Sunspots are relatively dark areas on the surface of the Sun and are thus cooler than its average surface. The number of sunspots correlates with the intensity of solar radiation. The variation is small and was only established once satellite measurements of solar variation became available in the 1980s. Based on work by Abbot, realized that higher values of radiation are associated with more sunspots. Nimbus 7 and the Solar Maximum Mission detected that because the areas surrounding sunspots are brighter, the overall effect is that more sunspots means a brighter sun.

Various studies have been made using sunspot number (for which records extend over hundreds of years) as a proxy for solar output and solar activity (for which good records only extend for a few decades). Also, ground instruments have been calibrated by comparison with high-altitude and orbital instruments. Researchers have combined present readings and factors to adjust historical data. Other proxy data - such as the abundance of cosmogenic isotopes - have been used to infer solar activity and thus likely brightness.

Sunspot activity has been measured using the Wolfer number for about 300 years. This index uses both the number of sunspots and the number of groups to compensate for variations in measurement.The level of solar activity during the past 70 years is exceptional - the last period of similar magnitude occurred over 8,000 years ago. The Sun was at a similarly high level of solar activity for only ~10% of the past 11,400 years, and almost all of the earlier high-activity periods were shorter than the present episode.

Everyone agrees the Sun has profound influence on our atmosphere. But what, exactly, is solar variation impact on climate? Researchers believe that changes in sunspot activity or other solar events may affect Earth in ways that are indirect but that can have a significant impact.

For example, the virtual disappearance of sunspots between 1645 and 1715 coincided with a period of intensely cold winters in Europe, part of the period dubbed the Little Ice Age. The lack of sunspots may have reduced solar radiation by a small amount, perhaps a quarter of a percent?enough to contribute to famines in Europe and allow glaciers to expand.

Although sunspots send comparatively little solar radiation into space, they are surrounded by bright areas with a high energy output. As a result, periods of sunspot activity see more overall solar radiation reaching Earth. Satellite measurements have detected a 0.1 swing in the Sun's total output during the course of an 11-year sunspot cycle.

That change appears to be too small to significantly affect global average temperatures in the lower atmosphere. But the ebb and flow of solar radiation can heat and cool the stratosphere enough to change its circulation patterns, which may have significant impacts on regional climate. In the case of the Little Ice Age, for example, Europe and North America felt the temperature drop most strongly.

The Sun may have other, more subtle climate impacts. Some researchers speculate that energy from the Sun may influence global temperatures indirectly by affecting the formation of clouds. Others speculate that plant growth, which appears to vary during solar cycles, may respond to variations in solar energy.
NCAR researchers develop powerful computer models to simulate the impact of the Sun on our climate. One such effort, the Whole Atmosphere Community Climate Model (WACCM), helps researchers home in on interactions among different levels of the atmosphere, ranging from the surface of Earth to the upper atmosphere and the edge of space. The modeling work is combined with analyses of data from observing instruments aboard satellites to track the impacts of solar energy throughout the atmosphere.