In addition to electromagnetic radiation, the sun is a source of hot plasma, in the form of the solar wind. The solar wind is highly variable, and its properties influence the flux of galactic cosmic rays entering the earth's magnetosphere on time scales ranging from days to decades. A relationship between surface winds deduced from lake varves and cosmic-ray flux deduced from 14C data has recently been suggested by Anderson [1992] for a 2,000-year period in the mid-Holocene during which the geomagnetic field was relatively weak, allowing increased fluxes of cosmic-ray particles to reach the atmosphere at middle latitudes. Since the higher-energy cosmic-ray particles can penetrate into the troposphere, they provide at least a conceptual means of influencing weather and climate at the earth's surface. Several suggestions of a linking mechanism involving cloud nucleation have been made in the past (e.g., Dickinson [1975]). If high-altitude cirrus formation could be shown to be at least partially controlled by ionizing radiation, the resultant changes in cloud formation rate could lead to radiative forcing that would affect the radiation balance of the lower atmosphere.
A somewhat different possible linkage has been suggested by Tinsley and Deen [1991] and Tinsley et al. [1994] (see also Detwiler [1993] and Tinsley [1993]), who pointed out a number of correlations between solar-wind and cosmic-ray variations and changes in various atmospheric parameters, such as Atlantic storm tracks, surface temperatures, and area-integrated storm intensity. They speculated that the linkage might come about through variations in the rate of electrical charging of supercooled water in high-altitude clouds, leading to the formation of ice crystals, which would subsequently sediment downward and affect latent heat release in midlevel clouds. The changes in diabatic heating could then alter tropospheric circulation on a wide scale. The idea has been further developed qualitatively by Tinsley and Heelis [1993], who suggest that the rate of production of charged aerosols near the upper and lower boundaries of a cloud is influenced by a vertical polarization electric field in the cloud. The field is a necessary consequence of the need to maintain continuity in the flow of current between the earth's surface and the ionosphere, which is a function of the earth-ionosphere potential difference and the atmospheric conductance, and hence of the properties of the solar wind.
While these mechanisms are interesting, and deserve further consideration, they appear to depend on individual steps that remain highly speculative. Even if the processes themselves can be shown to be valid, the question of whether or not they are sufficiently important to cause a significant effect in the presence of many competing processes is likely to be unanswered for some time.
In summary, the 1991-1994 quadrennium has given us some tantalizing glimpses of possible connections between solar variability and global climate. The generally accepted most likely mechanism for such a connection is the variation of the sun's total irradiance, which if it is large enough must undoubtedly affect climate. Since the earth's climate is dominated by the oceans, however, and since the oceans possess a large thermal inertia that will damp out high-frequency variations in solar radiation, the observable effects are likely to be confined to time scales of decades to centuries. The situation regarding effects at shorter periods, from the 11-year solar magnetic cycle to the time scale of synoptic weather systems, remains unsettled. There is now little doubt that the atmosphere displays variability on a 10--12 year scale, but while some of the atmospheric parameters show remarkable synchronization with the solar cycle, the possibility that the decadal-scale variability is a result of internal forcing from the ocean, or even some kind of stochastic variation in the highly nonlinear climate system, cannot be dismissed. If a generally acceptable mechanism linking short-term solar variability to weather and climate could be found, the situation would immediately become much clearer. While some plausible and imaginative suggestions have been made, much more work is needed to raise them to a level at which they are likely to be widely accepted by the climate community.