NEW LONDON, Conn. — President Obama used a commencement address on Wednesday at the Coast Guard Academy to cast his push for urgent action to combat climate change as a national security imperative, saying that the warming of the planet poses an “immediate risk” to the United States.

The speech was part of an effort by Mr. Obama to make a multipronged case for his ambitious climate-change agenda, which he has identified as a top priority for the remainder of his time in office and as a central element of his legacy. Instead of promoting his plan strictly in environmental terms, he has pitched it as beneficial for the economy, necessary to protect public health and vital to the nation’s security.

“I am here today to say that 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,” Mr. Obama told about 4,200 people on an athletic field overlooking the water here, including about 200 graduates in crisp white dress uniforms. “And so we need to act, and we need to act now.”

Mr. Obama repeated arguments he cites often to promote his climate change effort, including a litany of grim facts and figures about rising temperatures, swelling seas and vanishing sea ice, dismissing skeptics of the phenomenon or those who refuse to act on it as guilty of “negligence” and “dereliction of duty.”