What was done
The author reviews what we know about possible relationships between variations in climate and the flux of cosmic rays incident upon the earth, which is modulated by the solar wind, the planet's geomagnetic field and the galactic environment.

What was learned
Kirkby reports that "diverse reconstructions of past climate change have revealed clear associations with cosmic ray variations recorded in cosmogenic isotope archives, providing persuasive evidence for solar or cosmic ray forcing of the climate." Despite the increasing evidence of its importance, however, he says that "solar-climate variability is likely to remain controversial until a physical mechanism is established." In this regard he discusses two different classes of microphysical mechanisms that have been proposed to connect cosmic rays with clouds, which interact significantly with fluxes of both solar and thermal radiation and, therefore, climate: "firstly, an influence of cosmic rays on the production of cloud condensation nuclei and, secondly, an influence of cosmic rays on the global electrical circuit in the atmosphere and, in turn, on ice nucleation and other cloud microphysical processes." Kirkby reports, in this regard, that "considerable progress on understanding ion-aerosol-cloud processes has been made in recent years, and the results are suggestive of a physically-plausible link between cosmic rays, clouds and climate."

What it means
Although the evidence for a cosmic ray-climate connection grows greater by the day, concrete proof remains elusive; but "with new experiments planned or underway, such as the CLOUD facility at CERN," in the words of Kirkby, "there are good prospects that we will have some firm answers to this question within the next few years." This effort is extremely important, for as Kirkby rightly notes, "the question of whether, and to what extent, the climate is influenced by solar and cosmic ray variability remains central to our understanding of the anthropogenic contribution to present climate change."