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The sun has different cycles, and the Earth’s climate changes over time in response to these cycles. The sun goes through irradiance cycles, when the amount of solar radiation reaching the Earth varies. Scientists were only able to measure these cycles with precision after 1978 and the advent of satellites. Two of these cycles seem to exist, one running for 11 years and the other running for 22 years.

The solar cycles do affect climate in the short term, but the U.S. National Oceanic and Atmospheric Administration (NOAA) reports that the impact from the light intensity of the sun versus the impact from GHG emissions is a ratio of approximately 9 to 40. So, GHGs have more than four times the effect of solar cycles.

Other cycles that concern the sun are the Milankovitch cycles. Although they sound like Mr. Milankovitch’s bike collection, Milankovitch cycles are actually natural cycles of the Earth — one of these cycles, for example, is the way in which the planet tilts toward or away from the sun. These cycles may explain the glacial cycles — the ins and outs of ice ages. (We talk about the Milankovitch cycles in more detail in the section “Making the Case for Carbon,” later in this chapter.)

Although they’re important, the Milankovitch cycles have minimal effect on climate in comparison to the effects from GHG emissions when you look at them in terms of relatively short timescales — from decades to centuries. Overall, the IPCC says that the sun likely has little to do with global average temperature rises since 1950. In fact, computer models suggest that the Earth would be cooling if not for increases in GHGs.