Failure would produce a turbulent 21st century of weather extremes, spreading drought and disease, expanding oceans and displaced coastal populations, it said.
"The increasing numbers of environmental refugees as sea levels rise and storm surges increase will be in the tens of millions," panel co-chair Rosina Bierbaum, a University of Michigan ecologist, told reporters here.
After a two-year study, the 18-member group, representing 11 nations, offered scores of recommendations: from pouring billions more dollars into research and development of cleaner energy sources, to mobilizing U.N. and other agencies to help affected people, to winning political agreement on a global temperature "ceiling."
Their 166-page report, produced at U.N. request and sponsored by the private United Nations Foundation and the Sigma Xi Scientific Research Society, was issued just three weeks after the Intergovernmental Panel on Climate Change (IPCC), an authoritative U.N. network of 2,000 scientists, made headlines with its latest assessment of climate science.
The IPCC expressed its greatest confidence yet that global warming is being caused largely by the accumulation of carbon dioxide and other heat-trapping gases in the atmosphere, mostly from man's burning of coal, oil and other fossil fuels. If nothing's done, it said, global temperatures could rise as much as 11°F by 2100.
Temperatures rose an average 1.3°F over the past 100 years. Tuesday's report said the world's nations should agree to limit further rises this century to no more than 3.6°F.
Beyond that, "we would be in a regime where the danger of intolerable and unmanageable impacts on human well-being would rise very rapidly," said panel member John P. Holdren, director of Massachusetts' Woods Hole Research Center.
Separately, a powerful group of developing nations on Tuesday said wealthy nations must take responsibility for causing climate change instead of laying blame on others.
While emissions of greenhouse gases and other pollutants are increasing in booming Asian economies like India and China, "most of the environmental degradation that has happened has been historically caused by ... the industrial world," said Munir Akram, Pakistan's U.N. ambassador and chairman of the Group of 77, an organization grouping 132 mainly developing countries and China.
"There is now unfortunately a sort of propaganda effort to try and shift the blame for the environmental degradation on these fast-growing developing economies," Akram told reporters after a meeting of the group at the Rome-based U.N. Food and Agriculture Organization
The experts panel said global carbon dioxide emissions should be leveled off by 2015-2020, and then cut back to less than one-third that level by 2100, via a vast transformation of global energy systems — toward greater efficiency, away from fossil fuels, and toward biofuels, solar, wind and other renewable energy sources.
That changeover would be spurred by heavy "carbon taxes" or "cap-and-trade" systems, whereby industries' emissions are capped by governments, and more efficient companies can sell unused allowances to less efficient ones.
Such schemes — already in use in Europe under the Kyoto Protocol climate pact — have been proposed in the U.S. Congress, but are opposed by the Bush administration, which rejects Kyoto because it does not include developing countries such as China and India.
The White House points to what it says is spending of almost $3 billion a year on energy-technology research and development as its major contribution to combatting climate change. But Holdren said other calculations put spending at under $2 billion, and it's "far from proportionate to either the size of the challenge or the size of the opportunities."
Tuesday's report said such research budgets worldwide are badly underfunded, and require a tripling or quadrupling, to $45 billion or $60 billion a year.
Billions more should go toward work on cellulose as a biofuel, overcoming the problems of nuclear energy, reducing solar electricity's cost, and developing other cleaner energy sources, Holdren said. He said intensified research is particularly needed for carbon capture and sequestration — technology to capture carbon dioxide in power-plant emissions and store it underground.
In fact, the experts panel urged governments to immediately ban all new coal-fired power plants except those designed for eventual retrofitting of sequestration technology.
The panel's other co-chair is biodiversity expert Peter H. Raven, Missouri Botanical Garden director and past president of Sigma Xi.
