Willie Soon, a climatologist at the Harvard-Smithsonian Center for Astrophysics, is cranking up the heat within the global warming community. In a recent paper, he challenges a much-quoted United Nations expert panel, which concluded that the 1990s were the warmest decade on record and that Earth heated up more in the 20th century than in any other century during the past millennium. "All of these statements about 20th-century climate change cannot be backed up with data," he says. Michael Mann, a climate scientist at the University of Virginia who was one of the U.N. panel's lead authors, counters that Soon's paper is "a thinly disguised publicity stunt" that is "not legitimate science."
The controversy stems from a meta-analysis in which Soon and his coauthors combed through hundreds of recent studies of long-term climate indicators, including tree rings, ice cores, sedimentary fossils, and the movements of glaciers. The data affirm that atmospheric carbon dioxide levels have reached a record high, but Soon says they reveal no clear consequences: "There aren't any bad effects. The climate is not responding as people have expected." In fact, temperatures around the world were higher from A.D. 800 to 1300, a span known as the medieval warm period, than they are now. Soon theorizes that the present warm phase is part of an ongoing cycle driven by fluctuations in the sun's activity. "The natural variability in climate systems is very large and could have triggered the 20th-century change," he says.
Most climate researchers are not buying it. Mann says Soon's analysis focuses on the early part of the 20th century while it ignores unmistakable signals of warming in the latter part of the century. He also disputes Soon's claim that temperatures during the medieval era topped today's. "His argument is based on the interpretation of evidence that often reflects precipitation or drought, rather than temperature," Mann says. Change the selection criteria slightly and one could reclassify those years as a cold period. Soon brushes off the criticisms: "Our papers are strictly in the interest of making progress, however little that may come from a clarification of the geographic pattern of climatic changes that operate on timescales of several decades to centuries."