A paper published last week in the Journal of Atmospheric and Solar-Terrestrial Physics claims stratospheric ozone is the most important driver of recent climate, accounting for 75% of Earth's temperature variations during the period 1926-2011. Ozone is in turn controlled by natural variations in galactic cosmic rays & solar activity, rather than man-made chlorofluorocarbons or 'greenhouse gases.' The Svensmark hypothesis relates variations in solar activity to amplified variations of galactic cosmic rays, which in turn result in changes in cloud cover. This new paper may provide a second mechanism by which variations in solar activity are amplified by the effect on galactic cosmic rays and ozone.
Climate sensitivity to the lower stratospheric ozone variations