See Inside March 2005

Behind the Hockey Stick

Seven years ago Michael Mann introduced a graph that became an iconic symbol of humanity's contribution to global warming. He has been defending his science ever since

Michael Mann knows his students and his subject. The topic of the graduate seminar: El Ni¿o and radiative forcing. The beer he will be serving: Corona, "because I'm going to be talking about tropical climate." Not surprisingly, attendance is high.

Mann is most famously known for the "hockey stick," a plot of the past millennium's temperature that shows the drastic influence of humans in the 20th century. Specifically, temperature remains essentially flat until about 1900, then shoots up, like the upturned blade of a hockey stick. The work was also the first to add error bars to the historical temperatures and allow for regional reconstructions of temperature.

That stick has become a focal point in the controversy surrounding climate change and what to do about it. Proponents see it as a clear indicator that humans are warming the globe; skeptics argue that the climate is undergoing a natural fluctuation not unlike those in eras past. But Mann has not been deterred by the attacks. "If we allowed that sort of thing to stop us from progressing in science, that would be a very frightening world," says the 39-year-old climatologist in his University of Virginia office overlooking the hills of Monticello, the home of Thomas Jefferson.

To construct the hockey-stick plot, Mann, Raymond S. Bradley of the University of Massachusetts Amherst and Malcolm K. Hughes of the University of Arizona analyzed paleoclimatic data sets such as those from tree rings, ice cores and coral, joining historical data with thermometer readings from the recent past. In 1998 they obtained a "reconstruction" of Northern Hemisphere temperatures going back 600 years; by the next year they had extended their analysis to the past 1,000 years. In 2003 Mann and Philip D. Jones of the University of East Anglia in England used a different method to extend results back 2,000 years.

In each case, the outcome was clear: global mean temperature began to rise dramatically in the early 20th century. That rise coincided with the unprecedented release of carbon dioxide and other heat-trapping gases into the earth's atmosphere, leading to the conclusion that industrial activity was boosting the world's mean temperature. Other researchers subsequently confirmed the plot.

The work of Mann and his colleagues achieved special prominence in 2001. That is when the Intergovernmental Panel on Climate Change (IPCC), an international body of climate experts, placed the hockey-stick chart in the Summary for Policymakers section of the panel's Third Assessment Report. (Mann also co-authored one of the chapters in the report.) It thereby elevated the hockey stick to iconic status--as well as making it a bull's-eye. A community skeptical of human-induced warming argued that Mann's data points were too sparse to constitute a true picture, or that his raw data were numerically suspicious, or that they could not reproduce his results with the data he had used. Take down Mann, it seemed, and the rest of the IPCC's conclusions about anthropogenic climate change would follow.

That led to "unjustified attack after unjustified attack," complains climatologist Gavin A. Schmidt of the NASA Goddard Institute for Space Studies. Although questions in the field abound about how, for example, tree-ring data are compiled, many of those attacking Mann's work, Schmidt claims, have had a priori opinions that the work must be wrong. "Most scientists would have left the field long ago, but Mike is fighting back with a tenacity I find admirable," Schmidt says. One of Mann's more public punch backs took place in July 2003, when he defended his views before a congressional committee led by Senator James M. Inhofe of Oklahoma, who has called global warming a "hoax." "I left that meeting having demonstrated what the mainstream views on climate science are," Mann asserts.

More recently, Mann battled back in a 2004 corrigendum in the journal Nature, in which he clarified the presentation of his data. He has also shown how errors on the part of his attackers led to their specific results. For instance, skeptics often cite the Little Ice Age and Medieval Warming Period as pieces of evidence not reflected in the hockey stick, yet these extremes are examples of regional, not global, phenomena. "From an intellectual point of view, these contrarians are pathetic, because there's no scientific validity to their arguments whatsoever," Mann says. "But they're very skilled at deducing what sorts of disingenuous arguments and untruths are likely to be believable to the public that doesn't know better."

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