One of the bunnies said why bother posting these, the blogs have been all over this stuff. . . Well, for one thing Eli doesn't have to write them, for another, picking out the gems is fun and for anther, if anyone is looking for a good answer to one of the everbrowns, well Eli is happy to oblige. Today it's Svensmarks turn in the barrel, and there is a neat, new point down there in bold.
UPDATE: But before we start, in the comments, the AGW Observer, Ari Jokimäki, points to his article on a 2007 paper by Evan, et al., which claims that viewing angle has a strong effect on the cloud data (ISCCP) that Svensmark and Friis Christensen used. If corrected, most of the variation in cloudiness with time vanishes, and so does any claimed correlation with cosmic ray flux. UPDATED: The Evan result is not without its doubters, a summary is here but there is more reading to do. On the one hand there are problems with the ISCCP data set, on the other the problems may not be as deep as the critics maintain
Back to Eli's post
Many commenters (0153, 0245, 0509, 0591, 1017.1, 1187, 2953, 3722, 3729.1) claim that temperature is better correlated with solar activity patterns than with greenhouse forcing, some of whom reference researchers such as Svensmark or Shaviv that attribute the mechanism not to solar irradiance but rather solar wind or solar-magnetic flux (2917, 3205.1, 3324.1, 4632, 5058) and interactions with cosmic rays seeding low-lying clouds (0542, 0646, 0798, 1616.1), or length of solar cycles (0543) or sunspots (1219.1).
One commenter (7031) indicates that solar impacts on climate have received scant research attention and are minimized in the IPCC Fourth Assessment Report (2007a) and the climate model community, even though the IPCC authors rank the level of scientific understanding of solar-climate interactions as very low. The assumption is that variations in TSI are the only significant solar impact on global climate. The commenter also posits:
Recent studies have shown strong correlations between solar-modulated cosmic ray fluxes and low-level cloud cover and its subsequent impact on global temperatures. Experimental verification of a cosmic-ray cloud seeding mechanism was recently completed by Svensmark et al. , and the CLOUD (Cosmics Leaving OUtdoor Droplets) experiments at CERN (the European Organization for Nuclear Research) over the next few years will provide definitive measurements of cloud seeding by cosmic rays.
The commenter concludes it is clear that solar variations have much larger impacts on global climate than what is estimated based solely on TSI variations.
One commenter (3446.2) requests that the TSD include a rigorous presentation of sunspot activity and temperature over the past century, and notes objection to the lack of sunspot discussion in Karl et al. (2009). Another (3397) requests more discussion of solar activity as a climate forcer.
The contention that cosmic rays could provide the mechanism by which changes in solar activity affect climate is not supported by the literature. Solomon et al. (2007) address this topic, noting that “the cosmic ray time series does not appear to correspond to global total cloud cover after 1991 or to global low-level cloud cover after 1994.” More recent research continues to question the ability of this mechanism to play a significant role in climate change. Pierce and Adams (2009) use calculations to show that potential impacts on clouds from cosmic rays and “conclude that the hypothesized effect is too small to play a significant role in current climate change.” Erlykin et al. (2009) found that the evidence showed that connections between solar variation and climate were more likely to be mediated by direct variation of insolation rather than cosmic rays, and concluded: “Hence within our assumptions, the effect of varying solar activity, either by direct solar irradiance or by varying cosmic ray rates, must be less than 0.07 ◦C since 1956, i.e. less than 14% of the observed global warming.” Carslaw (2009) and Pittock (2009) review the recent and historical literature in this field and continue to find that the link between cosmic rays and climate is tenuous, though they encourage continued research.
The CLOUD experiments at CERN are interesting research but do not provide conclusive evidence that cosmic rays can serve as a major source of cloud seeding. Preliminary results from the experiment (Duplissy et al., 2009) suggest that though there was some evidence of ion mediated nucleation, for most of the nucleation events observed the contribution of ion processes appeared to be minor. These experiments also showed the difficulty in maintaining sufficiently clean conditions and stable temperatures to prevent spurious aerosol bursts. There is no indication that the earlier Svensmark experiments could even have matched the controlled conditions of the CERN experiment. We find that the Svensmark results on cloud seeding have not yet been shown to be robust or sufficient to materially alter the conclusions of the assessment literature, especially given the abundance of recent literature that is skeptical of the cosmic ray-climate linkage reviewed in the previous paragraph.
Therefore the TSD summary of the assessment literature on this issue is well founded: that the lack of a proven physical mechanism and the plausibility of other causal factors make the association between galactic cosmic ray-induced changes in aerosol and cloud formation controversial.