5.2: The Sun
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Another argument sometimes heard is that the big uptick in temperature of the last century or so was caused by increasing solar output. Looking at Figure 4.4.1, one would wonder why such a big change in our sun did not happen before. More to the point, since about 1980 scientists have been measuring with exquisite precision the amount of energy coming from the sun, using instruments placed in satellites high above the influence of our atmosphere. During that period, when much warming occurred, the satellites actually measured a slight decrease in solar output. While there is evidence that variations in solar output have caused climate to change in the past, these do not appear to explain the recent warming.