In a contretemps indicative of the political struggle over global climate change, a recent study suggested that humans may not be warming the earth. Greenhouse skeptics, pro-industry groups and political conservatives have seized on the results, proclaiming that the science of climate change is inconclusive and that agreements such as the Kyoto Protocol, which set limits on the output of industrial heat-trapping gases, are unnecessary. But mainstream climatologists, as represented by the Intergovernmental Panel on Climate Change (IPCC), are perturbed that the report has received so much attention; they say the study's conclusions are scientifically dubious and colored by politics.

Sallie Baliunas and Willie Soon of the Harvard-Smithsonian Center for Astrophysics reviewed more than 200 studies that examined climate "proxy" records--data from such phenomena as the growth of tree rings or coral, which are sensitive to climatic conditions. They concluded in the January Climate Research that "across the world, many records reveal that the 20th century is probably not the warmest nor a uniquely extreme climate period of the last millennium." They said that two extreme climate periods--the Medieval Warming Period between 800 and 1300 and the Little Ice Age of 1300 to 1900--occurred worldwide, at a time before industrial emissions of greenhouse gases became abundant. (A longer version subsequently appeared in the May Energy and Environment.)

In contrast, the consensus view among paleoclimatologists is that the Medieval Warming Period was a regional phenomenon, that the worldwide nature of the Little Ice Age is open to question and that the late 20th century saw the most extreme global average temperatures.

Scientists skeptical of human-induced warming applaud the analysis by Soon and Baliunas. "It has been painstaking and meticulous," says William Kininmonth, a meteorological consultant in Kew, Australia, and former head of the Australian National Climate Center. But he acknowledges that "from a purely statistical viewpoint, the work can be criticized."

And that criticism, from many scientists who feel that Soon and Baliunas produced deeply flawed work, has been unusually strident. "The fact that it has received any attention at all is a result, again in my view, of its utility to those groups who want the global warming issue to just go away," comments Tim Barnett, a marine physicist at the Scripps Institution of Oceanography, whose work Soon and Baliunas refer to. Similar sentiments came from Malcolm Hughes of the Laboratory of Tree-Ring Research at the University of Arizona, whose work is also discussed: "The Soon et al. paper is so fundamentally misconceived and contains so many egregious errors that it would take weeks to list and explain them all."

Rather than seeing global anomalies, many paleoclimatologists subscribe to the conclusions of Phil Jones of the University of East Anglia, Michael Mann of the University of Virginia and their colleagues, who began in 1998 to quantitatively splice together the proxy records. They have concluded that the global average temperature over the past 1,000 years has been relatively stable until the 20th century. "Nothing in the paper undermines in any way the conclusion of earlier studies that the average temperature of the late twentieth century in the Northern Hemisphere was anomalous against the background of the past millennium," wrote Mann and Princeton University's Michael Oppenheimer in a privately circulated statement.

The most significant criticism is that Soon and Baliunas do not present their data quantitatively--instead they merely categorize the work of others primarily into one of two sets: either supporting or not supporting their particular definitions of a Medieval Warming Period or Little Ice Age. "I was stating outright that I'm not able to give too many quantitative details, especially in terms of aggregating all the results," Soon says.

Specifically, they define a "climatic anomaly" as a period of 50 or more years of wetness or dryness or sustained warmth (or, for the Little Ice Age, coolness). The problem is that under this broad definition a wet or dry spell would indicate a climatic anomaly even if the temperature remained perfectly constant. Soon and Baliunas are "mindful" that the Medieval Warming Period and the Little Ice Age should be defined by temperature, but "we emphasize that great bias would result if those thermal anomalies were to be dissociated" from other climatic conditions. (Asked to define "wetness" and "dryness," Soon and Baliunas say only that they "referred to the standard usage in English.")

Moreover, their results were nonsynchronous: "Their analysis doesn't consider whether the warm/cold periods occurred at the same time," says Peter Stott, a climate scientist at the U.K.'s Hadley Center for Climate Prediction and Research in Bracknell. For example, if a proxy record indicated that a drier condition existed in one part of the world from 800 to 850, it would be counted as equal evidence for a Medieval Warming Period as a different proxy record that showed wetter conditions in another part of the world from 1250 to 1300. Regional conditions do not necessarily mirror the global average, Stott notes: "Iceland and Greenland had their warmest periods in the 1930s, whereas the warmest for the globe was the 1990s."

Soon and Baliunas also take issue with the IPCC by contending that the 20th century saw no unique patterns: they found few climatic anomalies in the proxy records. But they looked for 50-year-long anomalies; the last century's warming, the IPCC concludes, occurred in two periods of about 30 years each (with cooling in between). The warmest period occurred in the late 20th century--too short to meet Soon and Baliunas's selected requirement. The two researchers also discount thermometer readings and "give great weight to the paleo data for which the uncertainties are much greater," Stott says.

The conclusion of Soon and Baliunas that the warming during the 20th century is not unusual has engendered sharp debate and intense reactions on both sides--Soon and Baliunas responded primarily via e-mail and refused follow-up questions. The charges illustrate the polarized nature of the climate change debate in the U.S. "You'd be challenged, I'd bet, to find someone who supports the Kyoto Protocol and also thinks that this paper is good science, or someone who thinks that the paper is bad science and is opposed to Kyoto," predicts Roger Pielke, Jr., of the University of Colorado. Expect more of such flares as the stakes--and the world's temperatures--continue to rise.

David Appell is based in Lee, N.H.