"I've come here personally, as the leader of the world's largest economy and the second-largest emitter," Obama said, "to say that the United States of America not only recognizes our role in creating this problem, we embrace our responsibility to do something about it."

In a speech interrupted by repeated beeps warning that he had exceeded his time limit, Obama said in Le Bourget that the climate conference represented an important turning point in world history because the leaders attending the meeting now recognize the urgency of the problem.

"No nation — large or small, wealthy or poor — is immune," he said.

The greatest threat to reaching a binding climate accord may be a loose coalition of developing nations, led by India, who argue that they should not be asked to limit their economic growth as a way of fixing a problem that was largely created by the others, and Obama conceded that point.

"We know the truth that many nations have contributed little to climate change but will be the first to feel its most destructive effects," he said.

He promised money to help the poorest nations transition to economies that depend less on burning fossil fuels, but he said a delay was not acceptable.

"For I believe, in the words of Dr. Martin Luther King, Jr., that there is such a thing as being too late," Obama said. "And when it comes to climate change, that hour is almost upon us."

Obama also repeated an argument, lampooned by some Republicans, that the climate conference was a fitting response to the terrorist attacks that cost the lives of 130 people in and around Paris on Nov. 13.

"What greater rejection of those who would tear down our world than marshaling our best efforts to save it," he said.

About 150 world leaders were expected to gather at the opening of the talks in a heavily guarded convention center as a show of encouragement and support for efforts to forge a historic agreement to jointly curb greenhouse-gas emissions, in an effort to stave off the worst effects of climate change.

Obama has staked much of his legacy on ensuring success here, spending much of the past year courting the leaders of China, India and other major emitters in hopes they would finally agree to slow their rapidly rising use of coal and other carbon-intensive fuels.

President Francois Hollande of France greeted Obama just eight hours after the two paid a surprise late-night visit to the Bataclan, the concert hall where dozens of people were killed on Nov. 13, as part of a coordinated series of attacks in and around Paris.

At the brief visit last night, Obama, Hollande and Anne Hidalgo, the mayor of Paris, each laid a white rose before standing in silence in front of the building for several minutes.

Shortly after his arrival, Obama met with President Xi Jinping of China in a meeting of the leaders of the world's two largest carbon-polluting countries.

Citing climate change as "a huge challenge," Xi said it was "very important for China and the United States to be firmly committed to the right direction of building a new model of major country relations," including by "partnering with each other to help the climate conference deliver its expected targets."

The Breakthrough Energy Coalition, a group of business and philanthropy leaders led by the Microsoft founder Bill Gates who have a combined total of $350 billion in private wealth, have pledged to invest in moving clean-energy technologies from laboratories to the marketplace.

It is hoped that the pledge, along with one by 19 countries, including the United States, to double their investments in energy technologies to $20 billion by 2020 will help convince poor countries that they will be given significant help in transitioning to a new economic model that relies less on the use of carbon.In one of many such expected announcements, the State Department pledged $248 million to help the world's least-developed countries move toward a future that is less reliant on carbon, but there were already signs, however, that the world's poor are not yet mollified.

Prime Minister Narendra Modi of India said on the opening day of the summit meeting that poor nations have the right to burn carbon to grow their economies.

"Justice demands that, with what little carbon we can still safely burn, developing countries are allowed to grow," he wrote in a column published in The Financial Times. "The lifestyles of a few must not crowd out opportunities for the many still on the first steps of the development ladder."

Citing statistics showing that carbon pollution last year was equal to the year before while economic growth continued, Obama rejected arguments that cleaning up the world's air would be too costly or lead to poorer lifestyles.

"We have proved that strong economic growth and a safer environment no longer have to conflict with one another," he said.