For 20 years, as this crowded planet grew warmer, nations have gathered annually to try to do something about it. History now brings them to this chilly northern capital, and to a crossroads.
The world looks to Copenhagen "to witness what I believe will be an historic turning point in the fight against climate change," says Yvo de Boer, United Nations organizer of the two weeks of talks opening Monday.
It may witness, instead, history put on hold.
The change in U.S. administrations a year ago had aroused hopes the long-running climate talks might finally produce an all-encompassing package in 2009 to combat global warming and help its victims.
Too little time and too little agreement, however, especially between rich and poor countries, mean the 192-nation Copenhagen conference is likely to produce, at best, a framework _ a basis for continuing talks and signing internationally binding final agreements next year.
Two key building blocks for that framework may take shape here:
_Setting targets for controlling emissions of carbon dioxide and other global-warming gases, including by the leading contributors, China and the United States.
_Agreeing on how much rich countries should pay for poor nations' clean energy technology and for seawalls, irrigation and other projects to counter a changing climate.
Under the grand roof of Copenhagen's modern Bella Center, delegates will also deal with a heavy agenda of other issues: the technicalities of protecting forests, measuring emissions, setting rules for "carbon credits," enforcing an eventual treaty, and other concerns.
Underlining Copenhagen's importance, at least 100 national leaders, led by President Barack Obama, will converge on the Danish capital to offer high-level backing to the talks.
On Friday the White House announced Obama would come to Copenhagen on Dec. 18, the conference's last scheduled day. That's when the U.N. talks perennially go into overtime in last-minute wrangling and when other leaders are planning to take part.
The U.S. chief executive's change in plans indicated the Americans see a chance for important political agreements in those final hours.
Slow progress has marked climate talks since the 1992 Rio treaty calling for voluntary controls on greenhouse gases. It took five more years to get the Kyoto Protocol, which ordered emissions cuts by 37 industrialized nations, an accord the U.S. rejected. American resistance through eight years under President George W. Bush then blocked most progress.
While diplomacy has inched along, climate change hasn't waited.
Global temperatures are rising by 0.19 degrees Celsius (0.34 degrees Fahrenheit) per decade and twice as fast in the far north, melting Arctic sea ice at record rates. In the Copenhagen talks' final days, the World Meteorological Organization is expected to confirm this was the warmest decade on record.
Oceans, expanding from warmth and melting glaciers, are rising faster than predicted. The world's power plants, automobiles, burning forests and other sources are producing 29 percent more carbon dioxide than in 2000. Not in 2 million years has so much CO2 built up in the atmosphere, says the Global Carbon Project, an international research group.
That emissions path could drive temperatures by 2060 to at least 4 degrees C (7 degrees F) higher than preindustrial levels, scientists say. That would push the world deeper into a time of climate disruption, unusual droughts and powerful storms, species die-offs, spreading tropical diseases, coastal flooding and other, unpredictable consequences.
From the Arctic, from threatened Pacific islands, from industrial capitals, it's that fear that's bringing 15,000 delegates, environmentalists, business lobbyists, scientists, journalists and others to this quiet gray city of parks and bicycling commuters.
It will also draw hundreds of police reinforcements and protesters, activists demanding "climate justice," deeper emissions cuts by the wealthy, whose smokestacks first overloaded the skies with greenhouse gases, and richer compensation for poorer nations. Wary of confrontation, authorities have sealed off the conference site with massive concrete blocks topped by 6-foot-high metal fences.
The emissions cuts offered this time around, to follow Kyoto reductions expiring in 2012, have disappointed scientists and poorer nations facing damaging climate change. They say greenhouse gases, by 2020, must be reduced by 25 to 40 percent below 1990 output. That would keep temperatures in the less dangerous range of 2 degrees C (3.6 degrees F) above preindustrial levels, they say.
The European Union approaches that target, pledging to cut emissions by 20 percent below 1990 levels, and more if others agree. Awaiting U.S. congressional action, however, the Obama administration could make only a provisional offer of a 17 percent reduction by 2020, compared with a different baseline year, 2005. Against 1990, that represents only a 3 to 4 percent cut, experts say.
The developing world, for the first time, is offering its own actions _ not straight reductions, but clean energy projects and other steps to slow the growth of their emissions.
China says it will, by 2020, reduce gases by 40 to 45 percent below "business as usual," that is, judged against 2005 figures for energy used versus economic output. India offers a 20 to 25 percent slowdown in emissions growth.
An analysis Thursday by European research organizations found the industrialized nations' targets together amount to only 8 to 12 percent below 1990 levels, far short of what scientists urge. This track would produce global warming of well over 3 degrees C (5.4 degrees F) by 2100, it said.
De Boer hopes the Americans and others will up the ante once the talks start.
"The first thing I hope for is a good conversation among industrialized countries about how they can increase their level of ambitions if they are sure others are pulling their weight as well," the Dutch diplomat told reporters last week. He would also look for stronger offers from developing nations, he said.
On the second building block, financing, the industrialized nations are talking about a three-year, $10 billion-a-year package to help poorer nations adapt to climate change and to install clean-energy sources.
That's barely a start on what expert studies by the World Bank and others project: that hundreds of billions of dollars a year, in public and private money, will be needed to build those seawalls, water systems, wind farms and other projects in a new, warmer world. China says $300 billion or more will be needed each year.
Behind the Bella Center's closed doors, richer nations will be pressed to raise their bids on financing. That pressure is already felt in Washington, where appropriations moving through Congress would extend little more than $1 billion next year.
"I think the United States needs to come up with $2.5 or $3 billion to put on the table for an immediate jump start," reporters were told last week by U.S. Sen. John Kerry, the Massachusetts Democrat sponsoring the first legislation capping U.S. emissions.
Beyond the "jump start," the developing world will seek assurances about long-term financing, a framework of stable sources for many billions more.
Finally, rich and poor will argue over the legal structure of an eventual deal, as poorer nations resist any effort to bind them legally, and subject them to close scrutiny, as they pursue greener economies. They cite the 1992 treaty, which distinguished between two worlds, recognizing an obligation by the rich to undo the climate damage they've done, and by the poor to raise their peoples from poverty.
While delegates haggle in hushed rooms, a carnival of causes will unfold around them in the convention hall and elsewhere in Copenhagen _ protesters, propagandists and politicians, scientists and economists, religious leaders and salesmen warning of a challenging new world and promoting their way to confront it.
Even those in denial, who don't "believe" in climate change, will be here in search of a hearing, along with "believers" Al Gore, Arnold Schwarzenegger and Prince Charles.
Copenhagen was viewed in 2007, at the start of a new two-year negotiating track, as the deadline for decisive action, to replace those expiring Kyoto quotas. That looks very likely now to slip to late 2010 and a Mexico City conference.
One veteran of the talks counsels against despair.
Says former U.S. climate negotiator Eileen Claussen, now with the Pew Center on Global Climate Change:
"I think the most important thing we can do here is get started in the most ambitious way we can."
EDITOR'S NOTE _ Special Correspondent Charles J. Hanley reported from New York and Jan Olsen from Copenhagen.
AP and other news agencies' Facebook ClimatePool page: http://www.facebook.com/TheClimatePool
U.N. climate treaty secretariat: http://unfccc.int/2860.php
Denmark's Copenhagen climate conference site: http://en.cop15.dk/