Climate myths: The 'hockey stick' graph has been proven wrong
In fact, later studies support the key conclusion: the world is warmer now than it has been for at least 1000 years.
Michael Le Page04 September 2009
The "hockey stick" graph was the result of the first comprehensive attempt to reconstruct the average northern hemisphere temperature over the past 1000 years, based on numerous indicators of past temperatures, such as tree rings. It shows temperatures holding fairly steady until the last part of the 20th century and then suddenly shooting up. It provided yet more evidence that the rise in greenhouse gases due human activity is causing warming, although the case for this was already very strong. The conclusion that we are making the world warmer certainly does not depend on reconstructions of temperature prior to direct records.
2006 report of the US National Academy of Science (pdf). The academy was asked by Congress to assess the validity of temperature reconstructions, including the hockey stick. The report states: "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 1000 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".
Update: as suggested by the academy in its 2006 report, Michael Mann and his colleagues have reconstructed northern hemisphere temperatures for the past 2000 years using a broader set of proxies than was available for the original study and updated measurements from the recent past. The new reconstruction has been generated using two statistical methods, both different to that used in the original study. Like other temperature reconstructions done since 2001 (see graph), it shows greater variability than the original hockey stick. Yet again, though, the key conclusion is the same: it's hotter now than it has been for at least 1000 years. In fact, independent evidence, from ice cores and sea sediments for instance, suggest the last time the planet approached this degree of warmth was during the interglacial period preceding the last ice age over 100,000 years ago. It might even be hotter now than it has been for at least a million years.
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