Huang's study "reinforces the forecast for the 21st century that we've heard before: significant warming head," Jonathan T. Overpeck of the University of Arizona writes in "News and Views" commentary in the same Nature issue.

"The results also provide unsettling indications that human alteration of the climate system over the last century is going to make the reliable prediction of future climate change one of society's most important challenges," Overpeck says in his commentary, "The Hole Story."

A number of recent "proxy" record studies suggest that the late 20th century has been the warmest in the past 400 to 1,000 years, Overpeck adds. Huang's study is further independent confirmation of tree-ring, ice-core, coral and sediment records that show the global warming trend.

"In both the southern and northern hemispheres, the borehole records match the instrumental records of the last century, and confirm that the 20th century is the warmest of the last 500 years," Overpeck said. "Moreover, each of six continents investigated has warmed faster over the 20th century than during any of the previous four centuries."

Earth is starting the new millennium warmer than at any time during the previous millennium. What is at least as worrisome, Overpeck adds:

**Politicians, corporate industrialists and private citizens "need to know how climate is going to change, where and when" -- and not just in 50 or 100 years from now. Shifts in seasonal, decadal to centennial climate, for example, can have a big impact on economies and livelihoods. Given that society appears destined to further global warming, decision-makers need good forecasts for climate changing on several time scales.

**"Even if the effects of human (trace-gas) forcing on climate turn out to be modest, climate researchers are faced with a serious challenge." The challenge is to verify if -- as the Huang study suggests -- previous studies underestimate how greatly climate can vary over decades and centuries.

"We've come to the point where we know that natural variability isn't trivial, and that humans can change the course of climate," Overpeck says. "Now we have to get more serious about using our resources to develop the knowledge needed to tell society's decision-makers what Mother Nature is going to do in the seasons, years and decades ahead."

Overpeck is director of the Institute for the Study of Planet Earth and a professor of geosciences at the University of Arizona.

Lori Stiles <lstiles@U.ARIZONA.EDU>
Jonathan T. Overpeck, 520-622-9065, jto@u.arizona.edu