Climate change is not "like" a third world war. It is World War III and "we are losing," the head of a prominent environmental activist group said Monday.

"World War III is well and truly underway," said Bill McKibben, founder of the climate activist group "And we are losing."

"We're used to war as metaphor: the war on poverty, the war on drugs, the war on cancer," he said in a op-ed published in this week's New Republic magazine. "But this is no metaphor. By most of the ways we measure wars, climate change is the real deal: Carbon and methane are seizing physical territory, sowing havoc and panic, racking up casualties and even destabilizing governments."

McKibben's group has taken the lead in advancing the "Keep it in the Ground" movement to end the nation's use of fossil fuels and transition to 100 percent renewables by mid-century.

He has been a leading critic of Republican nominee Donald Trump for characterizing climate change as a hoax and has been funding a political action fund to stop Trump from being elected in November.

McKibben also has been piling increased pressure on Republicans in Senate races to resist Trump or face disaster in November.

On Thursday, activists wearing Trump masks ran alongside Sen. Kelly Ayotte, R-N.H., who is in a tough re-election battle against the state's Democratic goveror, at a 5K road race in Manchester to confront her support for the Republican presidential nominee. "By supporting Trump, she's supporting racism, climate denial and violence. Voters need to reject Trump and reject Ayotte," the group's state director said.

McKibben's war theme could be the latest move in the climate camp's call for taking action to confront global warming. McKibben blames the Earth's warming temperatures for causing increased global instability, the refugee crisis, the spread of extremism in Africa and the rise of the Islamic State.

Many scientists blame greenhouse gas emissions from the burning of fossil fuels for causing the temperature of the planet to rise, resulting in sea-level rise, more flooding and drought. The Obama administration has called climate change as great a threat to the country as Islamic extremism and terrorism.

President Obama on Saturday called for taking more action to combat the threat, calling climate change "one of the most urgent challenges of our time." But the administration hasn't gone as far as to characterize it as World War III.

"It's not that global warming is like a world war," McKibben emphasized in his op-ed. "It is a world war."

"Its first victims, ironically, are those who have done the least to cause the crisis," he added. "But it's a world war aimed at us all. And if we lose, we will be as decimated and helpless as the losers in every conflict — except that this time, there will be no winners, and no end to the planetwide occupation that follows."

Now the question before the U.S. and others is: "Will we fight back?" he asked. "And if we do, can we actually defeat an enemy as powerful and inexorable as the laws of physics?"