When CNN hosted a climate-change town hall for Democratic presidential candidates last week, former vice president Joe Biden brought up one of his favorite campaign topics: Barack Obama. "The first thing that happened when President Obama and I were elected, we went over to what they call the Tank, in the Pentagon, sat down and got the briefing on the greatest danger facing our security. Know what they told us it was? The military? Climate change. Climate change. Climate change is the single greatest concern for war and disruption in the world, short of a nuclear exchange."

He's right. A recent Department of Defense report found that climate change "will affect the Department of Defense's ability to defend the nation and poses immediate risks to U.S. national security." That includes risks to the physical safety of service members. Since 2008, 17 individual troops at U.S. bases have died from heat exhaustion during training exercises, according to a Pentagon report. In 2018, 2,792 active-duty service members suffered heat stroke, a 60 percent increase over the previous decade. Not coincidentally, the past five years have been the hottest in human history, largely a result of human-driven climate change. Earlier this year the Department of Defense found that two-thirds of the military's operationally critical facilities are threatened by climate change, including flooding, droughts, and wildfires.

But the Department of Defense isn't some passive victim in the coming climate catastrophe. While climate change threatens the U.S. military as much as it threatens everything else, the U.S. military is one of the single biggest climate-change contributors in the world.

According to the Costs of War, an ongoing project from the Watson Institute for International and Public Affairs at Brown University, since the global war on terror began in 2001, the U.S. military has produced 1.2 billion metric tons of greenhouse-gas emissions, or as much as 257 million passenger cars annually, roughly as many registered vehicles as there are in the entire U.S. That's a higher annual output than whole countries like Morocco, Sweden, and Switzerland. The total emissions from war-related activity in Afghanistan, Iraq, Pakistan, and Syria is estimated at more than 400 million metric tons of carbon dioxide alone.

It's difficult to get a full picture of the military's fuel consumption and greenhouse-gas emissions. Even though the U.S. never officially ratified the Kyoto Protocol, a 1992 international agreement between world powers to fight climate change, it pushed to exempt the military from the environmental standards laid out in the agreement. That includes having to document and report on carbon dioxide emissions. The 2015 Paris climate accords closed that loophole, but since Donald Trump has pulled the U.S. out, the military once again has carte blanche to burn all the fuel that it wants.

In a report out earlier this summer, Costs of War broke down where all that fuel is going. About 30 percent of the energy use goes to infrastructure, and the Department of Defense spent an estimated $3.5 billion in heating, cooling, and electricity costs in 2017 alone. The remaining 70 percent is "operational," meaning the actual fighting and all the hardware it takes to support that, including fuel for tremendously fuel-inefficient vehicles, planes, and ships.

The Department of Defense has been taking steps to "green" some of its bases, though that's less about carbon footprints and more about freeing those bases from relying on costly fuel convoys that are prone to attack. Similarly, gas-electric hybrid battleships need less fuel and therefore fewer refueling stops, so they're strategically preferable. But even those reductions don't go far enough. For 2017 alone, the U.S. military bought 269,230 barrels of oil a day and spent more than $8.6 billion on fuel for the Air Force, the Army, the Navy, and the Marines, and the military remains the single largest consumer of fossil fuels on the planet, according to the Union of Concerned Scientists.