Two years ago, more than 180 nations made a bold promise: By the end of 2009, they would draft a sweeping treaty to slow climate change.
Yvo de Boer, the United Nations' top climate-change official, called the agreement "a real breakthrough," and British Prime Minister Gordon Brown expressed confidence it would produce "a deal ... in 2009 to address the defining challenge of our time."
Now the deadline is nearing, and hope is fading. The treaty is supposed to be finalized at talks that start Dec. 7 in Copenhagen
, but diplomats have made almost no progress toward an agreement - a point made repeatedly by world leaders Tuesday at the U.N
. climate summit in New York
"As we head towards Copenhagen, there should be no illusions that the hardest part of our journey is in front of us," President Obama said at the U.N.
Swedish Prime Minister Fredrik Reinfeldt warned of a deadlock. "The negotiations are going far too slowly," he said.
The debate over climate change is wrapped in a range of political and economic conflicts.
Climate scientists, such as Rajendra Pachauri, head of a U.N.-organized group of thousands of climate experts,say the world is headed for dramatic changes unless nations slash emissions of carbon dioxide - from cars, power plants and factories - and other greenhouse gases soon.
The key question is how to do so without crippling the worldwide economy - and making such limits fair to industrialized nations such as the United States as well as developing nations and rising powers such as China. The U.S. Energy Department says China is the only nation that produces more carbon dioxide emissions from burning fossil fuels than the USA.
The notion that such emissions are dramatically changing the climate is widely accepted.
The U.N.'s Intergovernmental Panel on Climate Change published a report two years ago saying the odds are better than 90% that global warming is caused by humans. Even so, there are holdouts - notably Republicans in the U.S. Senate, which ratifies treaties involving the USA - who question whether emissions have much of a role in what they suggest is a natural warming of the planet.
Meanwhile, there are questions about whether even a dramatic reduction in emissions would avoid dangerous climate changes.
Against that backdrop, the scramble for a deal to combat global warming continues.
Key nations will take part in climate talks at a flurry of upcoming meetings, including the G-20 meeting in Pittsburgh of major economic powers Thursday and Friday and further U.N. talks starting Monday in Bangkok.
The Earth isn't waiting, scientists say.
If emissions keep increasing as they have, the planet's average temperature will rise 3 to 7 degrees by 2100, according to the U.N. climate panel's 2007 report.
A temperature rise of more than 3½ degrees could melt the ice covering Greenland - raising sea levels 20 feet, says climate scientist Jonathan Gregory of Britain's University of Reading.
If Antarctica's ice melts, too, cities such as Hong Kong and Miami would be threatened, MIT climate scientist John Reilly says.
Scientists at the Hadley Centre reported this year that above 3½ degrees, swaths of the Amazon rainforest will die.
Despite such dire scenarios, drafting a climate treaty will be "10 times more difficult" than it was in 1997, when diplomats meeting in Kyoto, Japan, agreed to the world's first mandatory climate pact, says John Prescott, Britain's former deputy prime minister and an important player in the Kyoto deal. More nations are involved now, Prescott says, and they are further apart on how much to cut emissions.
The worldwide financial crisis is adding to the economic concerns that surround talks on a climate-change treaty.
Obama acknowledged the economic pressures in Tuesday's speech, saying "every nation's most immediate priority is reviving their economy and putting their people back to work. And so all of us will face doubts and difficulties in our own capitals as we try to reach a lasting solution to the climate challenge."
The Bush administration and the Senate opposed the Kyoto Protocol because it exempted developing nations such as China and India from mandatory emissions cuts, which U.S. politicians feared would put the USA at an economic disadvantage. The Senate's lack of approval of the Kyoto treaty helped make the treaty ineffective in cutting emissions.
Today, many U.S. senators have the same worries they had in the 1990s: that a climate treaty would slow economic growth, make U.S. industry less competitive and drive up energy prices.
The Kyoto Protocol expires in 2012. If the deal making in Copenhagen leads to a new pact that would harm the U.S. economy, "no such treaty or agreement can be approved by the Senate," says Sen. James Inhofe, R-Okla.
Here are some of the problems confronting the global effort to cool the Earth's temperature:
Mistrust and finger-pointing
In Copenhagen, countries are supposed to settle which of them would be required to lower their emissions and by how much.
The battle over those issues is unlikely to end this year, says Eileen Claussen, head of the Pew Center on Global Climate Change and a former U.S. climate official. That's because the countries are far apart:
•The European Union suggested to the U.N. that industrialized countries, including its members, cut emissions by 30% by 2020. Meeting that limit plus another goal to use more solar power and other "renewable energy" would cost tens of billions of dollars in 2020, according to a European Commission study.
• China, which considers itself a developing country, wants developed countries to cut emissions 40% by 2020, according to the official Xinhua news agency. China insists it should not be subject to mandatory emissions cuts, pointing to its efforts to produce more renewable energy and to become more energy-efficient.
• A bill passed by the House of Representatives in June would use a carbon-trading system to help cut U.S. emissions 17% by 2020, according to an analysis by the World Resources Institute, an environmental research group. A similar bill is scheduled to be introduced in the Senate in the next few weeks.
The numbers being floated are partly bargaining positions, says former U.S. climate negotiator Rafe Pomerance, now president of Clean Air-Cool Planet, a non-profit climate-change group. But he said nations "end up creating a set of expectations that would be difficult to meet."
Demands for money
Several dozen industrialized nations agreed two years ago to dig deeper into their pockets for climate-related aid.
Developing countries would draw on the funds to cut their own emissions and to cope with the effects of a warming world, such as water-collection projects to guard against drought.
The thorny question is how much money wealthier nations will offer. At negotiations in August, developing countries requested $400 billion a year. A U.N. report released Sept. 1 pegged the need at more than $500 billion a year.
That far outstrips the money that developed nations are likely to put on the table, particularly at a time of budget deficits.
The European Union said Sept. 10 that it would contribute up to $22 billion a year, but the United States has kept quiet. Obama's proposed budget for foreign aid in 2010 is $36.5 billion for public health programs, anti-drug campaigns and other projects.
Developing nations have threatened to derail the talks if their demands aren't met.
If a treaty "is not consistent with our minimal position," Ethiopian Prime Minister Meles Zenawi said Sept. 3, according to Reuters, "we are prepared to walk out of any negotiations."
Twelve years ago, the Senate overwhelmingly opposed a global-warming treaty - 95 of the 100 senators voted for a resolution against any agreement that would harm the U.S. economy.
U.S. diplomats learned their lesson, Claussen says: Don't settle on a treaty that's likely to be rejected back home.
This time, U.S. climate negotiators have "made clear" that they won't finalize a treaty without "a very good sense of where Congress is," Claussen says.
The clock is working against the U.S. team. Introduction of a Senate version of the climate bill has been delayed repeatedly and may not take place until October, in part because of the health care debate. Senate Majority Leader Harry Reid, D-Nev., said last week that a bill may not come up for a vote until 2010.
The House version includes measures to soften economic impacts of reducing carbon dioxide emissions. Among them are financial aid to workers who lose their jobs because of the bill and barriers to importing goods from nations that don't cut emissions.
Even so, most Republican senators oppose a bill on climate change, and many moderate Democrats - especially those from coal and manufacturing states in the Midwest - are worried that such a plan would mean job losses in their states.
There's also a faction of senators who are skeptical that climate change is a problem at all.
"A lot of members of Congress ... are absolutely convinced" that humans are the primary cause of global warming, Sen. Chuck Grassley, R-Iowa, said last month. "I haven't reached that conclusion at this point."
Nearly all the news from scientists add up to one hard truth: It will be extremely difficult to avoid more warming of the Earth.
Even if carbon dioxide emissions fell to zero tomorrow, the Earth's temperature would continue to rise, says climate researcher Francisco de la Chesnaye of the Electric Power Research Institute.
That's because it takes decades for the climate to adjust fully to a new level of greenhouse gases in the atmosphere, he says.
In July, the European Union and 16 of the world's wealthiest nations - including China and the United States - signed a statement acknowledging the scientific opinion that the Earth's temperature rise "ought not to exceed" roughly 3½ degrees.
That's a tall order. The Earth already has warmed 1½ degrees since the late 19th century and will warm another 1 degree by 2100 based on the greenhouse gases already emitted.
To keep the temperature rise below 3½ degrees, greenhouse gases may have to be cleansed from the atmosphere, says Hugh Pitcher of the Pacific Northwest National Laboratory.
That might be accomplished, say Pitcher and other climate experts, with technology that doesn't yet exist: systems to suck carbon dioxide from the air and store it underground.
"We have got ourselves painted into a little bit of a corner on this one," he says.
Based on the amount of greenhouse gases now in the atmosphere, achieving the 3½ degree goal "will require huge efforts over the whole century," MIT climate expert Henry Jacoby says. "The kind of agreement that can be reached in Copenhagen can only get started on that task."
Though it will be difficult to prevent more warming, that doesn't mean "we might as well go home and forget it," Jacoby says. "If we get discouraged and don't do anything, the implications are very, very serious. The risks are very, very great."