The "Hockey Stick" has been one of the most beguiling arguments in the Kyoto proponents’ repertoire and it still appears on many websites. It originates with Michael Mann (1999) and co-workers and shows a rather flat temperature line over the last 1000 years, until in the late 1800s, when a sharp increase is evident, presumably because of man-made greenhouse gases.

Over the years, the graph has been subject to many criticisms from other scientists, for a number of reasons. Some complained that the well-documented Medieval Warm Period (approx. 1000-1400) and the even better known Little Ice Age (approx. 1450-1850) do not register on Mann’s chart. Others took issue with the tree ring proxy methods being used for the time span before thermometer readings were available, or with the inaccuracy of the temperature readings themselves. They maintain that instrument errors are often larger than the anomalies measured, or that the urban heat island issue skews surface data badly.

In this article in Energy & Environment (Vol 14/6, 2003) Canadians Stephen McIntyre and Ross McKitrick reject Mann’s methodology, point to numerous errors, unjustifiably truncated data and extrapolations, and other defects. They then use Mann’s original data and recognized methodology to prove that Mann’s graph shape is an artifact and that a proper interpretation finds that temperatures around 1400 were warmer than anything in the 20th century. The widely read work by McIntyre and McKitrick has elicited much discussion in scientific circles. A more definitive version was later published in Geophysical Research Letters.
The U.S. House of Representatives became aware and expressed their serious concerns over the reported flaws and data errors in Mann's work, and wrote this letter to the IPCC and appointed a committee to review the matter. The Wigmen committer was composed of Edward J. Wegman (George Mason University), David W. Scott (Rice University), and Yasmin H. Said (The Johns Hopkins University). The committee has reviewed the work published about the hockey stick. Read the Wegman summary and the classical full Wegman Report.