Barack Obama says denying climate change is like arguing the moon is made of cheese.
The US president issued a call to action to tens of thousands of graduates of the University of California who had gathered at Angel Stadium.
He told them Congress "is full of folks who stubbornly and automatically reject the scientific evidence" and say climate change is a hoax or fad.
"Many others duck the question by saying, 'Hey, I'm not a scientist'," Obama said in remarks prepared for delivery.
"Let me translate: what that means is, 'I accept that man made climate change is real, but if I admit it, I'll be run out of town by a radical fringe that thinks climate science is a liberal plot'."
Obama's address to about 8000 graduates from the Orange County campus comes two weeks after he announced a contentious plan to dramatically cut pollution from power plants.
Obama also used the address to announce a $US1 billion ($A1.08 billion) competitive fund for communities who have experienced natural disasters to rebuild and prepare for the impact of extreme weather.
He described a political system consumed by "small things" but said Americans should be determined to do "big things" like addressing climate change, despite the type of opposition he faces from Congress.
"No matter what you do in life, you will run up against a stubborn status quo and people determined to stymie your best efforts, who say you can't do something and shouldn't bother trying. I've got some experience with this myself," Obama said.
"I've got to admit, though, it's pretty rare that you'll encounter someone who says the problem you're trying to solve doesn't even exist," he continued.
"When President Kennedy set us on a course for the moon, I'm sure some made a serious case that it wouldn't be worth it. But I don't remember anyone ignoring science. I don't remember anyone saying the moon wasn't real, or that it was made of cheese."
Obama said today's young dreams are fed a steady diet of cynicism but argued they have a right to be optimistic.
Obama's spending the rest of the Father's Day weekend on holiday at the Rancho Mirage home of White House decorator Michael Smith and his partner, US Ambassador to Spain James Costos.
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