UNITED NATIONS - Chinese President Hu Jintao announced new goals on Tuesday to slow the growth in his country's carbon dioxide emissions while U.S. President Barack Obama warned time was short to act on global warming.

The two men, whose countries represent some 40 percent of the world's greenhouse gas emissions, sought at a climate change summit to reinvigorate stalled U.N. talks to reach an international deal to fight global warming.

Hu laid out a new climate change plan for China. Obama outlined his administration's efforts so far this year but did not offer new proposals.

Hu, who will meet with Obama directly later on Tuesday, said China would vigorously develop renewable and nuclear energy and promised emissions would grow slower than economic growth in future.

"We will endeavor to cut carbon dioxide emissions per unit of GDP by a notable margin by 2020 from the 2005 level," Hu said, according to a prepared text of his remarks.

The pledge, while short of an absolute cap on output, was seen as an attempt to counter critics, especially in Washington, who say Beijing is doing too little.

Obama said the United States had done more over the eight months of his presidency to reduce carbon pollution than at any time in history and urged all nations to act together.

"Our generation's response to this challenge will be judged by history, for if we fail to meet it -- boldly, swiftly, and together -- we risk consigning future generations to an irreversible catastrophe," Obama said.

"The time we have to reverse this tide is running out."

Momentum?

Activists hoped the United States and China would inject momentum, 2-1/2 months before 190 nations gather in Copenhagen aiming to complete a deal to slow climate change.

But environmentalists were upset at the lack of specifics in Obama's first presidential speech to the U.N.

"We are really very, very disappointed about what Obama has said," said Thomas Henningsen, climate coordinator for Greenpeace International.

"It is really more of a step back than a step forward," he said, adding Obama had not spelled out any concrete steps compared to what Japan and other nations were prepared to do.

Europeans, who had welcomed Obama's commitment to fight climate change as a positive development after his predecessor George W. Bush, are growing impatient.

A climate change bill mandating cuts in U.S. emissions is unlikely to be passed by the U.S. Senate by December.

French President Nicolas Sarkozy proposed that heads of state from major economies meet in November ahead of the Copenhagen talks.

U.N. Secretary-General Ban Ki-moon, who called Tuesday's meeting, said talks were moving too slowly.

"Failure to reach broad agreement in Copenhagen would be morally inexcusable, economically short-sighted and politically unwise," Ban said.

Talks leading to the December 7-18 meeting have put developed and developing countries at odds over how to distribute emissions curbs. Poorer nations are pressing richer ones to contribute hundreds of billions of dollars a year to help them cope with rising temperatures.

Obama's legislative initiatives to reduce U.S. emissions have been overshadowed by his push for healthcare reform.