UNITED NATIONS (AP) — President Barack Obama and Chinese President Hu Jintao each vowed urgent action Tuesday to cool an overheating planet, even as prospects dimmed for a full treaty by the end of the year.

The world’s two biggest greenhouse-gas polluting nations were the focus at the U.N.’s unprecedented daylong climate change summit, which drew more than 50 presidents and 35 prime ministers, along with many environment ministers and at least one prince.

U.N. Secretary-General Ban Ki-moon opened the gathering with an appeal to leaders to set aside national interests.

Think about the future of the globe — and a rebuke for their foot-dragging thus far.

“The climate negotiations are proceeding at glacial speed.

The world’s glaciers are now melting faster than human progress to protect them — and us,” the U.N. chief said.

Failure to reach a new international pact on climate change “would be morally inexcusable, economically shortsighted and politically unwise,” Ban warned.

“The science demands it. The world economy needs it.”

Tuesday’s U.N. gathering and the G-20 summit in Pittsburgh this week are seen as an attempt to pressure rich nations to commit to a global climate treaty at Copenhagen, Denmark, in December.

And to pay for poorer nations to burn less coal and preserve their forests.

With a mere 76 days to go before the pivotal conference, it appeared an interim agreement might be the most that could be expected in December, leaving difficult details for later talks.

“We are on the path to failure if we continue to act as we have,” French President Nicolas Sarkozy cautioned.

Much attention was fixed on Obama’s first U.N. speech, in which he pledged the United States is “determined to act.”

“The threat from climate change is serious, it is urgent, and it is growing,” Obama said, after receiving loud applause.

“And the time we have to reverse this tide is running out.”

But while Obama campaigned for the presidency vowing to push through stringent cuts in U.S. emissions, he has run up against stiff resistance among Republicans.

The Senate most likely won’t have written climate legislation until after the Copenhagen meeting.

By comparison, Hu runs a command economy and was unencumbered by political opposition.

He outlined an ambitious program that included plans to plant enough forest to cover about 150,000 square miles.

An area the size of Montana — and generate 15 percent of its energy needs from renewable sources within a decade.

He said the communist nation would also take steps to improve energy efficiency and reduce “by a notable margin” its growth rate of carbon pollution as measured against economic growth — though he did not give any specific numerical targets.

“At stake in the fight against climate change are the common interests of the entire world,” Hu said.

“Out of a sense of responsibility to its own people and people across the world.China fully appreciates the importance and urgency of addressing climate change.”

Still, China and other developing nations “should not … be asked to take on obligations that go beyond their development stage,” Hu said.

China and India, the world’s fifth-biggest greenhouse gas emitter, both want to link emissions to their growth in gross domestic product, meaning they still may increase emissions even if they take fundamental steps to curb them in the long run.

Experts were watching China closely because it has in the past largely ignored global efforts to diminish emissions.

The United States, under former President George W. Bush’s administration, stayed away from international commitments citing inaction by major developing nations like China and India.

China and the U.S. each account for about 20 percent of all the world’s greenhouse gas pollution, created when coal, natural gas or oil are burned.

The European Union is next, generating 14 percent, followed by Russia and India, which each account for 5 percent.

In his speech Tuesday, Obama detailed the steps his administration is taking to reduce America’s carbon footprint.

Including doubling the generating capacity from wind and other renewable resources in three years.

Launching offshore wind energy projects and spending billions to capture carbon pollution from coal plants.

Obama previously had announced a voluntary target of returning to 1990 levels of greenhouse emissions by 2020, but action awaits Congress passing legislation to make those goals law.

By contrast, the EU has urged other rich countries to match its pledge to cut emissions by 20 percent from 1990 levels by 2020, and has said it would cut up to 30 percent if other rich countries follow suit.

On Tuesday, Japan’s new prime minister, Yukio Hatoyama, whose nation generates more than 4 percent of the world’s greenhouse gases, pledged his nation would seek a 25 percent cut in greenhouse gas emissions from 1990 levels by 2020.

• John Heilprin