Anne M. Waple is a paleoclimatology researcher at the Climatology Laboratory of the Department of Geoscience at the University of Massachusetts in Amherst. She offers this answer.
Since the telescope was invented in 1610, people have been regularly recording differences in the appearance of the sun. Although some observations were made earlier, it was really the telescope that allowed consistent sightings of dark blotches on the sun's surface. It was not until 1843, however, that Heinrich Schwabe, an amateur astronomer, declared that these sunspots appeared to come and go in a regular 11-year cycle.
Following Schwabe's announcement, there were many attempts at making correlations with the sunspot cycles. Everything from the economy to crop yields as well as weather was linked to 11-year cycles. But when the statistics were closely examined, there was little evidence that these apparent correlations were based in reality.
It was not until 1976 that a paper published in Science by John A. Eddy of Saginaw Valley State University renewed interest in the sun-climate relationship with a comprehensive analysis of many different historical records of solar observation. Once this evidence had been synthesized, it became much clearer that there were robust correlations between the temperature and sunspots.
From 1645 to 1715, a period known as the Maunder Minimum, there were virtually no sunspots observed, indicating a "quiet" period in the sun's activity. This period coincides with the height of a time known as the Little Ice Age, which was a period of lower temperatures in Europe and perhaps globally.
Image: Mt. Wilson Observatory
Perhaps the most important development in the solar-climate link came in 1978. At that time, satellites that could measure the Total Solar Irradiance (TSI) received by the Earth from the Sun without atmospheric disturbance were launched. TSI is shown to be directly related to these "activity" cycles that Schwabe noticed 150 years ago. At the maxima of these cycles, there are more sunspots (which are magnetic phenomena that decrease total irradiance), but new instruments show that these dark sunspots are more than compensated for by bright areas on the sun, called faculae. Therefore, the overall irradiance increases in correspondence with higher numbers of sunspots.
The satellite data have now been able to capture two complete cycles (21 and 22). When Richard C. Willson of the Jet Propulsion Laboratory and Columbia University's Center for Climate Systems Research analyzed the data in the September 26, 1997 issue of Science, he noticed an increase in TSI of 0.036 percent from the previous minimum in 1986.
There are certain issues regarding the accuracy of the satellite data. When the Active Cavity Radiometer Irradiance Monitor (ACRIM I) satellite ended its mission, there was a delay in launching ACRIM II, which meant that data from the Earth Radiation Budget Satellite (ERBS) satellite data had to be used during the intervening period. ERBS produces different absolute values from its measurements and is not able to calibrate itself as well as the ACRIM satellites. Still, it seems as though the precision of the data is sufficient to believe this increase in the minimum values.