CANCUN, Mexico -- Mexican President Felipe Calderon called on climate negotiators Monday to put national interests aside and act on behalf of all humanity to fight global warming.

Opening a two-week conference, Calderon urged delegates to overcome the deep divide between rich and poor countries that has stymied efforts to negotiate a new climate treaty for three years.

"It would be a tragedy if our inability to see beyond our personal interests or national interests makes us fail," Calderon said in a speech to 15,000 delegates, businessmen, activists and journalists. "The atmosphere is indifferent to the sovereignty of states."

The annual U.N. conference in this coastal resort comes amid mounting evidence that the Earth's climate already is changing in ways that will affect both sides of the wealth divide among nations.

After a disappointing summit last year in Copenhagen, no hope remains of reaching an overarching deal this year setting legal limits on how much major countries would be allowed to pollute. Such an accord was meant to describe a path toward slashing greenhouse gas emissions by mid-century, when scientists say they should be half of today's levels.

During the talks, delegates hope to conclude agreements that will clear the way to mobilize billions of dollars for developing countries and give them green technology to help them shift from fossil fuels affecting climate change.

The gathering started with renewed warnings from scientists over the failure to act.

"Climate change is the most serious challenge our society has ever confronted," said Mario Molina, a Mexican chemist who won the Nobel Prize in 1995 for his studies on the ozone layer.

The tools are at hand to limit the planet's warming at little cost, but it could mean "astronomical costs for future generations" if nothing is done, he said in the convention's inaugural address.

Rajendra Pachauri, chairman of the U.N.'s expert panel on climate change, reminded the delegates of its findings that global warming is "unequivocal" and could lead to "abrupt and irreversible" changes in the Earth's climate system.

Eighty-five countries have made specific pledges to reduce emissions or constrain their growth, but those promises amount to far less than required to keep temperatures from rising to potentially dangerous levels.

The recriminations that followed the Copenhagen summit have raised questions over whether the unwieldy U.N. negotiations, which require at least tacit agreement from every nation, can ever work.

Adopting scaled back ambitions for Cancun, if successful, could restore confidence in the process.

"As is the case with any large puzzle with over 1,000 pieces and over 190 players, one needs to start with the edges and work inwards," said Jennifer Morgan, of World Resources Institute, a Washington-based environmental think tank.

Christiana Figueres, the top U.N. climate official, said world capitals are aware of both a growing environmental and political urgency. "Governments need to prove that the intergovernmental process can deliver," she said Sunday.

"They know that they can do it. They know that they need to compromise. I'm not saying it's a done deal. It's still going to be a heavy lift," she said.

Delegates are convening at a resort complex under elaborate security precautions, including naval warships a few hundred yards (meters) offshore in the Gulf of Mexico.

While they haggle over the wording, timing and dollar figures involved in any agreement, scientists and political activists at the conference will be offering the latest indications of the planet's warming. Some 250 presentations are planned on the sidelines of the negotiations.

Meteorologists are likely to report that 2010 will end up tied for the hottest year globally since records began 131 years ago. And agronomists are due to report on shifting weather patterns that are destabilizing the world's food supply and access to clean water, and that could lead to mass migrations as farmers flee drought or flood-prone regions.

As often during the three-year process, attention will focus on the United States and China, two often antagonistic nations in the climate talks.

President Barack Obama has drawn fire for his failure to win passage of domestic climate legislation and what is often described as the feeble U.S. pledge to reduce carbon emissions 17 percent by 2020 from 2005 levels.

The U.S. has insisted it will agree to binding pollution limits only if China also accepts legal limitations. China, now the world's biggest polluter but also the biggest investor in renewable energy, rejects international limits, saying it still needs to overcome widespread poverty and bears no historic responsibility for the problem.

U.S. negotiators may feel further constrained from showing flexibility toward the Chinese after the Republican swing in this month's congressional elections, which brought dozens of new legislators who doubt the seriousness of climate change.