Couldn't the Sun be the cause of global warming?

Author: 
January 23, 2014

No, there has been no significant net change in the sun's energy output from the late 1970s to the present, which is the period of the most rapid warming.

Sunspot time series

The peaks and valleys in solar geomagnetic activity since 1900, based on the number of sunspots observed on the face of the Sun each day (orange dots). The Sun's activity increased in the early half of the twentieth century, but it can't be responsible for warming over the past 50 years. Graph by NOAA Climate.gov, based on data from the WDC-SILSO, Royal Observatory of Belgium.

Also, if the sun had intensified its energy output then all layers of Earth's atmosphere would warm. But such warming hasn't been observed. Rather, warming has occurred in the lower atmosphere (troposphere) and cooling in the upper atmosphere (stratosphere)—which is exactly what would be expected if the warming was due to an increase in heat-trapping gases near the surface. This evidence from temperature records is regarded as a "smoking gun" linking today's global warming to the increase in heat-trapping gases in the lower atmosphere.

References

Kopp, G. and J.L. Lean (2011): "A New, Lower Value of Total Solar Irradiance: Evidence and Climate Significance." Geophys. Res. Letters, Frontier Articles. v38, L01706, doi: 10.1029/2010GL045777.