The authors further note that the total variation in low cloud amount over a solar cycle is about 1.7%, which corresponds to a change in the planet's radiation budget of about one watt per square meter (1 Wm^-2).  This change, they say, "is highly significant when compared ... with the estimated radiative forcing of 1.4 Wm^-2 from anthropogenic CO2 emissions."  However, because of the short length of a solar cycle (11 years), the large thermal inertia of the world's oceans dampens the much greater global temperature change that would have occurred as a result of this radiative forcing if it had been spread out over a much longer period of time, so that the actual observed warming is something a little less than 0.1°C.

Much of Carslaw et al.'s review focuses on mechanisms by which cosmic rays might induce the synchronous low cloud cover changes that have been observed to accompany their intensity changes.  They begin by briefly describing the three principle mechanisms that have been suggested to function as links between solar variability and changes in earth's weather: (1) changes in total solar irradiance that provide variable heat input to the lower atmosphere, (2) changes in solar ultraviolet radiation and its interaction with ozone in the stratosphere that couple dynamically to the lower atmosphere, and (3) changes in cloud processes having significance for condensation nucleus abundances, thunderstorm electrification and thermodynamics, and ice formation in cyclones.

Focusing on the third of these mechanisms, the researchers note that cosmic rays provide the sole source of ions away from terrestrial sources of radioisotopes.  Hence, they further refine their focus to concentrate on ways by which cosmic-ray-produced ions may affect cloud droplet number concentrations and ice particles.  Here, they concentrate on two specific topics: what they call the ion-aerosol clear-air mechanism and the ion-aerosol near-cloud mechanism. Their review suggests that what we know about these subjects is very much less than what we could know about them, which further suggests that these areas are much deserving of funding for future research, in view of the likelihood that new knowledge in these areas may be the key to resolving the primary question asked in the title of our Editorial; for as Carslaw et al. forthrightly report, many scientists believe that "it is inconceivable that the lower atmosphere can be globally bombarded by ionizing radiation without producing an effect on the climate system."

So, do solar-mediated changes in cosmic ray intensities influence climate on centennial and millennial time scales?  In a provocative plot that suggests a positive answer to this question, Carslaw et al. depict a composite history of cosmic ray intensities derived from four independent proxies, two of which extend all the way back to 1700.  Comparing this plot with what we believe to be the most accurate temperature history of the Northern Hemisphere, i.e., that derived by Esper et al. (2002), we note that for almost all of the 18th century, cosmic ray intensity declined modestly, while air temperature slowly rose.  Then came a sharp rise in cosmic ray intensity that was immediately followed by a sharp drop in temperature.  This change, in turn, was followed by a sharp decline in cosmic ray intensity that was immediately followed by a sharp upturn in temperature.  Thereafter, the cosmic ray intensity leveled off, rose slightly and then declined in undulating fashion to the end of the record, while temperature leveled off, dropped slightly and then rose in undulating fashion to the end of the record, as would be expected to occur in light of what is currently known about the cosmic ray-cloud connection.

Commenting on the last century of change, Carslaw et al. point out that the cosmic ray intensity declined by about 15% over this period, "owing to an increase in the solar open magnetic flux by more than a factor of 2."  They further report that "this 100-year change in intensity is about the same magnitude as the observed change over the last solar cycle."  In addition, we note that the cosmic ray intensity was already much lower at the start of the 20th century than it was just after the start of the19th century, when the Esper et al. record indicates the planet began its nearly-two-century-long recovery from the chilly depths of the Little Ice Age.

Clearly, these observations strongly suggest that solar-mediated variations in the intensity of cosmic rays bombarding the earth are indeed responsible for the temperature variations of the past three centuries.  They provide a much better fit to the temperature data than do atmospheric CO2 data; and as Carslaw et al. remark, "if the cosmic ray-cloud effect is real, then these long-term changes of cosmic ray intensity could substantially influence climate."  It is this possibility, they say, that makes it "all the more important to understand the cause of the cloudiness variations," which is basically the message of their paper, i.e., that we must work hard to deepen our understanding of the cosmic ray-cloud connection, as it may well hold the key to resolving what they call this "fiercely debated geophysical phenomenon."  We therefore propose that funding for relevant research be an integral part of the next five-year thrust of the U.S. Global Change Research Program.

References
Carslaw, K.S., Harrizon, R.G. and Kirkby, J.  2002.  Cosmic rays, clouds, and climate.  Science 298: 1732-1737.

Esper, J., Cook, E.R. and Schweingruber, F.H.  2002.  Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability.  Science 295: 2250-2253.