(Richard Drew / AP Photo)

Even as he battles ISIS and Ebola, President Obama today declared climate change the threat that will "define the contours of this century more than any other."

"Climate is changing faster than our efforts to address it. The alarm bells keep ringing. Our citizens keep marching. We cannot pretend we don't hear them," Obama told delegates at the U.N. Climate Summit in New York, referencing the massive demonstrations over the past few days.

"We have to tackle this global threat before it's too late."

Obama cast himself as a global champion of efforts to reduce greenhouse gas emissions, highlighting the administration's efforts to boost fuel efficiency of American cars and trucks, curb carbon pollution from coal-fired US power plants and planned phase-out the use of HFC coolant used in refrigerators and air conditioners.

The White House also unveiled an Executive Order the president will sign requiring government agencies to take climate change into account when funding international development work. The administration also said it was providing new tools - including improved NASA satellite data, National Oceanic and Atmospheric Administration meteorological forecasting tools and other measures - to help poorer, developing communities manage the impact of climate change.

Obama warned the U.S. cannot solve the crisis alone.

"We will do our part. We will help developing nation's do theirs. But we can only succeed combating climate change if we are joined by every nation, developed and developing alike. Nobody gets a pass," he said.

"None of this is without controversy. In each of our countries, there are interests that will be resistant to action. And in each country there is a suspicion that if we act and other countries don't then we will be at an economic disadvantage," he said. "But we have to lead."

Overshadowing the climate summit was the list of big names that chose not attend, including the leaders of some of the world's biggest polluters - China, India, and Russia.

Today's meeting was not part of formal negotiations for a global climate treaty to be signed in Paris next year, but it's been billed as a highly symbolic show aimed at pressuring visiting heads of state to make commitments to help curb greenhouse gas emissions.