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The Discovery of Global Warming                      December 2009
An example of the evidence put forth to argue that changes in solar activity were responsible for the 20th century's warming. Solar activity such as sunspots rose, fell, and then rose again in roughly the same fashion as global temperature. An especially striking match was found by two scientists who focused on the length of the sunspot cycle:

It turned out that an error in arithmetic made the match look much better than it really was for the 1980s, but never mind that. Like hundreds of other correlations between sunspots and weather that had been reported for generations, this one fell apart as new data came in. By the 2000s, sunspot activity had fallen back to the level of the 1950s while global temperatures had soared. The mismatch with sunspot cycle length was especially striking:

Friis-Christensen and K. Lassen (1991), p. 699. Figure copyright © 1991 American Association for the Advancement of Science. Page copyright © 2009 American Instute of Physics