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The United States must reaffirm its commitment to combating climate change, President Barack Obama said Saturday, challenging college graduates to step up to skeptics with scientific data.

He even compared denying climate change to saying the moon is made of cheese.

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“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,” Obama said in his University of California, Irvine, commencement address in Anaheim, California. “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, in some of his sharpest words yet against climate change critics, announced plans for a roughly $1 billion competitive fund to prepare for the effects of environmental impacts such as rising sea levels and extreme weather. Obama's address to about 8,000 graduates follows his previous announcement of a plan to decrease pollution from power plants.

President Barack Obama speaks during the commencement ceremony for the University of California Irvine at Angel Stadium in Anaheim, California, USA, 14 June 2014. Obama's remarks to some 7,000 graduates comes 50 years to the week that former President Lyndon B. Johnson dedicated the grounds where UC Irvine would be built.PAUL BUCK / EPA

UC Irvine has a long history of supporting environmental research. A university professor was part of a group of scientists that won the 1995 Nobel Prize in Chemistry for their research on how pollutants damage the ozone layer.

For the president, climate change has become a cause that may end up defining his final years in office.

If action isn't taken, Obama said, “we will fail one of our primary reasons for being on this world in the first place: That is to leave the world a little better for the next generation.”

He cited a “longer, harsher” wildfire season on the West Coast, droughts affecting California farmers and frequent flooding in Norfolk, Virginia, and Miami as climate change examples. Additionally, he added, “the 18 warmest years on record have all happened since you graduates were born.”

These anecdotes served as ammunition in his attacks on opponents in Congress who have challenged his environmental initiatives. Obama also pointed to Republican predecessors who fought for environmental causes, including Theodore Roosevelt, who created the first national parks, and George H.W. Bush, who acknowledged the threat of climate change.

Obama lamented that “nothing’s happening” today, placing the blame for that squarely on “a radical fringe that thinks climate science is a liberal plot.”

For the graduates in attendance, Obama began and ended his speech with a call to ignore the cynicism they encounter as they enter the real world to tackle major problems.

“When life gets you down or somebody tells you that you can’t do something,” the president said, “then do something better.”

— Jacob Passy