Israeli Astrophysicist Dr. Nir Shaviv posted a guest essay at Luboš Motl The Reference Frame titled: The CLOUD is clearing
In a nutshell he’s saying that cosmic ray flux modulated by solar variability has a strong place right alongside CO2, and may in fact be a larger forcing.
The results are very beautiful and they demonstrate, yet again, how cosmic rays (which govern the amount of atmospheric ionization) can in principle have an effect on climate.
What do I mean? First, it is well known that solar variability has a large effect on climate. In fact, the effect can be quantified and shown to be 6 to 7 times larger than one could naively expect from just changes in the total solar irradiance. This was shown by using the oceans as a huge calorimeter (e.g., as described here). Namely, an amplification mechanism must be operating.
As a consequence, anyone trying to understand past (and future) climate change must consider the whole effect that the sun has on climate, not just the relatively small variations in the total irradiance (which is the only solar influence most modelers consider). This in turn implies, that some of the 20th century warming should be attributed to the sun, and that the climate sensitivity is on the low side (around 1 deg increase per CO2 doubling)
Read the entire essay here
h/t to Dr. Indur Goklany
Also, William Briggs has an excellent summary as well.