US president Barack Obama has said denying climate change is like arguing that the moon is made of cheese.
Mr Obama issued a call for action on global warming as he addressed graduates of the University of California, Irvine.
He told the tens of thousands gathered at Angel Stadium that Congress "is full of folks who stubbornly and automatically reject the scientific evidence" and say climate change is a hoax or a fad, while others avoid the question.
"They say, 'Hey, look, I'm not a scientist,'" Mr Obama said.
"I'll translate that for you: what that really means is: 'I know that man-made climate change really is happening but if I admit it, I'll be run out of town by a radical fringe that thinks climate science is a liberal plot.'"
Mr Obama's address to about 8,000 graduates at the Orange County campus comes two weeks after he announced a contentious plan to dramatically cut pollution from power plants.
The US president also used the address to announce a one billion dollar (£589 million) 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.
He added: "There's going to be a stubborn status quo and people determined to stymie your efforts to bring about change. There are going to be people who say you can't do something. There are going to be people who say you shouldn't bother trying. I've got some experience with this myself.
"It's pretty rare that you'll encounter somebody who says the problem you're trying to solve simply doesn't exist. When president Kennedy set us on a course to the moon, there were a number of people who made a serious case that it wouldn't be worth it.
"But nobody ignored the science. I don't remember anybody saying the moon wasn't there, or that it was made of cheese."
The president said today's young dreamers are fed a steady diet of cynicism, but argued they have a right to be optimistic.
"Consider this: since the time most of you graduated from high school, fewer Americans are at war.
"More have health insurance. More are graduating from college. Our businesses have added more than nine million new jobs. And the number of states where you're free to marry who you love has more than doubled. That's just some of the progress you've seen."
While in Orange County, the president also raised money for the Democratic National Committee at a closed-door fundraiser at the Laguna Beach home of Getty oil heiress Anne Earhart. About 25 people paid up to 32,400 dollars (£19,105) to participate in a round-table discussion with the president.
He is spending the rest of the Father's Day weekend on vacation at the Rancho Mirage home of White House decorator Michael Smith and his partner, US Ambassador to Spain James Costos.