Carbon emissions cuts pledged at U.N. climate talks would put the world on "an unsustainable pathway" toward average global warming 50 percent higher than industrial countries want, a confidential U.N. draft document showed Thursday.
The document, obtained by The Associated Press, forecast that the average global temperature would rise in coming decades by 3 degrees Celsius (5.4 degrees Fahrenheit) compared to pre-industrial levels.
The world has already warmed a bit, so that would mean an additional 2.3 degrees Celsius (4.1 degrees Fahrenheit) warming from the present day.
Scientists say such rises in average temperatures could lead to catastrophic sea level rises, which would threaten islands and coastal cities, kill off many species of animals and plants, and alter the agricultural economies of many countries.
U.N. climate officials' internal tally of rich and developing nations' targets, both required and voluntary, shows they must cut emissions 10.5 billion tons a year by 2020. This would enable the world to prevent average temperatures from rising more than 2 degrees Celsius (3.6 degrees Fahrenheit) _ the target most industrialized nations use at Copenhagen.
"All the pledges to date do not add up to what the science requires," said Robert Orr, a U.N. assistant secretary-general for policy, confirming the document's preliminary figures.
But Orr said the pledges "will create significant carbon markets (and) significant funding streams that will change the equation that we currently see today."
Environmental groups said the leaked document offered a reality check for measuring the conference's success.
"The stark message for world leaders at Copenhagen is that the proposals on the table _ especially from industrialized countries _ fall far short of what the world needs," said Keith Allott, head of climate change for WWF in Britain.
Rajendra Pauchauri, chairman of the Intergovernmental Panel on Climate Change, the U.N.-sponsored science network, said he didn't know about the U.N. document. But he said emissions growth must peak before 2015 to avoid the Earth heating up 2.3 degrees Celsius.
The world is pumping about 38 billion tons a year of greenhouse gases into the atmosphere, and that is projected to grow to about 54 billion tons by 2020. The internal draft says that must be kept to 44 billion tons to limit climate change to below 2 degrees Celsius.
Even if rich and developing nations fulfill the pledges they have made in Copenhagen, they still would have to reduce emissions by another 4.2 billion tons a year by 2020 to keep from putting themselves on "an unsustainable pathway" that could dangerously increase atmospheric concentrations of carbon dioxide, the document said.
The draft assessment covers 40 rich nations' pledges to reduce their emissions and the announcements by major developing nations such as China, India and Brazil to reduce voluntarily their rates of growth in greenhouse gases.
Orr noted that a lot can still happen _ nations might at the last minute ramp up their targets to cut carbon emissions _ and "there is no road map for this, there is no precedent. We're on entirely new terrain."
The United States has pledged cutting emissions by 17 percent from 2005 levels by 2020. That represents a 3.5 percent cut from 1990 levels, the benchmark used by most countries.
The European Union plans to cut emissions by 20 percent by 2020, and is willing to increase its target to 30 percent if other developed countries make comparable commitments.
China, the world's biggest carbon polluter, has pledged to cut "carbon intensity" _ a measure of carbon dioxide emissions per unit of production _ by 40 percent to 45 percent by 2020 versus 2005 levels.
Bill McKibben, an environmentalist and author, said the leaked U.N. document proves that conference negotiators are failing to limit temperature increases to the levels demanded by most industrialized and island nations.
Associated Press Writers Seth Borenstein and Michael Casey contributed to this report.