A new paper is currently in press in the journal Comptes Rendus Geoscience, which reviews the evidence for a connection between Cosmic Rays and Climate.
The invited review authored by Ilya G. Usoskin and Gennady A. Kovaltsov is entitled: ‘Cosmic rays and climate of the Earth: possible connection’ is available via Ilya Usoskin’s personal website for free download (as a corrected proof).
The Abstract states:
Despite much evidence relating climatic changes on Earth to solar variability, a physical mechanism responsible for this is still poorly known. A possible link connecting solar activity and climate variations is related to cosmic rays and the physical-chemical changes they produce in the atmosphere. Here we review experimental evidence and theoretical grounds for this rela tion. The cosmic ray – climate link seems to be a plausible climate driver which effectively operates on different time scales, but its exact mechanism and relative importance still remain open questions.
The paper concludes:
We have reviewed the experimental evidence and theoretical models relating cosmic ray variations to the terrestrial climate changes.
On short time scale of a few days, there exists much evidence that CR changes may affect the process of cyclogenesis via the changing transparency and pressure, particularly in the North Atlantic during cold seasons. Although each individual piece of evidence is barely significant, in aggregate, they suggest that the relation can be real.
A link between low clouds and CR appears statistically significant on the inter-annual time scale since 1984 in limited geographical regions, the largest being North Atlantic + Europe and South Atlantic. We note that many reconstructions of the past climate are based on European data, where the CR-cloud relation is the most pronounced. Extension of this relation to the global scale may be misleading.
A relation between the geomagnetic field changes and climatic variations provides evidence favoring the possible CR influence on climate. A study of regional climate variations in relation to the geomagnetic dipole axis migration over the last millennium is also promising.
There is an indication of the climate changes synchronously with the CR flux on Mega-yr time scales, but this result is not straightforward to interpret. Large uncertainties make it only indicative.
Essential progress has been recently achieved in theoretical modelling of both ionizing effect of CR and physical mechanisms relating CRII to cloud variations, but the link between micro- and macro-physics is still missing. A new experimental evidence, obtained by the SKYexperiment team, confirm that enhanced ionization notably facilitates the production of small ion clusters in realistic atmospheric conditions.
In conclusion, a CR-climate link seems to be a plausible climate driver, as supported by the bulk of statistical studies and existing theoretical models. However, further studies, in particular a clear case study as well as improved model development, are foreseen to improve our understanding of the link between cosmic rays and the climate on Earth.