By NEIL MacFARQUHAR
UNITED NATIONS — Some 100 heads of state gathered at the United Nations on Tuesday for an unprecedented daylong conference on combating climate change, which President Obama and other leaders acknowledged was a difficult issue to agree on but was crucial for the future health of the planet.
The world “cannot allow the old divisions that have characterized the climate debate for so many years to block our progress,” Mr. Obama told the leaders gathered at the General Assembly hall, acknowledging that forging any kind of consensus will come slowly.
“It is a journey that will require each of us to persevere through setback and fight for every inch of progress, even when it comes in fits and starts,” he said. He said the world had been too slow to recognize the gathering danger from rising temperatures. “It is true of my own country as well; we recognize that.”
Negotiators trying to hammer out a deal to cut global emissions by December in Copenhagen have largely stalled, and the United Nations organizers are hoping that gathering the leaders will give the talks new political momentum.
China, followed by the United States, are the largest emitters, accounting for about 40 percent split evenly between them. Speaking shortly after Mr. Obama, President Hu Jintao of China committed his country to reducing carbon emissions, but he qualified his commitment, saying that the size of the cuts would depend on the growth of his nation’s economy, rather than an absolute number.
Mr. Hu said China would develop nuclear power and increase the use of renewable energy to 15 percent of the power used in China. He said China would work to increase forests by nearly 99 million acres. But he also emphasized his country’s position that the world should address the needs of developing countries by providing financing and technology to help them reduce emissions.
“We should make our endeavor on climate change a win-win for both developed and developing countries,” he said.
The conference on Tuesday, which is not a negotiating session but is designed to push toward a strategy, is focused on four outstanding hurdles.
Industrialized nations, while agreeing on cutting emissions in the long term — by 2050 — have failed to agree on a crucial midterm target for carbon emissions cuts by 2020. They have pledged to go roughly halfway toward meeting the ambitious target set by the Intergovernmental Panel on Climate Change — a 25 percent to 40 percent reduction from 1990 levels by 2020 — which environmental advocates say is not enough.
Developing powerhouses like China and India have agreed on the need to trim emissions, but they reject mandatory limits and demand financial and technical support in exchange.
Efforts to reach any kind of consensus around the issue of aid for the poorest countries to adapt to the impact of climate change are faltering. Finally, there is no agreement on what institutions would verify that targets are being met and supervise the financial and emissions targets.
The organizers at the United Nations said they had never been involved in such a high-level summit meeting where the outcome was not predetermined. The main hurdle is coming up with a plan over the next decade that will keep the temperature rise to about 2 degrees Celsius, or 3.6 degrees Fahrenheit, above pre-industrial levels. Even countries like India, which largely blames the developed world for the problem, but has announced a package of cuts, admit that looking ahead to 2050 is not good enough.
“It is the height of dishonesty to have a target for 2050 because none of us will be around to be held accountable,” Jairam Ramesh, the environment minister of India, told a news conference late Monday.
Some blocs of nations have their own targets. The small island states of the Pacific and the Caribbean want to limit the temperature rise to 1.5 degrees because they fear being inundated by the sea rise that climate change could bring. Those states, along with many in Africa, are demanding billions of dollars in aid to assuage the damage they are already suffering.
The French environment minister, Jean- Louis Borloo, told reporters that developed countries would probably support direct aid for projects that counter the affects of global warming, but he rejected the idea of “damages.”
“They have to show what it will pay for,” he said.
During the speeches on Tuesday, the change in language coming from America was stark. Gone was the Bush administration’s questioning about whether global warming is caused by mankind. Mr. Obama was quick to take responsibility on behalf of said mankind.
“John F. Kennedy once observed that ‘our problems are man-made, therefore they may be solved by man,’ ” Mr. Obama said. “It is true that for too many years mankind has been slow to respond to or even recognize the magnitude of the climate threat. It is true of my own country as well; we recognize that.”
He said he was committed to the United States making its largest-ever investment in renewable energy, new standards for reducing pollution from vehicles and making clean energy profitable, among other initiatives. He said developing nations must also provide financial and technical assistance to help the rest adapt to the impact of climate change and pursue low-carbon development.
“We understand the gravity of the climate threat,” Mr. Obama said, but he noted that the push for change comes in the midst of a global recession. “And so all of us will face doubts and difficulties in our own capitals as we try to reach a lasting solution to the climate challenge.”
In his remarks, Ban Ki-moon, the United Nations secretary general, also called for unity in addressing the problem of climate change, appealing to the leaders to set aside their national interests and think about the future of the globe.
“Instead of demanding concessions from others, let us ask how we can contribute to the greater good,” he said, describing the talks as moving at “glacial” speed. “The world’s glaciers are now melting faster than human progress to protect them — and us.”
The United States ultimately rejected the last global agreement negotiated at 1997 in Kyoto, Japan, because its pollution caps did not apply to China and other developing countries. Its own new carbon reduction law is stuck in Congress behind health care. Negotiators conceded that the world will probably have to give the Americans more time, but they hope to hear from Mr. Obama at least what his goals are across a variety of environmental issues, like cars.
Various ministers and other officials said that if major powers like China, Brazil and Indonesia all make conciliatory gestures Tuesday at the United Nations, that would likely help Mr. Obama overcome domestic opposition.