The primary source of Earth’s heat is the Sun, so relatively small changes in solar output can affect our climate. Low solar activity may well have caused a series of very cold European winters during the 1700s, known as the ‘Little Ice Age’.
Satellite observations since the late 1970s have shown a slight decrease in the Sun’s total energy output. Instead of cooling however, Earth has warmed over this period.
Furthermore, warming from the Sun would heat all parts of the atmosphere, including the lowest few kilometres (the troposphere) and the layer above (the stratosphere). Observations show that the stratosphere is in fact cooling while the troposphere warms, consistent with greenhouse gas heating.
Trends in stratospheric temperature since systematic measurements began, from eight different datasets