A United Nations panel on Friday released its most comprehensive strategy to avoid the catastrophic effects of global warming, but experts said that political and economic realities likely doom it to failure.
Although more than 100 countries backed the report, experts said that its call for a global, multi-trillion dollar effort to reduce greenhouse gas emissions is unrealistic.
The United States and China – which account for more than 40 percent of the world’s emissions – approved the report but have given no indication that they would reverse their long-held opposition to mandatory reductions in emissions.
“It’s not realistic from a political standpoint, and it’s not realistic because those targets are incredibly expensive,” said Robert Mendelsohn, an economist at Yale University.
Even supporters of the plan were daunted by the speed and scale of action required by the report to stabilize carbon emissions at roughly current levels.
“It’s hard to imagine,” said Jae Edmonds, an economist at the Joint Global Change Research Institute, located at the University of Maryland. “So many things have to happen so fast, and they are so big.”
The report, by the U.N.’s Intergovernmental Panel on Climate Change, was released in Bangkok, Thailand, after a week of contentious debates.
Its primary instrument for reducing greenhouse gas concentrations is a system in which governments would place a cap on emissions and charge polluters for every ton of carbon dioxide beyond that point. That would force companies to cut emissions and invest in energy efficiency and alternative fuels.
The price per ton would reach as high as $100 by 2030. By then, the system could cost up to 3 percent of the world’s gross domestic product, the report said.
The Bush administration quickly denounced the restrictions as too expensive.
“It would cause a global recession,” said James Connaughton, chairman of the White House Council on Environmental Quality.
“Our goal is reducing emissions and growing the economy,” he said during a news conference in Bangkok.
But Robert Socolow, a carbon mitigation expert at Princeton University, said that taking a cheaper and slower path could be costly. A study by the British government last year found that damage from global warming – flooding, starvation, drought and other calamities – would easily top 5 percent of global GDP annually.
The U.N. report looked at a variety of scenarios, but only the most expensive would avoid the worst perils of rising temperatures.
That course requires annual emissions to peak by 2015 and fall 50 percent, to 85 percent of 2000 levels, by mid-century. It would limit the temperature increase to 3 degrees Fahrenheit.
The concentration of greenhouse gases would be stabilized between 445 and 490 parts per million carbon dioxide equivalents – a measure that factors in the warming effects of all greenhouse gases. The current concentration is about 425 parts per million.
The United States has opposed calls for mandatory emissions cuts and carbon taxes, instead placing its hopes on voluntary reductions and future technologies that would be cleaner and cheaper.
China also has opposed mandatory reductions, saying they would derail its economic growth.
The world’s most populous country unsuccessfully fought to delete the most stringent emissions scenario from the report, participants in the conference said. India, the world’s fifth-biggest polluter, joined in the effort.
The most prominent supporters of the plan were European nations.
“The report shows – and this is encouraging – that ambitious climate protection is economically manageable,” said German government spokesman Ulrich Wilhelm.
The U.N. report is the third of four installments being issued this year. With the input of more than 2,000 scientists and the approval of more than 100 governments, they are the closest thing the world has to a consensus on global warming.
The release in February of the first report, which definitively blamed humans for global warming, helped galvanize world opinion after years of debate and shifted the focus of activity toward fixing the problem.
The second report, released in April, looked at the potential impacts of global warming. It said that rising temperatures, if left unchecked, would lead to widespread coastal flooding, starvation and species extinctions.
The current report is as much about policy as it is about science, asking how much the world is willing to pay to stem global warming.
Among its options are several more affordable scenarios. For example, one plan would stabilize greenhouse gas levels between 590 and 710 parts per million. It would cost 0.2 percent of global GDP in 2030, compared with 3 percent in the most stringent plan.
Annual emissions would continue to rise until 2060, increasing more than 60 percent above 2000 levels.
But under that scenario, temperatures would rise about 6 degrees, which the U.N. panel has described as calamitous.
The crux of the problem is deciding who should pay to contain global warming.
China and other developing countries argue that industrialized nations should foot the bill, since they are responsible for the bulk of carbon dioxide accumulation in the atmosphere over the last two centuries.
That view is reflected in the Kyoto Protocol, which requires participants to reduce their greenhouse gas emissions by 2012 by an average of 5 percent below 1990 levels.
The Bush administration has refused to join the Kyoto pact on the grounds that it does not restrict emissions in the developing world.
China, which opens a new coal-fired power plant once a week, is on track to surpass the United States as the biggest polluter as soon as this year.
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