Back in 1998, a little-known climate scientist named Michael Mann and two colleagues published a paper [PDF] 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.”

And then all hell broke loose.

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IPCC Third Assessment Report / WikipediaClick to embiggen.

Mann tells the full story of the hockey stick — and the myriad unsuccessful attacks on it — in his 2012 book The Hockey Stick and the Climate Wars: Dispatches From the Front Lines; Mann will appear at a Climate Desk Live event on May 15 to discuss this saga. But to summarize a very complex history of scientific and political skirmishes in a few paragraphs:

The hockey stick was repeatedly attacked, and so was Mann himself. Congress got involved, with demands for Mann’s data and other information, including a computer code used in his research. Then the National Academy of Sciences weighed in in 2006, vindicating the hockey stick as good science and noting:

The basic conclusion of Mann et al. (1998, 1999) was that the late 20th century warmth in the Northern Hemisphere was unprecedented during at least the last 1,000 years. This conclusion has subsequently been supported by an array of evidence that includes both additional large-scale surface temperature reconstructions and pronounced changes in a variety of local proxy indicators, such as melting on ice caps and the retreat of glaciers around the world.

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It didn’t change the minds of the deniers, though — and soon Mann and his colleagues were drawn into the 2009 “Climategate” pseudo-scandal, which purported to reveal internal emails that (among other things) seemingly undermined the hockey stick. Only, they didn’t.

In the meantime, those wacky scientists kept doing what they do best — finding out what’s true. As Mann relates, over the years other researchers were able to test his work using “more extensive data sets, and more sophisticated methods. And the bottom line conclusion doesn’t change.” Thus the single hockey stick gradually became what Mann calls a “hockey team.” “If you look at all the different groups, there are literally about two dozen” hockey sticks now, he says.