Additional info from NASA
Changes in the brightness of the sun can influence the climate from decade to decade, but an increase in solar output falls short as an explanation for recent warming. NASA satellites have been measuring the sun’s output since 1978. The total energy the sun radiates varies over an 11-year cycle. During solar maxima, solar energy is approximately 0.1 percent higher on average than it is during solar minima.
Each cycle exhibits subtle differences in intensity and duration. In early 2010, the solar brightness was slightly lower, not higher, than it was during the previous 11-year minimum in solar activity, which occurred in the late 1990s (Figure 1). This implies that the sun’s impact between 2005 and 2010 might have been to slightly decrease the warming that greenhouse emissions alone would have caused.
Figure 1: Satellite measurements of daily (light line) and monthly average (dark line) total solar irradiance since 1979 have not detected a clear long-term trend. Source: NASA. Graph by Robert Simmon, based on data from the ACRIM Science Team.
Scientists theorize that there may be a multi-decadal trend in solar output, though if one exists, it has not been observed yet. Even if the sun were getting brighter, however, the pattern of warming observed on Earth since 1950 does not match the type of warming the sun alone would cause. When the sun’s energy is at its peak (solar maxima), temperatures in both the lower atmosphere (troposphere) and the upper atmosphere (stratosphere) become warmer. Instead, observations show the pattern expected from greenhouse gas effects: Earth’s surface and troposphere have warmed, but the stratosphere has cooled (Figure 2).
Figure 2: Satellite measurements show warming in the troposphere (lower atmosphere, green line) but cooling in the stratosphere (upper atmosphere, red line). This vertical pattern is consistent with global warming due to increasing greenhouse gases, but inconsistent with warming from natural causes. Source: NASA. Graph by Robert Simmon, based on data from Remote Sensing Systems, sponsored by the NOAA Climate and Global Change Program.
The stratosphere gets warmer during solar maxima because the ozone layer absorbs ultraviolet light; more ultraviolet light during solar maxima means warmer temperatures. Ozone depletion explains the biggest part of the cooling of the stratosphere over recent decades, but it can’t account for all of it. Increased concentrations of carbon dioxide in the troposphere and stratosphere together contribute to cooling in the stratosphere.