A number of blogs were excited after having leaked the second-order draft of IPCC document, which they interpreted as a “game-changing admission of enhanced solar forcing”.
However, little evidence remains for a link between galactic cosmic rays (GCR) and variations in Earth’s cloudiness. Laken et al. (2012) recently provided an extensive review of the study of the GCR and Earth’s climate, from the initial work by Ney (1959) to the latest findings from 2012.
The story is quite remarkable from a point of view on public discourse and climate change – so much hype for so little (the recent excitements from the leaked IPCC drafts, a point in case).
Laken et al.‘s review indicates that there never really were any strong correlations, and subsequent investigation found that those which looked interesting, didn’t stand up to scrutiny. They provide an account of the work on GCR-climate connections which I feel is well in line with the views presented here on RealClimate.
For instance, there have been some claims of statistically significant relationships between the GCR variations over solar cycles as well as during Forbush Decrease events (FD) (here). Laken et al. provide an overview of others who subsequently repeated the analyses behind these claims, and report that errors were discovered pointing to inflated results in the first place.
There were many problems. The data describing the clouds were not sufficiently reliable for studying long-term variations on a global scale. Especially not for the low-level clouds, which often would be obscured by layers of clouds aloft.
Lab experiments such as CLOUD and Sky produced some interesting findings, where charging affected the nucleation rate of the miniscule particles (more here), but the concentrations of these tiny particles do not have a strong influence on the concentration of substantially larger cloud condensation nuclei (CCN) (here and here).
The hype in the past was prompted by films, such as the ‘Cloud Mystery’, and proponents who gave the impression that GCRs can explain the global warming. There was never any solid support behind this claim: in fact, as noted long ago, there has been no trend in GCR. Furthermore, any resemblance between GCR/clouds and the global mean temperature is lacking (see the grey curve in the figure below).
In my opinion, Laken et al. provide an accurate comprehensive review of the hypothesised effect of GCRs on our climate through moderating the clouds. There is still no evidence suggesting that the GCR influence our climate in significant ways.
We wish all our readers a Merry Christmas!
- B.A. Laken, E. Pallé, J. Čalogović, and E.M. Dunne, "A cosmic ray-climate link and cloud observations", J. Space Weather Space Clim., vol. 2, pp. A18, 2012. http://dx.doi.org/10.1051/swsc/2012018