the world's oceans by the Special Sounder Microwave Imager, liquid water cloud fraction data obtained by the Moderate Resolution Imaging Spectroradiometer, and data on IR detection of low clouds over the ocean by the International Satellite Cloud Climate Project -- as well as FD data obtained from 130 neutron monitors world-wide and the Nagoya muon detector -- Svensmark et al. found what they describe as "substantial declines in liquid-water clouds, apparently tracking the declining cosmic rays and reaching minima some [~7] days after the GCR minima." Concurrently, they also found that "parallel observations by the aerosol robotic network AERONET reveal falls in the relative abundance of fine aerosol particles, which, in normal circumstances, could have evolved into cloud condensation nuclei."
The Danish scientists say their results "show global-scale evidence of conspicuous influences of solar variability on cloudiness and aerosols," and that "the loss of ions from the air during FDs reduces the cloud liquid water content over the oceans." In fact, "so marked was the response to relatively small variations in the total ionization," they concluded that "a large fraction of earth's clouds could be controlled by ionization."
These observations tend to support Svensmark's theory that solar-activity-induced decreases in GCR bombardment of the earth lead to decreases in low (<3.2 km) clouds as a result of reduced atmospheric ionization and, therefore, less fine aerosol particles that under normal circumstances could have evolved into cloud condensation nuclei that could have resulted in more low-level clouds that could have cooled the planet (but, obviously, were not there to do so under conditions of decreased GCR bombardment). This theory thus goes a very long way towards explaining why so many climatic phenomena appear to be related to numerous solar cycles of various frequencies. And it provides a rational explanation for the bulk of the post-Little Ice Age warming of the earth that appears to have continued all the way through the 20th century.