My attention was brought by this week's issue of Science onto an article by Vincent Courtillot and co-workers from the Institut de Physique du Globe de Paris entitle "Are there connections between the Earth's magnetic field and climate ?" (Earth and Planetary Science Letters, v. 253, 2007, pp. 328-339).
In this interesting article, the authors argue that there are correlations between variations in the geomagnetic field and Earth mean temperature on different time scales ranging from secular variations (10-100 years) to geological variations (1,000-1,000,000 years). They also propose physical mechanisms to explain the relations between the geomagnetic field and climate:
"Three mechanisms are thought to link solar variability with climate: (1) changes in solar irradiance leading to changes in heat input to the lower atmosphere; (2) solar ultraviolet radiation coupled to changes in ozone concentration heating the stratosphere; and (3) galactic cosmic rays [...] These are modulated by longterm solar magnetic activity, by changes of the source of galactic cosmic rays as well as by changes of the Earth's magnetic field. Cosmic rays could in turn act on climate in three ways: (1) through changes in the concentration of cloud condensation nuclei (CCN); (2) thunderstorm electrification; and (3) ice formation in cyclones. [...] Higher cosmic ray flux would lead to more low clouds and thus higher albedo and lower Earth surface temperatures. The cosmic ray variation over one solar cycle translates as a change of energy input to the atmosphere on the order of 1.5 W/m^2, which is not negligible compared for instance to the estimated radiative forcing from anthropogenic CO2 emissions (~2 W/m^2)."
Unfortunately, they do not give a reference for their estimate of 1.5 W/m^2, and I wonder how they obtained it. If it is simply from empirical correlations, it is not robust since "different direct and indirect physical processes may operate simultaneously", as recalled in Climate Change 2007, the physical basis, Fourth Assessment Report of the Intergovernmental Panel on Climate Change (p. 193). Also according to the IPCC report:
"Whether solar wind fluctuations or solar-induced heliospheric modulation of galactic cosmic rays also contribute indirect forcings remains ambiguous. [...] Because of the difficulty in tracking the influence of one particular modification brought about by ions through the long chain of complex interacting processes, quantitative estimates of galactic cosmic-ray induced changes in aerosol and cloud formation have not been reached." (pp. 192-193).
No numerical model can yet predict clouds formation at the level of details required to give quantitative estimates of the proposed mechanism, therefore it remains speculative, and observed correlations do not prove anything, since two quantities can be correlated because they both depend on a third one, without having a direct link between them.
Therefore this is an interesting speculation, which deserves further research, but is not likely to question the consensus on the causes of climate change, since as even Courtillot et al. say:
"The observed correlation between temperature and magnetism fails after the mid-1980s, when solar irradiance and magnetic activity drop, whereas temperature continues an accelerated rise. This is when anthropogenically-induced global warming might first become apparent. Having lost the « Sun-Magnetism-Climate connection », which seems to have prevailed over geological until very recent times, may be a worrying loss..."