Jan Esper

In 2001, the U.N. Intergovernmental Panel on Climate Change (IPCC) featured a graph of Northern Hemisphere temperature history from a 1999 study by Profs. Michael Mann, Raymond Bradley, and Malcolm Hughes. Because of its shape, the graph became known as the “hockey stick.” From A.D. 1,000 to about 1915, the graph depicts a gradual decline in Northern Hemisphere temperatures (the hockey stick handle) followed by an abrupt upturn in hemispheric temperatures during the remainder of the 20th century (the blade).

The graph appears in the IPCC 2001 report’s Summary for Policymakers, Technical Summary, and chapter 2 on Observed Climate Variability and Change. Based on the Mann-Bradley-Hughes (MBH) study, the IPCC famously concluded that, “The 1990s are likely to have been the warmest decade of the millennium in the Northern Hemisphere and 1998 is likely to have been the warmest year” (chapter 2, p. 102). The IPCC also asserted that, “Evidence does not support the existence of globally synchronous periods of cooling or warming associated with the ‘Little Ice Age’ and ‘Medieval Warm Period’.” The hockey stick instantly became the poster child for pro-Kyoto advocacy, touted as seeing-is-believing evidence that late 20th century warmth was unprecedented during the past 1,000 years, and that mankind’s fuelish ways must be to blame.

Soon after its PR boost from the IPCC, the hockey stick became embroiled in a controversy that persists to this day. Books both pro and con have been written on the subject. Two leading critics, mining consultant Steve McIntyre and economist Ross McKitrick, argued that MBH’s computer program generates hockey stick-shaped graphs from random data. As for the IPCC’s dismissal of the Medieval Warm Period as a European phenomenon, the Center for the Study of Carbon Dioxide and Global Change maintains a large and growing archive of studies indicating that the Medieval Warm Period was global and/or warmer than recent decades.

A recent study published in Nature Climate Change further undermines the credibility of the hockey stick. The study, “Orbital forcing of tree-ring data,” by Jan Esper of Johannes Gutenberg University, in Germany, and colleagues from Germany, Switzerland, Finland, and Scotland, used X rays to measure changes in the cell-wall density of trees in Northern Finland over the past 2,000 years. The analysis examined both “living and subfossil pine (Pinus sylvestris) trees from 14 lakes and 3 lakeshore sites.” [click to continue…]