Climatologist Michael E. Mann is most famous for what he calls one of the “least interesting” aspects of his work. In the 1990s he used data from tree rings, coral growth bands and ice cores as proxies for ancient temperatures, combining them with modern thermometer readings. This annual record of temperature variations over the past millennium offered insights into natural climate cycles. As an “afterthought,” he included a graph of average temperatures in the Northern Hemisphere going back to the 1400s in a 1998 paper (he later extended it to A.D. 1000). That “hockey stick” graph, which shows temperatures bouncing up and down before rapidly rising more recently, became an icon of climate change.