Obama "climate change is defining threat of this century".

President Barack Obama declared climate change to be the defining threat of this century as he began a historic visit to the far northwestern state of Alaska.

Speaking at a summit on Arctic environmental issues, President Obama said the world is "not acting fast enough" to address climate change.

"On this issue of all issues, there is such a thing as being too late,'' Obama said. "And that moment is almost upon us."

"Human activity is disrupting the climate, in many ways, faster than we previously thought. The science is stark. It is sharpening. It proves that this once distance threat is now very much in the present."

"We've got to come together around an ambitious framework to protect the one planet that we've got while we still can."