  The variation with time from 1956-2002 of the globally averaged rate of
ionization produced by cosmic rays in the atmosphere is deduced and shown to
have a cyclic component of period roughly twice the 11 year solar cycle period.
Long term variations in the global average surface temperature as a function of
time since 1956 are found to have a similar cyclic component. The cyclic
variations are also observed in the solar irradiance and in the mean daily sun
spot number. The cyclic variation in the cosmic ray rate is observed to be
delayed by 2-4 years relative to the temperature, the solar irradiance and
daily sun spot variations suggesting that the origin of the correlation is more
likely to be direct solar activity than cosmic rays. Assuming that the
correlation is caused by such solar activity, we deduce that the maximum recent
increase in the mean surface temperature of the Earth which can be ascribed to
this activity is $\lesssim14%$ of the observed global warming.
