"The journey is hard. And we don't have much time left to make it," Obama said in brief remarks at a high-level climate summit convened by U.N. Secretary-General Ban Ki-moon.
Obama sought to show 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 at the United Nations that also was to include a three-way meeting with Israeli and Palestinian leaders in an effort to nudge forward the Mideast peace process.
"We understand the gravity of the climate threat. We are determined to act," Obama said. "And we will meet our responsibility to future generations."
He spoke after Ban admonished leaders to put aside differences and move more quickly on global warming.
Obama is under pressure to put political capital behind getting a serious clean-energy law at home and show that the U.S., an economic giant, 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.
In his first presidential visit to the United Nations, Obama also sought to show a clear break from former President George W. Bush without referring to his predecessor by name. Bush's critics said he didn't take climate change seriously enough.
"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," Obama said. "We recognize that."
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," Obama said.
Obama said his administration has made the "largest-ever" American investment in renewable energy. And he called on other nations - the rich and the developing countries alike - to rise to the challenge. He said undertaking costly environmental clean up work is difficult at a time when the world is trying to recover from a recession, but that it has to be done.
"All of us will face doubts and difficulties in our own capitals as we try to reach a lasting solution to the climate challenge," 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.
Obama sought repeatedly to hold everyone accountable. He said developed nations such as the United States have a "responsibility to lead" but rapidly-growing nations must do their part.
As for Obama's Mideast diplomacy efforts, there were no expectations of a breakthrough from Tuesday's three-way meeting. But it was seen as a crucial step for the president nonetheless.
After seeing Israeli Prime Minister Benjamin Netanyahu and Palestinian President Mahmoud Abbas separately, Obama was bringing the two together for the first Israeli-Palestinian meeting since Netanyahu took office in March.
Even if little more than a photo opportunity, it will probably be the most-watched portion of a marathon day of international diplomacy for Obama, a 12-hour sprint through many high-profile global problems and disputes.
The Israeli-Palestinian sit-down wasn't announced until Saturday and comes with the two sides still far apart on what it would take to resume peace talks that broke off in 2008.
U.S. envoy George Mitchell failed last week to bridge the gap between the two sides on the issue of Jewish settlements in Palestinian territory, putting the long hoped-for three-way meeting in doubt. Obama has asked Israel to freeze all settlement construction, a condition for Abbas to resume negotiations. But Israel has only committed to a partial halt.
Still, the sides decided to go ahead, even though Obama is considered unlikely to resolve the settlement showdown and announce a relaunching of peace talks.
"We have no grand expectations out of one meeting," White House spokesman Robert Gibbs said.
One reason to have the meeting is the need to get momentum going.
"The U.S. wants to and the U.S. needs to negotiate in public," said Jon Alterman, a senior fellow in Middle East policy at the Center for Strategic and International Studies and a former State Department official in President George W. Bush's first term. "There's a perceived need for the U.S. to visibly be involved in making progress on Arab-Israeli issues."
Obama's agenda on Tuesday also included meeting Chinese President Hu Jintao at a fraught time in the Washington-Beijing relationship; playing luncheon host, as America's first black president, to sub-Saharan African leaders for talks on boosting opportunities for young people in their poverty-stricken nations; delivering key speeches to former President Bill Clinton's Global Initiative and to a U.N. heads-of-state session on the stalled issue of climate change; and ending the day with a U.N.-sponsored leaders dinner.