Variations in solar output, possibly amplified by cloud feedbacks, may have contributed to recent warming. A difference between this mechanism and greenhouse warming is that an increase in solar activity should warm the stratosphere while greenhouse warming should cool the stratosphere. Cooling in the lower stratosphere has been observed since at least 1960, which would not be expected if solar activity were the main contributor to recent warming (Reduction of stratospheric ozone also has a cooling influence).

Two researchers at Duke University have estimated that the Sun may have contributed about 40–50% of the global surface warming over the period 1900–2000, and about 25–35% between 1980 and 2000.