As the Sun is the Earth's primary energy source, changes in incoming sunlight directly affect the climate system.
[85] Solar irradiance has been measured directly by
satellites,
[86] and indirect measurements are available beginning in the early 1600s.
[85] There has been no upward trend in the amount of the Sun's energy reaching the Earth, so it cannot be responsible for the current warming.
[87] Physical climate models are also unable to reproduce the rapid warming observed in recent decades when taking into account only variations in solar output and volcanic activity.
[88] Another line of evidence for the warming not being due to the Sun is how temperature changes differ at different levels in the Earth's atmosphere.
[89] According to basic physical principles, the greenhouse effect produces warming of the lower atmosphere (the troposphere), but cooling of the upper atmosphere (the stratosphere).
[90] If solar variations were responsible for the observed warming, warming of both the troposphere and the stratosphere would be expected, but that has not been the case.
[91]