In a new report by the National Research Council, researchers said they were highly confident the mean global surface temperature was higher in the past 25 years than any comparable period during the previous four centuries.

They had less confidence the past quarter-century was hotter than any comparable period in the years from 900 to 1600, but found that plausible. For the years before 900, the scientists said they had very little confidence about what the Earth's mean surface temperatures were.

They did not dispute multiple measurements that showed the world warmed up by about 1 degree F (0.6 C) over the course of the 20th century, a quick rise compared with previous centuries.

The scientists also noted that temperature reconstructions for periods before the Industrial Revolution -- when levels of climate-warming greenhouse gases were much lower -- supported the notion the current global climate change was caused by human activities, rather than natural variations in climate.

CLIMATE VARIABILITY

"Natural climate variability is something that we'd like to know about," said Kurt Cuffey of the University of California-Berkeley, who served on the council's committee and spoke at a Webcast about the report.

"But if we did know for example that the climate was as warm at 1000 AD as it is now, it would have no essential impact on our understanding of climate change in the 20th century, the role of humans in causing it and the need to think seriously about how that may evolve in the next few centuries," he said.

The human causes of global warming have been under dispute, especially by a skeptical Bush administration, but are generally accepted by scientists as a key factor in climate change.

Figuring out global temperatures over the past 150 years is relatively simple, since reliable records exist. But for the years and centuries before that, researchers must read clues left by the growth rings on trees, the retreat of glaciers and even old paintings and diaries that document climate.

Such clues are called proxies, and scientists began using them in sophisticated ways in the 1990s to estimate Earth's surface temperature in past eras.

The council's report was prompted by a request from the US Congress, spurred by a controversial 1998 report in the journal Nature that used a number of sources, including proxies, to estimate temperatures in the Northern Hemisphere over the last 1,000 years.

That report concluded the hemisphere was warmer during the late 20th century than at any other time in the past millennium, and that the 1990s were the warmest decade and 1998 was the warmest year during that whole period.