Obama talks with fellow world leaders during a family photo for the opening day of the World Climate Change Conference

U.S. President Obama told a gathering of almost 200 world leaders they must act now on combating climate change or it will be too late Monday.

Speaking at the COP21 climate summit in Paris, Obama said he saluted the people of Paris for insisting the talks go on despite the recent terrorist attacks. "[It is] an act of defiance that proves nothing will deter us from building the future we want for our children," he said.

"Nearly 200 nations have assembled here this week. A declaration that for all the challenges we face, the growing threat of climate change could define the contours of this century more dramatically than any other."

He added that "this summer, I saw the effects of climate change firsthand in our northernmost state, Alaska, where the sea is already swallowing villages and eroding shorelines, where permafrost thaws and the tundra burns, where glaciers are melting at a pace unprecedented in modern times.

"And it was a preview of one possible future -- a glimpse of our children's fate if the climate keeps changing faster than our efforts to address it. Submerged countries. Abandoned cities. Fields that no longer grow."