The next time you’re on a flight worrying about destroying the planet, rest easy knowing that at least you’re not in a fighter jet. The airline industry is responsible for 2.5 percent of global CO2 emissions, but the world’s militaries are responsible for more than double that, at 5.5 percent. 

When nations boost military budgets, they also boost their carbon emissions. With a bump of $157 billion, thanks to the budget the Trump administration passed earlier this month, the United States now spends $1 trillion each year on defense. That’s more than three times as much as China, the next highest spender, as well as the entire European Union. If combined, the world’s armed forces would have the fourth highest carbon footprint, behind India, the U.S., and China. 

Yet it’s been maddeningly difficult for researchers to monitor the emissions of militaries, which aren’t required to report these things. “There’s a guessing game involved,” said Nick Buxton, who has coauthored reports on military emissions from the Transnational Institute, an international research and advocacy group. “One of the overwhelming calls for everyone working in the sector is just for more open and transparent data, so we can come up with some reliable figures.”

To that end, using what data the Department of Energy has made publicly available between 1975 to 2022, researchers have calculated that if the U.S. consistently decreased military spending — by even a little — instead of increasing it, it’d be saving as much energy as Delaware and Slovenia use in a year. A decrease of less than 7 percent each year over about a decade would theoretically reduce energy consumption from about 640 trillion to 394 trillion British thermal units (a measurement of heat energy produced from burning fuels).

The study gives observers not just a better idea of how much carbon the American military is spewing, but also how effective it would be to reduce its funding. “We realize that the feasibility of military spending reductions taking place anytime soon within the U.S. context is probably quite questionable, to put it mildly,” said Andrew Jorgenson, professor of sociology and founding director of the University of British Columbia’s Climate and Society Lab and coauthor of the study, which was published June 2 in the journal PLOS Climate. “But it does highlight that it is a possible pathway to decarbonization and climate mitigation, just with very modest reductions in military spending.”

The researchers note that between 2010 and 2019, the Department of Defense’s emissions were over 636 million metric tons of atmosphere-warming emissions. (The DOD did not respond to a request to comment for this story.) And that’s a conservative and necessarily incomplete estimate, Jorgenson said. Fuel use can give researchers a general idea of how much carbon the armed forces are directly sending into the atmosphere, but there are also all kinds of indirect emissions that come with operating a military. Vegetables, for instance, took energy to grow and ship to bases, to say nothing of all the other supplies flowing around a military’s supply chain: bullets, blankets, boots. 

“If anything, our findings are then perhaps undercounting and underestimating the actual scope of the U.S. military’s contribution to energy consumption and carbon emissions and climate change,” Jorgenson said. “That’s a speculative statement — I just want to be clear about that.”

All these variables not only make it difficult for researchers to accurately determine the climate costs of war — governments themselves can be in the dark too. “Militaries are decades behind in their ability to even understand their emission sources and where they’re coming from,” said Ellie Kinney, military emissions campaigner at the nonprofit Conflict and Environment Observatory. “There is this lag compared to other industries, because no one’s asked them to.”

Calculations get even more complicated when a military actually goes to war. More jet flights require more fuel, and even missiles produce their own emissions. The resulting fires in conflict zones, like the ones that have been devastating Ukraine’s forests, release still more carbon into the atmosphere. While the U.S. spends an outsized amount of money on its armed forces, other nations, particularly those involved in active wars, seem intent to catch up. Russia is now spending a third of its federal budget on defense as its invasion of Ukraine drags on. Last year, Israel’s military spending jumped by 65 percent to $46.5 billion as the country assaulted Gaza. 

Last month, at President Trump’s urging, NATO allies committed to investing 3.5 percent of their gross domestic product each year on defense, and a further 1.5 percent on domestic security like new infrastructure, by 2035. That combined 5 percent is more than double their previous agreement to spend 2 percent of GDP. And on Monday, NATO secretary general Mark Rutte joined Trump in the Oval Office to announce a deal in which “billions of dollars’ worth of military equipment” will be purchased from the U.S. and delivered to Ukraine to support its defense against Russia. 

According to a Transnational Institute report, if every NATO state actually reaches its new military spending goal, by 2030 the alliance’s annual military carbon footprint would be 2.3 billion metric tons CO2 equivalent. (The group published the report prior to the formalization of the June agreement, hence the discrepancy of using 2030 in their modeling instead of 2035.) That’s nearly 700 million metric tons extra than if 2024 levels of military spending were sustained until that time. 

“We’re moving to a world which is readying itself constantly for war, which often makes war much more inevitable,” Buxton said. “And when war happens, emissions just skyrocket.”

All this additional military investment can create a feedback loop, Buxton and Kinney warn. Military leaders in the U.S. and elsewhere recognize climate change as a “threat multiplier,” meaning that it exacerbates existing hazards and conflicts. But with more investment in defense comes more emissions, and more warming, and more threats, which encourages more investment in armed forces. That also means less money for investing in renewable energy and adaptation measures: The richest nations are spending 30 times more on their militaries than on climate finance for the world’s most vulnerable countries.

“An escalation beyond control feels like the situation that we’re heading into,” Kinney said. “This is obviously deeply concerning from a broader security perspective, but really concerning from a climate perspective.”