The 'hockey stick' graph refers to the temperature graph shown above. It shows that temperatures in the 20th century are higher than at any point in the last 1,000 years. A version of this graph was first published in a paper by Mann, Bradley and Hughes in 1998, covering the period from 1998 back to the year 1400. It became known as the ‘hockey stick’ because it resembles the shape of an ice-hockey stick. Controversy around this graph has been one of the longest and most detailed in climate change science.
Much of the controversy is about the reliability of the information used. In re-constructing temperatures from the period before reliable thermometer readings became available, a whole range of other indicators have been used including data from tree-rings, lake sediments, borehole temperatures and ice cores. The accuracy and value of many of these has been challenged.
Most researchers think that temperature reconstructions using other indicators are an important way of finding out what past climate conditions were like and are working to increase the accuracy of the reconstructions. A minority think that the data and methods cannot be used to provide useful information.
However, the U.S. National Academy of Science was asked by the U.S. Congress to assess the validity of temperature reconstructions, including the hockey stick. Their report on Surface Temperature Reconstructions for the Last 2,000 Years (2006) 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'.
The report also says it has very high confidence that the last few decades of the 20th Century were warmer than any comparable period in the last 400 years. However, it added that climate estimates between the years AD 900 and AD 1600 were less reliable, and less still before AD 900. It called for more research to gain better proxy data for these periods.
For more information about the hockey stick debate visit the following websites: New Scientist - Climate myths, Real Climate - myth vs fact, and the paper by Ross McKitrick, 'What is the ‘Hockey Stick’ Debate About?'
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Photo credit: The Third Assessment Report of The Intergovernmental Panel on Climate Change, Chapter 2, Figure 2.20 http://www.grida.no/climate/ipcc_tar/wg1/pdf/WG1_TAR-FRONT.PDF