Solar Activity and Climate

Space weather may also 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 global average temperatures, and these energy sources appear to be quite constant. 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 temperatureat 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 above:

The red curve illustrates the solar activity, which is generally
increasing through an interval of 100 years, since the cycle length
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 blue 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).