SOLAR VARIABILITY AND CLIMATE-CHANGE - GEOMAGNETIC AA INDEX AND GLOBAL SURFACE-TEMPERATURE

During the past similar to 120 years, Earth's surface temperature is c orrelated with both decadal averages and solar cycle minimum values of the geomagnetic aa index. The correlation with aa minimum values sugg ests the existence of a long-term (low-frequency) component of solar i rradiance that underlies the 11-year cyclic component. Extrapolating t he aa-temperature correlations to Maunder Minimum geomagnetic conditio ns implies that solar forcing can account for similar to 50% or more o f the estimated similar to 0.7-1.5 degrees C increase in global surfac e temperature since the second half of the 17th century. Our analysis is admittedly crude and ignores known contributors to climate change s uch as warming by anthropogenic greenhouse-gases or cooling by volcani c aerosols. Nevertheless, the general similarity in the time-variation of Earth's surface temperature and the low-frequency or secular compo nent of the aa index over the last similar to 120 years supports other studies that indicate a more significant role for solar variability i n climate change on decadal and century time-scales than has previousl y been supposed. The most recent aa data for the current solar minimum suggest that the longterm component of solar forcing will level off o r decline during the coming solar cycle.