COPENHAGEN: The largest and most important U.N. climate change conference in history opened Monday, with organizers warning diplomats from 192 nations that this could be the last, best chance for a deal to protect the world from calamitous global warming.

Negotiations have dragged on for two years and only recently have shown signs of breakthroughs with new commitments from major emitters such as the United States, China and India to control greenhouse gas emissions.

In a signal the Obama administration is prepared to act without congressional action if needed, officials said the U.S. Environmental Protection Agency has concluded that greenhouse gases are endangering Americans' health and must be regulated.

The two-week conference convened in an upbeat mood after a series of promises by rich and emerging economies to curb their greenhouse gases. Still, major issues have yet to be resolved.

At stake is a deal that aims to wean the world away from fossil fuels and other pollutants to greener sources of energy, and to transfer hundreds of billions of dollars from rich to poor countries every year over decades to help them adapt to climate change.

Scientists say without such an agreement, the Earth will face the consequences of ever-rising temperatures, leading to the extinction of plant and animal species, the flooding of coastal cities, more extreme weather events, drought and the spread of diseases.

With the commitments remaining short of scientists' demands, the pressure was on those major emitters for bigger cuts. Swedish Environment Minister Anders Carlgren, speaking for the European Union, said it would be "astonishing" if President Barack Obama came for the final negotiation session "to deliver just what was announced in last week's press release."

U.S. EPA Administrator Lisa Jackson was expected to announce the endangerment finding later Monday, officials told The Associated Press, speaking privately because the announcement had not been made.

American business groups have strongly argued against tackling global warming through the Clean Air Act, saying it is less flexible and more costly than the bill being considered before Congress. On Monday, some of those groups questioned the timing of the EPA announcement, calling it political.

Climate activists in Copenhagen said the decision could help the Obama administration move ahead on climate policy without waiting for action from Congress.

"The question is will they use it that way, or are they using it as a bargaining chip to threaten action, and get Congress to act instead," said Damon Moglen, of Greenpeace USA.

Conference president Connie Hedegaard said the key to an agreement is finding a way to raise and channel public and private financing to poor countries for years to come to help them fight the effects of climate change.

Hedegaard — Denmark's former climate minister — said if governments miss their chance at the Copenhagen summit, a better opportunity may never come.

"This is our chance. If we miss it, it could take years before we got a new and better one. If we ever do," she said.

The conference opened with video clips of children from around the globe urging delegates to help them grow up without facing catastrophic warming. On the sidelines, climate activists competed for attention to their campaigns on deforestation, clean energy and low-carbon growth.

Mohamad Shinaz, an activist from the Maldives, plunged feet-first into a tank with nearly 200 gallons (750 liters) of frigid water to illustrate what rising sea levels were doing to his island nation.

"I want people to know that this is happening," Shinaz said as the water reached up to his chest. "We have to stop global warming."

Leah Wickham, a 24-year-old from Fiji, broke down in tears as she handed a petition from 10 million people asking the negotiators at Copenhagen to come up with a deal to save islands like hers.

"I'm on the front lines of climate change," she said.

Denmark's prime minister said 110 heads of state and government will attend the final days of the conference. Obama's decision to attend the end of the conference, not the middle, was taken as a signal that an agreement was getting closer.

"The evidence is now overwhelming" that the world needs early action to combat global warming, said Rajendra Pachauri, the head of the Intergovernmental Panel on Climate Change, an U.N. expert panel.

He defended climate research in the face of a controversy over e-mails pilfered from a British university, which global warming skeptics say show scientists have been conspiring to hide evidence that doesn't fit their theories.

"The recent incident of stealing the e-mails of scientists at the University of East Anglia shows that some would go to the extent of carrying out illegal acts perhaps in an attempt to discredit the IPCC," he told the conference.

The first week of the conference will focus on refining the complex text of a draft treaty. But major decisions will await the arrival next week of environment ministers and the heads of state in the final days of the conference, which ends Dec. 18.

"The time for formal statements is over. The time for restating well-known positions is past," said the U.N.'s top climate official, Yvo de Boer. "Copenhagen will only be a success it delivers significant and immediate action."

Among those decisions is a proposed fund of $10 billion each year for the next three years to help poor countries create climate change strategies. After that, hundreds of billions of dollars will be needed every year to set the world on a new energy path and adapt to new climates.

"The deal that we invite leaders to sign up on will be one that affects all aspects of society, just as the changing climate does," said Danish Prime Minister Lars Loekke Rasmussen. "Negotiators cannot do this alone, nor can politicians. The ultimate responsibility rests with the citizens of the world, who will ultimately bear the fatal consequences if we fail to act."

A study released by the U.N. Environment Program on Sunday indicated that pledges by industrial countries and major emerging nations fall just short of the reductions of greenhouse gas emissions that scientists have said are needed to keep average temperatures from rising more than 2 degrees C (3.6 F) by the end of the century.

In Vienna, another senior U.N. official warned that the fight against climate change must not "cannibalize" development financing.

Kandeh Yumkella, director-general of the U.N Industrial Development Organization, said poor countries need "fresh money" to combat global warming, not funds diverted from efforts to improve maternal health or fight world hunger.