US President Barack Obama says global warming poses economic and security risks that have to be tackled immediately, but insists the climate problem could be solved.
If global warming continues, "then before long we are going to have to devote more and more of our economic and military resources not to growing opportunity for our people but to adapting to the various consequences of a changing planet," Obama said.
"Climate change is a massive problem, it's a generational problem," Obama said, speaking on the sidelines of the November 30-December 11 UN conference north of Paris.
"And yet despite all that, the main message I've got is, I actually think we're going to solve this thing."
Turning to plans for a UN climate pact, Obama said it was crucial for the deal to incorporate regular and transparent reviews of progress in curbing greenhouse-gas emissions.
"If we have these periodic reviews built in, what I believe will happen is that by sending that signal to researchers and scientists and investors and entrepreneurs and venture funds, we'll actually start hitting the targets faster than we expected," he said.
"We can be even more ambitious. The thing about human ingenuity is, it responds when it gets a strong signal of what needs to be done."
Obama accepted it was difficult to create political momentum for action, particularly in the United States, where many opposition Republicans reject the scientific consensus on the risks from carbon emissions.
"Climate change ... is a problem that by definition is just about the hardest thing for any political system to absorb.
"People don't feel it immediately so there's not a lot of constituency pressure on politicians to do something about it right away. It kind of creeps up on you."
But he said he was "optimistic".
"I think we're going to solve it. I think the issue is just going to be the pace and how much damage is done before we are able to fully apply the brakes."