Students listen as U.S. President Barack Obama speaks during the commencement ceremony for the University of California, Irvine at Angels Stadium in Anaheim, California June 14, 2014. REUTERS/Larry Downing

U.S. President Barack Obama Saturday made climate change the centerpiece of his commencement address at the University of California-Irvine, calling it "one of the most significant long-term challenges that our country and our planet faces."

Obama told graduates they already have witnessed "the 18 warmest years on record."

"We know what we see with our own eyes," he said. "Out West, firefighters brave longer, harsher wildfire seasons; states have to budget for that. Mountain towns worry about what smaller snowpacks mean for tourism. Farmers and families at the bottom worry about what it will mean for their water. In cities like Norfolk and Miami, streets now flood frequently at high tide. Shrinking icecaps have National Geographic making the biggest change in its atlas since the Soviet Union broke apart."

Obama said he wasn't making a policy speech on "one of the most significant long-term challenges that our country and our planet faces: the growing threat of a rapidly changing climate."

Obama warned lack of action will mean "we will fail one of our primary reasons for being on this world in the first place. And that is to leave the world a little bit better for the next generation."

Obama said climate change should not be a partisan issue and noted Republicans used to "lead the way on new ideas to protect our environment," citing Theodore Roosevelt's efforts to establish national parks, Richard Nixon's signing of the Clean Air Act and George H.W. Bush's remarks about human activities changing the atmosphere in unprecedented ways.

Obama said not even minor energy efficiency bills can make their way through Congress because lawmakers are thinking about politics rather than future generations.

"What's the point of public office if you're not going to use your power to help solve problems?" Obama asked.

Obama said he decided to deliver remarks at UC-Irvine after he was inundated by 10,000 postcards asking him to speak. He also joked he now knows why his personal assistant greets him every morning by shouting "Zot, zot, zot." The assistant is a graduate of the school, whose mascot is the anteater.

"Hello anteaters," Obama began his speech by saying. "That is something I never thought I'd say."