In a speech to the graduating class of cadets at the Coast Guard Academy here, Obama noted the dangers of rising sea levels that threaten health and safety in coastal areas and pointed to volatile new storm systems, droughts and wildfires that endanger the rest of the world.
"This is not just a problem for countries on the coast or for certain regions of the world," he said. "Climate change constitutes a serious threat to global security, an immediate risk to our national security, and make no mistake: It will impact how our military defends our country.
"We need to act," Obama said, "and we need to act now."
The call to action comes as Obama pursues a global agreement to reduce greenhouse gas emissions. The White House hopes to close an ambitious deal with sweeping goals at a December summit in Paris.
Crucial to that push is a good-faith display by the U.S. to cut its own emissions. Obama wants the country to cut greenhouse gases dramatically over the next decade.
In his address to the new class of Coast Guard officers, Obama made a sales pitch for his climate agenda: that national and global security depend on it.
It's a strategy that he has employed recently in an effort to sell his trade and economic policies, as he argues that U.S. security depends in part on building new and strong commercial and financial ties to the rest of the world.
The argument is based on the world's shared problem of climate change and related security concerns.
Drought and famine have made basic resources like food and water scarce, leading to instability around the world, he said. Violent storms force people from their homes.
The stakes, Obama argued Wednesday, are high.
Denying climate change or refusing to deal with it undermines American readiness, he said.
"We cannot and must not ignore a peril that can affect generations," Obama said, noting that some in Washington don't believe in taking action to deal with climate change.
Scientists know it's happening, Obama said, and "the Coast Guard knows it's happening."
Climate change poses a threat to the readiness of American forces, he said, ticking off floods at bases in Norfolk, Va., damage from thawing permafrost at military facilities in Alaska and the possibility of extensive droughts and wildfires threatening training areas in the West.
For its part, the Pentagon is assessing the vulnerability of more than 7,000 military bases, installations and other facilities around the world as a result of climate change.
The departments of Defense and Homeland Security have been studying how to deal with the security implications of melting sea ice in the Arctic.
If the military is taking steps to deal with climate change, said one member of Congress, it should inspire others to do the same.
“Our military takes the world as it is, not as ideologues would hope it to be,” said Rep.
Obama urged the officers to embrace the challenge before them.
"You are part of the first generation of officers to begin your service in a world where the effects of climate change are so clearly upon us," he told the graduating cadets. "Climate change will shape how every one of our services plan, operate, train, equip, and protect their infrastructure, today and for the long term."
The president's remarks were greeted with little applause in the Coast Guard stadium or from climate change skeptics in Congress.
“It’s no wonder that our military personnel's trust in their commander in chief is at an all-time low," said Sen.