  Among the most puzzling questions in climate change is that of solar-climate
variability, which has attracted the attention of scientists for more than two
centuries. Until recently, even the existence of solar-climate variability has
been controversial - perhaps because the observations had largely involved
temporary correlations between climate and the sunspot cycle. Over the last few
years, however, 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. However, despite the increasing evidence of its importance, solar
climate variability is likely to remain controversial until a physical
mechanism is established. Although this remains a mystery, observations suggest
that cloud cover may be influenced by cosmic rays, which are modulated by the
solar wind and, on longer time scales, by the geomagnetic field and by the
galactic environment of Earth. Two different classes of microphysical
mechanisms have been proposed to connect cosmic rays with clouds: 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. 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. However, a concerted
effort is now required to carry out definitive laboratory measurements of the
fundamental physical and chemical processes involved, and to evaluate their
climatic significance with dedicated field observations and modelling studies.
