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.