According to the Swedish Meteorological and Hydrological Institute (SMHI) the insolation reaching Sweden has increased from the middle of the 80’s to 2005-2006 with an amazing 8 percent annually. The same pattern is found across most of Europe. A prominent example of natural variability.
The explanation is changes in the cloud cover, and as expected not the TSI at TOA. When a cloud covers the sunbeams the insolation at the surface can change from 960 w/m2 to slightly above 0 w/m2 almost instantaneously.
This is particularly interesting since it’s commonly known that variations in the insolation alone cannot account for the recent observed warming, the change in TSI simply isn’t large enough. But there are theories stipulating additional effects from changes in the TSI, with a potentially larger impact on the climate. One such proponent is Henrik Svensmark a professor of physics at the Danish National Space Institute who believes that the power of the sun can change the cloud cover of the Earth.
According to Svensmark cosmic rays entering the atmosphere play an important role in the formation of clouds. Sunbeams act as a shield against cosmic rays, hence an increase in the power of the sun reduces the indecent cosmic rays and the cloud formation, thus lowering the albedo (reflection) of the Earth. A small change in the albedo makes a large difference in the insolation. For example, a -0,50 percentage point change in the albedo approximately corresponds to the direct effect of doubling co2!