Rising seas and thawing permafrost caused by warmer global temperatures threaten US military bases and will change the way the US armed services defend the country, President Barack Obama said last night.
In a commencement address at the United States Coast Guard Academy, Mr Obama underscored the risks to national security posed by climate change.
The issue is one of his top priorities for action in his remaining 19 months in office.
"The threat of a changing climate cuts to the very core of your service," Obama told the 224 graduating cadets, who studied the impacts of global warming as part of their curriculum.
"It will affect everything that you do in your careers," Mr Obama added, noting that sea levels were expected to rise by 30-120cm in the coming century.
The Pentagon is assessing the vulnerability to climate change of its 7,000 bases, installations and facilities, many of which are on the coast, the White House said.
Mr Obama highlighted damage to the navy and air bases at Norfolk, Virginia, from increasing floods.
Coast guard operations in Alaska have already begun dealing with changes, Admiral Paul Zukunft, commandant of the Coast Guard, told cadets.
Mr Obama pointed to severe drought in Nigeria as creating instability that was exploited by militant group Boko Haram, and said crop failures and high food prices in Syria fuelled unrest that led to civil war.
"We need to act, and we need to act now," he said.
"Denying it or refusing to deal with it endangers our national security," he added. "It undermines the readiness of our forces."
Mr Obama has been trying to build support for an international agreement to cut greenhouse gas emissions. Global talks on the deal are slated for Paris in December.
This summer, his administration will finalise regulations to cut carbon emissions from power plants, a step that has been opposed by Republicans who control Congress.
Likely Republican presidential contender Jeb Bush said of Mr Obama's comments: "If the president thinks this is the gravest threat to our national security, it seems like he would say: 'Let's expand LNG (liquefied natural gas) as fast as we can to get it into the hands of higher carbon-intense economies like China and other places.'"