Solar activity may in the long term affect the Earth's climate. Solar ultra-violet, visible and heat radiation are the primary factors for the Earth's climate, including the global average temperatures, and these energy sources appear to be quite constant on time scales of tens or a few hundreds of years. However, many scientists have observed corrrelations between the solar magnetic activity, which is reflected in the sunspot frequency, and climate parameters at the Earth. Sunspots has been recorded through several hundreds of years which makes it possible to compare their variable frequency to climate variations to the extent that reliable climatological records exists. One of the most striking comparisons was published by E. Friis-Christensen og K. Lassen, DMI, in "Science" in 1991. In their work they compared the average temperature at the northern hemisphere with the average solar activity defined through the interval between successive sunspot maxima. The more active the sun - the shorter the interval: the solar cycle runs more intense. Their results are displayed in the figure below:

The red curve illustrates the solar activity, which is generally increasing through an interval of 100 years, since the cycle lenght has decreased from around 11.5 years to less than 10 years. Within the same interval the Earth's average temperature as indicated by the black curve has increased by approximately 0.7 degree C. Even the finer structures in the two curves have similar appearances.
(Reference: Friis-Christensen, E., and K. Lassen, Length of the solar cycle: An indicator of solar activity closely associated with climate, Science, 254, 698-700, 1991).

These results have recently been updated, using data for the temperature and solar activity through the 1980's and 1990's. The results have been presented at the recent EGS conference in Nice (April 2000), and are accepted for publication in the journal JASTP - a preprint is available as a DMI report. (Click here for a PDF version, and here for a PostScript version). The main result of the reanalysis is seen in this figure, which is the New Scientist's adaptation of our results from the above report:

While the curves do not match perfectly at any time, they start to diverge noticeably by the 1980's. We interpret this widening gap as evidence for an additional influence on the temperature - over and above what the Sun is causing. We think this is likely to be due to the anthropogenic greenhouse effect. We base this interpretation on such modelling work as that by Mitchell, et al. (Nature, 1995, vol. 376, p. 501) in which the combined effects of greenhouse gasses and aerosols have the property seen above - an accelerating temperature increase from about the 1970's.

Some data for Sun spots and Climate are available here.