There have been claims that cosmic rays could have contributed significantly to the global warming over the past century. According to a new study, that is not the case. Instead, during the last 50 years, cosmic rays seemed to have caused warming of only about 0.002°C – a negligible amount compared to observed warming.
Cosmic rays have been claimed to be a significant source for the formation of cloud condensation nuclei, and through that mechanism they have been claimed to affect Earth’s climate significantly. There have been many studies debunking these claims, and currently it seems that the possible effect of cosmic rays on the climate is small (see Skeptical Science advanced rebuttal to cosmic ray hypothesis). It is however likely that there are some mechanisms by which cosmic rays do affect cloud formation at least a little. For example, recently a correlation was found in Europe between diurnal temperature range (difference between daily maximum and minimum temperatures) and strong cosmic ray flux changes (Forbush decreases and ground level enhancements).
A new study by Erlykin et al. evaluates the effects of cosmic rays to cloud cover. The paper discusses previous cosmic ray research, and performs some new analysis. They use cloud cover measurements of International Satellite Cloud Climatology Project (ISCCP). There have been criticisms of ISCCP data, but the researchers believe that ISCCP problems do not affect their results.