President Barack Obama says the United States, as the world's second biggest emitter of greenhouse gases, recognises its role in creating the problem of global warming and has embraced its responsibility to help fix it.
Addressing more than 150 world leaders on Monday at the start of two weeks of UN talks in Paris, Obama said the staging of the summit in the French capital was a rebuke to Islamist militants who killed 130 people there on November 13.
"As the leader of the world's largest economy and the second largest emitter ... the United States of America not only recognises our role in creating this problem, we embrace our responsibility to do something about it," Obama said.
China is the world's top emitter.
Obama set out the possible consequences of what he called "one possible future" of unchecked global warming.
"Submerged countries, abandoned cities, fields that no longer grow. Political disruptions that trigger new conflicts, leaving more floods of desperate people seeking sanctuary in nations not their own," he said.
Obama said the growing threat of climate change could define the contours of this century more dramatically than any other, adding 14 of the last 15 warmest years on record have occurred since 2000 - and 2015 was on pace to be the warmest.
Not only was inaction likely to lead to economic damage, action to limit emissions and shift to lower carbon energy had been proved to be compatible with economic growth, he said.
"We have broken the old arguments for inaction. We've proved that strong economic growth and a safer environment no longer have to conflict with one another. They can work in concert with one another," he said.
The UN summit will see two weeks of bargaining in a bid to reach a deal aimed at steering the global economy away from its dependence on fossil fuels.
The United States and China have in the past resisted signing up to a global deal to address climate change. This time, they have pledged to work together, although both nations have issues with the UN process and are expected to struggle to accept a legally binding global pact.
Chinese President Xi Jinping, in his address to the UN conference on Monday, said it had to acknowledge the difference between the developed and developing world, allowing poor countries less stringent targets.
Obama praised France for going ahead with the conference despite the attacks. "What greater rejection of those who will tear down our world than marshalling our best efforts to save it?" he said.
