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.