Ew. Cliver et al., SOLAR VARIABILITY AND CLIMATE-CHANGE - GEOMAGNETIC AA INDEX AND GLOBAL SURFACE-TEMPERATURE, Geophysical research letters, 25(7), 1998, pp. 1035-1038
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.