The theory of this Israeli astrophysicist has gained traction as the great white hope of climate skeptics. Below are some sources of background reading.
Shaviv champions the solar-wind modulated cosmic ray flux (CRF) hypothesis, which was suggested by Ney, discussed by Dickenson, and furthered by Svensmark (see CO2 Science). Evidence consistes of correlations between CRF variations and cloud cover, correlations between non-solar CRF variations and temperature over geological timescales, as well as experimental results showing that the formation of small condensation nuclei could be bottlenecked by the number density of atmospheric ions.
Basically, high CRF ionizes particles that seed more clouds, causing cooling. Low CRF produces brighter cloud free condition, resulting in warming.
Recently, he reports in GRL that three independent data sets show that the oceans absorb and emit an order of magnitude more heat than could be expected from just the variations in the total solar irradiance, implying the existence of an amplification mechanism. Shaviv, says this predicts the correct radiation imbalance observed in the cloud cover variations that are needed to produce the magnitude of the net heat flux into the oceans associated with the 11-year solar cycle.
The Reference Frame had an article about Shaviv recently too, noting significant pushback by RealClimate, proof the CRF theory is a viable alternative to the GHG warming as the main explanation for recent warmth.
By the way, despite all the huge pro-greenhouse bias in the journals and elsewhere, the Shaviv-Veizer paper has 91 citations right now, while the almost immediate alarmist reply by 11 authors, including RealClimate’s Rahmstorf, Archer, and Schmidt, only has 24 citations.
A very instructive exchange ensued in May 2006 at the RealClimate post “Thankyou for Emitting” where Shaviv challenged masterfully (starting at post 37), until the team eventually threw in the towel around post 125.
On the subject of Rahmstorf, Shaviv’s own blog site ScienceBits refers to RealClimate as WishfulClimate.org in a post More slurs from RealClimate. He pins them as bleeding hearts and intellectual lightweights as well.
Realclimate.org continues with its same line of attack. Wishfulclimate.org writers try again and again to concoct what appears to be deep critiques against skeptic arguments, but end up doing a very shallow job. All in the name of saving the world. How gallant of them.
Since there is no evidence which proves that 20th century warming is human in origin, the only logically possible way to convict humanity is to prove that there is no alternative explanation to the warming (e.g., see here). My motivation (as is the motivation of my serious colleagues) is simply to do the science as good as I can.
But Nir is not an extremist discounting all effects of greenhouse gasses.
In fact, my best estimate for climate sensitivity implies that anthropogenic radiative forcing explain about 1/3 of the 20th century warming, in particular over the past few decades.
Some of the flavor of the debate between them can be seen from the following two comments at Shaviv’s blog:
Rasmus: You are wrong about the motivation about our critisism, Shaviv; we are primarily interested in doing good sicence. We want to unravel the facts behind climate variability. In science, one challenge other views if one finds them strange or not credible. This is what we habve done. You make claims based on your own subjective belief og based on far-fetched speculations. The fact is that the claim that the recent global warming is due to GCR is not supported be any real evidence; there is no credible trend in the solar activity or GCR in the last ~50 years.
Shaviv: Perhaps you’re right. But if so, then it means you should have the integrity to add at the end of your post (and not buried in the discussion below), an addendum saying that this particular critique turned out to be wrong, as Kranz et al. is not applicable to the Milky Way. I for my part would add a similar addendum to my response, specifying that my comments about motives was wrong.
Second, over all, there was a large increase in the solar activity over the 20th century, even if you discard the Yakutsk data (used in the Ahluwalia plot), and this increase explains a large fraction of the 20th century temperature increase if the CRF/climate link is real. As for the temperature increase over the 1990′s, see my response above. Some of the warming is due to the fact that although there was a decrease in the indirect solar forcing over the last cycle, it is still notably above the current forcing/temperature equilibrium (and therefore causes warming), and of course, some of the warming is anthropogenic.
The scientific issues are not settled.