“It was really great to have the vision, but with just 70 days left to Copenhagen, it is time to put some substance on the table,” said Steve Howard, the founder of the Climate Group, an international organization pushing for a climate change agreement. “The two most important countries on this issue are being guarded in their positions.”

Image President Obama delivered remarks to the leaders gathered in the General Assembly hall on Tuesday. Credit... Doug Mills/The New York Times

Those two countries  the United States and China  account for more than 40 percent of the carbon emissions, roughly divided between both.

Speaking at the green marble lectern of the General Assembly chamber, President Obama told the audience of some 100 heads of state and government that “unease is no excuse for inaction.”

China’s president, Hu Jintao, spoke of reducing the “carbon intensity” of his fast-growing economy, or cutting emissions as a percentage of future economic output, by a “notable” margin that he did not specify.

Mr. Obama acknowledged that the United States once played down the issue, but now recognized its gravity. The world “cannot allow the old divisions that have characterized the climate debate for so many years to block our progress,” he said, adding that forging consensus would come slowly. “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.”