At the U.N. now, delegates are watching a film -- made by young people from around the world -- about climate change.

"That so many of us are here today is a recognition that the threat from climate change is serious, it is urgent, and it is growing," the president begins.

"The security and stability of each nation and all peoples -- our prosperity, our health, our safety -- are in jeopardy," Obama says. "And the time we have to reverse this tide is running out."

The president says his predecessor's policies no longer apply:

"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.   We recognize that.   But this is a new day.   It is a new era."

"We understand the gravity of the climate threat," Obama says of the United States. "We are determined to act.   And we will meet our responsibility to future generations."

Addressing the other leaders, Obama says that:

Difficulty is no excuse for complacency.   Unease is no excuse for inaction.   And we must not allow the perfect to become the enemy of progress.   Each of us must do what we can when we can to grow our economies without endangering our planet  -- and we must all do it together."