When there are more sunspots, the radiation coming from the
sun is slightly
increased. Solar activity follows
an 11 year cycle, with some variation from cycle to cycle. Based on astronomical records, we can reconstruct
the history of sunspot activity back to 1600 or so, when there appears
to be a sustained minimum in the number of sunspots. It has been hypothesized that the
cooler climate during this period is due to reduced solar activity.
However, the difference in incoming
sunshine between times of maximum and minimum solar activity is
only on the
order of a few tenths of a watt per meter squared,
which is unlikely to generate a large climate
anomaly by
itself.