Copenhagen, Denmark ? 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:

l Setting targets for controlling emissions of carbon dioxide and other global-warming gases, including by the leading contributors, China and the United States.

l 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 0.34 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 7 degrees 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.