Cosmic rays (and global warming)

''In 2009, cosmic ray intensities have increased 19 percent beyond anything we've seen in the past 50 years. The increase is significant, and it could mean we need to re-think how much radiation shielding astronauts take with them on deep-space missions'' [source]

''Cosmic rays ionize the atmosphere and an experiment performed at the Danish National Space Center has found that the production of aerosols in a sample atmosphere with condensable gases (such as sulphuric acid and water vapor) depends on the amount of ionization. Since aerosols work as precursors for formation of cloud droplets, this is an indication that cosmic rays affect climate.

Climate models only include the effects of the small variations in the direct solar radiation (infrared, visible and UV). The effects of cosmic rays on clouds are not included in models and the models do a rather poor job of simulating clouds in the present climate.'' [source]