The most controversial chart in history, explained

By Chris Mooney

Back in 1998, a little-known climate scientist named Michael Mann and two colleagues published a paper that sought to reconstruct the planet’s past temperatures going back half a millennium before the era of thermometers — thereby showing just how out of whack recent warming has been. The finding: Recent Northern Hemisphere temperatures had been “warmer than any other year since (at least) AD 1400.” The graph depicting this result looked rather like a hockey stick: After a long period of relatively minor temperature variations (the “shaft”), it showed a sharp mercury upswing during the last century or so (“the blade”).

The report moved quickly through climate science circles. Mann and a colleague soon lengthened the shaft [PDF] of the hockey stick back to the year 1000 AD — and then, in 2001, the U.N.’s Intergovernmental Panel on Climate Change prominently featured the hockey stick in its Third Assessment Report. Based on this evidence, the IPCC proclaimed that “the increase in temperature in the 20th century is likely to have been the largest of any century during the past 1,000 years.”