During a weekend trip to Yosemite National Park, President Obama warned that climate change is not only a threat, but a reality.

"The biggest challenge we're going to face in protecting this place and places like it is climate change. Make no mistake, climate change is no longer just a threat, it's already reality," Mr. Obama said in brief remarks at the park's Sentinel Bridge in California.

The president said at Yosemite, meadows are drying out, bird ranges are drifting northward and "Yosemite's largest glacier, once a mile wide, is now almost gone." He also mentioned longer, dangerous, more expensive wildfire seasons that are raging across the West.

Climate change, he said, could at some point "even threaten icons like the Statue of Liberty at Ellis Island."

"We can't treat these things as something that we deal with later, that it's somebody else's problem," Mr. Obama said.

The president visited the park with first lady Michelle Obama and daughters Malia and Sasha. They also visited Carlsbad Caverns National Park on Friday in New Mexico.

Earlier in the day on Saturday, the president and first lady stopped at a park facility where they surprised a group of fourth-grade students.

The first lady asked the kids, "How are you going to scare bears away?"

As all the kids started to scream, Mr. Obama responded, "Oh, I would get out of here!"