I hope everyone had a nice holiday season, for those to whom that might apply. Although for many it’s the start of a new year, I’ve got some decidedly not new “news”: cosmic rays aren’t causing global warming and climatic change. Previous studies debunking this meme discussed here, here, here, here, and here.
Yes, it’s probably true that most sane people and even many denialists are no longer trafficking in this particular myth. However, among the fever swamps of the denialosphere and their political allies, this canard remains predictably popular and the idea still garners an inexplicable amount of attention in some physics circles (e.g. arXiv).
Henrik and Jacob Svensmark’s last paper claimed to find a reduction in cloud water content after Forbush decreases (essentially a reduction in cosmic rays hitting Earth during solar flares), and asserted that they had established a global link between cosmic ray activity and climate. Basically, the Svensmarks propose that cosmic rays “seed” cloud cover via ionization, which in turn reduces (increases) the amount of incoming solar radiation received by the Earth, cooling (warming) the climate. Like so many prior incarnations of the cosmic ray idea, the only problem is that reality completely fails to conform to the hypothesis.
Others have pointed out that the Svensmark selection criteria for the Forbush decrease seemed, shall we say, “in need of further explanation”. Now Calogovic et al. have a paper in press [doi:10.1029/2009GL041327] at GRL entitled “Sudden Cosmic Ray Decreases: No Change of Global Cloud Cover” in which they explore this and other problems with Svensmark 2009′s ostensible results.
From the abstract (all following emphases mine):
Currently a cosmic ray cloud connection (CRC) hypothesis is subject of an intense controversial debate. It postulates that galactic cosmic rays (GCR) intruding the Earth’s atmosphere influence cloud cover. If correct it would have important consequences for our understanding of climate driving processes. Here we report on an alternative and stringent test of the CRC-hypothesis by searching for a possible influence of sudden GCR decreases (so-called Forbush decreases) on clouds. We find no response of global cloud cover to Forbush decreases at any altitude and latitude.
In a recent study Svensmark et. al. analyzed 26 Forbush decreases and, contrary to us, found a significant response in cloud cover and aerosol content. However, a closer inspection of Svensmark’s list of used Fd events revealed 5 Fd events which did not fulfill our selection criteria. For example, the third strongest Fd event in Svensmark’s list which occurred on January 20, 2005 was accompanied by one of the strongest solar proton events. Mironova et al.  analyzed this event and found a significant increases in the aerosol content for the Antartic region. Without further discussion we would like to state that a study as the one by Svensmark et al.  including Fd events which are associated with the solar proton events leads easily to questionable or even contradictory results (see also [Laken et al., 2009]).
The paper concludes:
All our tests did not provide any evidence for a response of the cloud cover to Fd events: 1. No significant global average correlation (Pavg) nor median maxima were found in independent analysis of every Fd event for all cloud layers (not shown). The geographical locations where the cloud cover correlates more positively with the CR intensity are different for each single Fd event, an indication of stochastic correlations. 2. Median values calculated for the frequency distributions of the correlation coefficients are all almost zero and independent of the lag time (not shown). 3. There are no indications for regional effects of CR changes on cloud cover. Pavg and median values obtained in the analysis of grid cells corresponding to particular geographical regions (high and low latitudes, grid cells over oceans and land, see details in AM) show no considerable difference in significance. In conclusion, our global and regional analysis does not indicate any significant response of the cloud cover to undisturbed Forbush decreases.
You’d think that at a certain point, the Svensmarks would just acknowledge defeat and move on. I have a feeling, though, that we’ll be assured that the cosmic ray-climate idea still has some bite to it.