The Hockey Stick
The latest version of the "hockey stick." The different curves represent different ways of estimating past temperatures: from corals, cave deposits, tree-rings, etc. The black curve shows measured temperatures, which began to become available in about 1880.
The National Academy of Sciences validated the hockey stick in 2006: "Large-scale surface temperature reconstructions contribute evidence that...global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries." A 2007 review by the National Center for Atmospheric Research left "entirely unaltered the primary conclusion of Mann et al. (as well as many other reconstructions) that both the 20th century upward trend and high late-20th century hemispheric surface temperatures are anomalous.”
Chart source: Mann, M. E., Z. Zhang, et al. (2008). "Proxy-based reconstructions of hemispheric and global surface temperature variations over the past two millennia." Proceedings of the National Academy of Sciences 105, p. 6.