Behind the hockey stick.

Michael Mann knows his students and his subject. The topic of the graduate seminar: El Niño 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 infl uence of humans in the 20th century. Specifi cally, temperature remains essentially fl at until about 1900, then shoots up, like the upturned blade of a hockey stick. The work was also the fi rst 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 fl uctuation 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 offi ce 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 confi rmed 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 MICHAEL MANN: DETECTING PAST CLIMATE