“The journey is hard. And we don’t have much time left to make it,” he said in brief remarks at a high-level climate summit convened by United Nations Secretary-General Ban Ki-moon.

Mr. Obama sought to show the U.S. resolve ahead of crucial talks in Copenhagen in December, when nations will try to reach a new global treaty to address climate change. He spoke at the start of a busy day of diplomacy ahead of the main U.N. General Assembly meetings.

“We understand the gravity of the climate threat. We are determined to act,” Mr. Obama said. “And we will meet our responsibility to future generations.”

He spoke after Mr. Ban admonished leaders to put aside differences and move more quickly.

Mr. Obama is under pressure to put political capital behind getting a serious clean-energy law at home and show that the U.S. will do its part to cut heat-trapping emissions.

The U.S. House passed a bill this summer that would set the first mandatory limits on greenhouse gases, but a Senate version appears increasingly unlikely this year.

Environmental experts warn of catastrophic changes, from rising sea levels to more drought, if industrial and developing nations cannot collectively address a warming planet.

“Our generation’s response to this challenge will be judged by history,” Mr. Obama said.

He pointed out that his administration had made the “largest-ever” investment in renewable energy. And he called on other nations — the rich and the developing countries alike — to rise to the challenge. Undertaking costly environmental clean-up work was difficult at a time when the world was trying to recover from a recession, but that it had to be done.

“Difficulty no excuse”

“All of us will face doubts and difficulties in our own capitals as we try to reach a lasting solution to the climate challenge,” Mr. Obama said. “But difficulty is no excuse for complacency.”

Tuesday’s U.N. summit and the G-20 summit in Pittsburgh later this week seek to put added pressure on rich nations to commit to greenhouse gas cuts and to pay for poorer nations to burn less coal and preserve their forests.

Mr. Obama repeatedly sought to hold everyone accountable. He said developed nations such as the U.S. have a “responsibility to lead” but rapidly-growing nations must do their part.