BBC News—Reports: Negotiators from 195 countries will try to reach a deal within two weeks aimed at reducing global carbon emissions and limiting global warming to 2C (3.6F).
Leaders from 147 nations have been addressing the meeting, known as COP21.
President Obama urged negotiators to deliver a meaningful deal, because the “next generation is watching”.
He told delegates: “Climate change could define the contours of this century more than any other (challenge).
“I came here personally to say the United States not only recognises the problem but is committed to do something about it.”
He added that recent years had shown that the global economy had grown while emissions had remained flat, breaking the old arguments for inaction “that economic growth and environmental protection were in conflict”.
Russian President Vladimir Putin also addressed the conference.
During negotiations for the preceding Kyoto Protocol, Russia was the last industrialised nation to ratify the global agreement, allowing the landmark deal to come into force in 2001.
Echoing President Obama, Mr Putin said: “We have demonstrated we can ensure economic development and take care of our environment at the same time.”
In a diplomatic play on semantics, probably to highlight the differing points of view between industrialised and emerging economies, Chinese President Xi Jinping told the conference he did not see the Paris talks as a turning point nor a “finish line, but a new starting point”.
He said that climate change went beyond national borders and that it was “a shared mission for all mankind”, before reiterating China’s pledge to start cutting its emissions from a peak in 2030.