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How the US became the biggest military emitter and stopped everyone finding out

The climate impact of Donald Trump’s geopolitical ambitions could deepen planetary catastrophe, triggering a global military buildup that accelerates greenhouse gas emissions, a leading expert has warned.

The Pentagon – the US armed forces and Department of Defense (DoD) agencies – is the world’s largest institutional greenhouse gas emitter, accounting for at least 1% of total US emissions annually, according to analysis by Neta Crawford, co-founder of the Costs of War project at Brown University.

Over the past five decades, US military emissions have waxed and waned with its geopolitical fears and ambitions. In 2023, the Pentagon’s operations and installations generated about 48 megatons of carbon dioxide equivalent (MtCO₂e) – more planet-warming gases than emitted by entire countries including Finland, Guatemala and Syria that year.

Now, once again, the US military carbon footprint is on the cusp of rising significantly as Trump upends the old geopolitical order in his second presidency. In the first 100 days of his second term, Trump threatened military action in Panama, Greenland, Mexico and Canada, dropped bombs on Yemen and increased military sales to Israel, which has intensified its military assault on Gaza, the West Bank, Yemen and Lebanon.

Trump has also aligned the US with former adversaries including Russia, while hurling direct or thinly veiled threats at former allies including Ukraine and the entire Nato alliance. Relations with China have sunk amid Trump’s chaotic trade war.

“If Trump follows through with his threats, US military emissions will absolutely rise, and this will cause a ripple effect,” said Crawford, author of the book The Pentagon, Climate Change, and War: Charting the Rise and Fall of US Military Emissions.

“We’re already seeing lots of escalatory rhetoric, with fewer off-ramps and less commitment to resolving conflicts. The allies or former allies of the US have increased their military spending, so their emissions will go up. As adversaries and potential adversaries of the United States increase their military activity, their emissions will go up. It’s very bad news for the climate.”

The Pentagon is the largest single fossil fuel consumer in the US, already accounting for about 80% of all government emissions. In March, the defence secretary, Pete Hegseth, wrote on X: “The @DeptofDefense does not do climate change crap. We do training and warfighting.”

Trump has promised $1tn in defence spending for 2026 – which if approved by Congress would represent a 13% rise on the 2025 Pentagon budget amid unprecedented cuts to almost every other federal agency, including those that research and respond to the climate crisis. His military ambitions sit alongside orders to terminate climate research at the Pentagon and a broader assault on climate action across government, while also taking steps to boost fossil fuel extraction.

“No one spends like the US on the military and they want to spend even more. If they neglect education, health and infrastructure and their economy weakens, they will get paranoid about rivals, let’s say China, and this fear will cause even more spending. It’s an escalatory downward spiral, which often doesn’t end well – especially for the country doing the escalating,” said Crawford.

“Of course, it depends on what they do and how they do it, and the DoD may slow-roll some of this, because it is, frankly, provocative, stupid and unnecessary, but we’re going exactly the wrong way. Emissions go up in step with military spending, and this is exactly the wrong time to do this.”

In 2024, worldwide military expenditure had its steepest rise since the end of the cold war, reaching $2.7tn as wars and rising tensions drove up spending, according to a recent report by the Stockholm International Peace Research Institute.

US military spending – and emissions – are both the highest in the world, by a long way. And it is thanks to the US that states are not required to account for military emissions to the UN. In the run-up to the Kyoto protocol, the 1997 international treaty that set binding targets for greenhouse gas reductions, the Pentagon lobbied the Bill Clinton White House to push for a blanket exemption for emissions generated by military fuel use.

US pressure on its friends and foes worked, and Kyoto was celebrated as a win for American ambitions. “We took special pains … to fully protect the unique position of the United States as the world’s only superpower with global military responsibilities,” Stuart Eizenstat, undersecretary of the state department, told Congress. “The Kyoto protocol did not limit the US.”

Crawford’s research began more than a decade ago after discovering there was no data to share with her undergraduate climate change students – despite the Pentagon having warned for decades about the threat of climate change to US national security.

She found that military spending and emissions rise when the US is directly at war or preparing for war. During Ronald Reagan’s anti-communism buildup in the 1980s, spending surged and with it fuel use and emissions. After the end of the cold war, spending and emissions fell throughout the 1990s, apart from a spike during the first Gulf war. After the 9/11 attacks, emissions again surged as the US launched wars in Afghanistan and Iraq.

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From 1979 to 2023, the Pentagon generated almost 4,000 MtCO₂e – about the same as the entire 2023 emissions reported by India, a country of 1.4 billion people. Its installations and 700 bases account for about 40%, while 60% are operational emissions, resulting from fuel use in war, training and exercises with other countries, according to Crawford’s analysis.

In addition, the military industry – US-based companies manufacturing weapons, planes and other equipment for warfare – generates more than double the greenhouse gases emitted by the Pentagon each year.

Still, the known US military climate impact is probably a significant undercount. Crawford’s figures do not account for greenhouse gases generated by dropping bombs, destroying buildings and subsequent reconstruction. The additional CO₂ released into the atmosphere as a result of destroying carbon sinks such as forests, farmland and even whales killed during naval exercises are also not included, nor are those generated by burning oil fields or blowing up pipelines during conflicts.

Significantly, the ripple effect of increased militarisation and operations by allies and enemies is also not counted. For instance, the emissions generated by the armed forces and death squads of Argentina, El Salvador and Chile during the US-backed dirty wars are not accounted for, nor those from China increasing its military exercises in response to US threats. Jet fuel shipped to Israel and Ukraine can be counted if transported on a military tanker, while commercial shipments of crude used for warfare are not.

“These are important but, as yet, not well understood climate consequences of military spending and war,” Crawford said. “We’ve long underestimated the impact of mobilisation, war and reconstruction.”

Yet the Pentagon has long warned that water scarcity, sea level rise and desertification in vulnerable regions could lead to political instability and forced migration, framing climate change as a “threat multiplier” to US interests. In 1991, former president George HW Bush formally acknowledged climate change as a national security threat.

More recently, the direct threat posed by floods, wildfires and land degradation to US military capabilities has become clear. In 2018, during the first Trump administration, flood water from Hurricane Michael destroyed an air force base in Florida, and then a few months later another storm significantly damaged the Strategic Command base in Nebraska, headquarters of the nation’s nuclear arsenal.

Overall, the US military has reduced its fuel use and emissions since 1975, thanks to base closures, fewer and smaller exercises, switching from coal, and increasingly efficient vehicles and operations. But according to Crawford, this is driven by improving fighter efficiency – not the environment.

“The Pentagon has framed migration from climate change as a threat in order to get more money, which shows a lack of compassion and a failure to think ahead. If they really believed their own rhetoric, they would of course work to reduce their contribution to climate change by reducing emissions. The irony is difficult to stomach,” she said.

The military ripple effect is playing out. In response to Russia’s ground invasion of Ukraine – and more recently, Trump’s shift towards authoritarianism and anti-Ukraine, anti-Europe rhetoric – the UK, Germany and other Nato countries have increased military spending.

Here lies a fundamental problem, Crawford argues. “We can’t let Ukraine fall, but that doesn’t mean you have to mobilise all of Europe’s militaries in this way and spend this much. Russia is not the threat that they were years ago, yet the current response is based around the same old aggressive military doctrine. It’s just nonsensical and bad news for the climate.

“There’s a less expensive, less greenhouse gas-intensive way of standing up to the Russians, and that would be to support Ukraine, and directly,” said Crawford, an expert in military doctrine and peace building, and the current Montague Burton professor of international relations at the University of Oxford.

Another global military trend that could have significant climate and environmental costs is the expansion of nuclear forces. The US and UK are considering modernising their submarine fleets, while China’s expanding nuclear force includes a growing arsenal of intercontinental ballistic missiles. The production of nuclear weapons is energy- and greenhouse gas-intensive.

“Nuclear modernisation is supposed to be making us safer, more stable, but usually leads to adversaries also increasing conventional forces as well,” said Crawford. “It’s part of a broader militarisation, all of which leads to an upward spiral in emissions. The threat inflation always leads to emissions inflation.”

The total military carbon footprint is estimated at about 5.5% of global emissions – excluding greenhouse gases from conflict and war fighting. This is more than the combined contribution of civilian aviation (2%) and shipping (3%). If the world’s militaries were a country, this figure would represent the fourth largest national carbon footprint in the world – higher than Russia.

The global military buildup could be catastrophic for global heating, at a time when scientists agree that time is running out to avoid catastrophic temperature rises.

And despite growing calls for greater military accountability in climate breakdown, Crawford fears the Trump administration will no longer publish the fuel data that she relies on to calculate Pentagon emissions. In addition to withdrawing from the Paris agreement, the Trump administration has failed to report the US’s annual emissions to the UN framework convention on climate change for the first time and has erased all mention of climate change from government websites.

“Getting a handle on the scale, scope and impact of the world’s military emissions is extremely important, so that there is accountability and a path toward reduction … but the US is shutting things down,” said Crawford. “It’s becoming a black hole of information. It’s authoritarianism.”

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