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RENO, Nev. (AP) - International experts at a gathering of more than 1,000 scientists studying climate change and the future of mankind said the threat of global warming is real and growing worse.

One leading researcher at the weeklong conference said it is "ludicrous" President George W. Bush's administration has refused to acknowledge the increasing danger of greenhouse gases.

"The voluntary measures the administration is proposing are going to get us nowhere," Raymond Bradley said Friday.

Bradley is the director of the University of Massachusetts' Climate System Research Center at Amherst, Mass.

"Right now, we have good, strong scientific evidence supported by the vast majority of scientists who studied the problem to say we are facing a serious problem," he said Friday.

Bradley criticized the White House decision this week to make the study of natural cycles in climate change the chief goal of a new 10-year plan addressing global warming.

Bush and his advisers maintain reducing emissions through costly near-term measures is unjustified. The White House argues forecasting climate change is too imprecise to agree to long-term, international, mandatory cuts in greenhouse-gas emissions.

"It is only imprecise if you choose to consider what I would describe as fringe science," Bradley said.

"Politicians are always faced with making decisions in the face of uncertainty but I think the uncertainty over this issue is relatively low."

Bradley co-authored a study of tree rings and ice cores that determined 10 of the hottest years globally over the last 600 years have come since 1990 - the hottest in 1998.

"We need to put our present state in perspective for politicians and others who are not yet convinced things need to be taken seriously," he said in a speech Thursday.

"Most of the major developments in this area have taken place in the last 30 years," Bradley said.

"One-half of all the greenhouse gases have been added since I was a grad student," he said, mostly in the form of emissions from carbon dioxide and methane.

"This change is clearly unprecedented, it is abrupt and it's of a magnitude larger than anything we have ever experienced. And whatever we've seen in the recent past, those changes are destined to be overshadowed by changes in the near future," he told the International Union for Quaternary Research.

INQUA was formed in 1928 by scientists seeking to understand environmental changes on Earth since the Quaternary Period, which spans approximately the last two million years.

Other papers presented at the conference include the findings of James Knox of the University of Wisconsin - that flooding of the Upper Mississippi River over the last 7,000 years was "strongly linked to relatively modest climate changes."

The high frequency of large floods on the (river) since about 1950 have occurred during a period of rapid global warming, he said.

David Sauchyn of the University of Regina said his research suggests global warming could result in Canada's prairie environment becoming much drier.

U.S. Energy Secretary Spencer Abraham has said the administration is "already engaged in an active, aggressive and multi-pronged campaign to address climate change."

The new program introduced this week "will find the answers to the many unanswered questions about climate change and identify the most promising areas for investment in future technology research and development," he said.

James Schlesinger, U.S. energy secretary under former president Jimmy Carter, said at a recent Energy Department symposium the idea the "science is settled" on global warming is "far from the truth.

"We cannot tell how much of the recent warming trend can be attributed to the greenhouse effect and how much to other factors. In climate change, we have only a limited grasp on the overall forces at work," he said.

Bradley said there were times in history where higher levels of carbon dioxide likely existed.

"But there weren't six billion people living on a knife's edge when those levels were reached in the past," he said.

"For the first time in history, human beings are having a global impact on the most remote parts of the planet. When you go to the South or North Pole, you see the evidence of what is happening 10,000 miles away."

The INQUA conference, the first in the United States since it met in Colorado in 1965, is hosted by the Desert Research Institute and continues through Wednesday.

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On the Net:

International Union for Quaternary Research Conference: http://inqua2003.dri.edu/

U.S. Energy Department: www.doe.gov

Desert Research Institute: dri.edu