UNITED NATIONS: China laid down a significant plan for curbing greenhouse gases on Tuesday, outlining ambitious goals of planting enough forest to cover an area the size of Norway and generating 15 percent of its energy needs from renewable sources within a decade.

Chinese President Hu Jintao also promised at the opening of the United Nations climate summit that the communist nation would take "determined and practical steps" to boost its nuclear energy, improve energy efficiency and reduce "by a notable margin" the growth rate of its carbon pollution as measured against economic growth.

Experts were watching the Chinese closely because it had in the past largely ignored global efforts to diminish emissions. The goals Hu outlined also were held in contrast to the United States, where the Senate has yet to take up climate legislation and likely will not have produced a new law by the time world leaders gather this December in Copenhagen, Denmark, to negotiate a treaty to replace the 1997 Kyoto pact.

"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."

Much attention also was fixed on U.S. President Barack Obama's first U.N. speech, where he said 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."

China's more specific ambitions topped the lofty speechmaking as U.N. Secretary-General Ban Ki-moon called on presidents, prime ministers and other leaders "to accelerate the pace of negotiations and to strengthen the ambition of what is on offer" for a new global climate pact at Copenhagen, Denmark in December.

"Failure to reach broad agreement in Copenhagen would be morally inexcusable, economically shortsighted and politically unwise," Ban warned. "The science demands it. The world economy needs it."

Actor Djimon Hounsou of Benin helped open the summit by quoting late astronomer Carl Sagan and showing his "Pale Blue Dot" photo of Earth taken in 1990 from Voyager 1 within the larger cosmos.

Tuesday's U.N. summit and the G-20 summit in Pittsburgh later this week seek to add pressure on rich nations to commit to a deal in Copenhagen for mandatory greenhouse gas cuts starting in 2013, and to pay for poorer nations to burn less coal and preserve their forests.

But China and some other major fast-developing economies will not agree to binding greenhouse-gas cuts. Developing nations "should not ... be asked to take on obligations that go beyond their development stage," Hu said.

Leaders said that with only about three weeks left for negotiations the likelihood was fast-growing for something less than a full-blown treaty at Copenhagen.

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

Obama said the U.S. is 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 has announced a target of returning to 1990 levels of greenhouse emissions by 2020, but action awaits Congress passing legislation to make those goals the law of the land.

The United States, under former President George W. Bush's administration, stayed away from international commitments citing inaction by 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.

The EU is urging 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.

But the Paris-based International Energy Agency expects global carbon emissions will drop by 2.6 percent this year, the biggest such decrease in more than 40 years, because of the world's recession that is slowing industrial activity, according to projections first reported Monday by The Financial Times.

Even with the economic slowdown, the dangers of climate-altering heat waves, droughts, melting glaciers, loss of the Greenland ice sheet and other calamities are fast approaching, said Rajendra Pachauri, chairman of the U.N. Intergovernmental Panel on Climate Change that shared the Nobel Peace Prize with former vice president Al Gore in 2007.

"The science leaves us with no room for inaction now," he said.

Pachauri said major greenhouse gas cuts must be made by 2015 to avoid many of these dangers.

Japanese's prime minister, whose nation generates more than 4 percent of the world's greenhouse gases, said his nation will seek a 25 percent cut in greenhouse gas emissions from 1990 levels by 2020.

"I will now seek to unite our efforts to address current and future climate change with due consideration of the role of science," said Yukio Hatoyama, six days after taking office. "I am resolved to exercise the political will require to deliver on this promise."

Hatoyama also said Japan is ready to contribute money and technical help for poorer countries to cut emissions. He called for a "fair and effective international framework" that allows all countries to make cuts.