Barack Obama has compared climate change deniers to people who believe the moon is "made of cheese".
Addressing thousands of graduates in California, the US president told them their generation had been "short changed by inaction" and said: "I know that manmade climate change really is happening."
Obama, who is locked in a battle with climate change sceptics in the US Congress, addressed an audience of around 8,600.
He told the audience of University of California at Irvine graduates: “Today’s Congress, though, is full of folks who stubbornly and automatically reject the scientific evidence about climate change,” he said. “They’ll tell you it’s a hoax, or a fad.”
He compared the battle to reverse climate change as similar to John F. Kennedy's pledge that Americans would go to the moon in the 1960s.
Obama said that, while many believed it impossible, few denied the moon's existence or claimed the moon was "made of cheese".
The president described this view as a “fairly serious threat to the future.” He added: “At least they have the brass to say what they actually think.”
He said: “I know that manmade 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. So, I’m not going to admit it.”
“I’m telling you all of this because I want to light a fire under you,” Obama said. “As the generation that is being shortchanged by inaction, you do not have to accept that this is the way that it has to be.”