UNITED NATIONS (AP) - President Barack Obama says the United States was slow to recognize the magnitude of climate change, but that Washington is moving swiftly to catch up.

Addressing the U.N. Climate Change Summit in New York, Obama said his administration has made the "largest-ever" American investment in renewable energy. And he called on other nations - the rich and the developing countries alike - to rise to the challenge.

Obama acknowledged that pursuing costly environmental clean up is difficult at a time when the world is trying to recover from a recession, but said it has to be done. The president said "we did not come here today to celebrate progress," saying the conference dramatizes the need for even more work.