Solar variability and climate change: Geomagnetic aa index and global surface temperature

During the past similar to 120 years, Earth's surface temperature is correlated with both decadal averages and solar cycle minimum values of the geomagnetic aa index. The correlation with aa minimum values suggests the existence of a long-term (low-frequency) component of solar irradiance that underlies the 11-year cyclic component. Extrapolating the aa-temperature correlations to Maunder Minimum geomagnetic conditions implies that solar forcing can account for similar to 50% or more of the estimated similar to 0.7-1.5 degrees C increase in global surface temperature since the second half of the 17th century. Our analysis is admittedly crude and ignores known contributors to climate change such as warming by anthropogenic greenhouse-gases or cooling by volcanic aerosols. Nevertheless, the general similarity in the time-variation of Earth's surface temperature and the low-frequency or secular component of the aa index over the last similar to 120 years supports other studies that indicate a more significant role for solar variability in climate change on decadal and century time-scales than has previously been supposed. The most recent aa data for the current solar minimum suggest that the longterm component of solar forcing will level off or decline during the coming solar cycle.