Greenhouse gas emissions from US military operations dwarf the entire national output of countries including Sweden and Portugal, a study found.

If the Pentagon were a country it would be the world’s 55th largest contributor, researchers said.

The US defence department emitted about 59 million metric tons of carbon dioxide and other greenhouse gases in 2017, according to the first report to compile such comprehensive data, published by Brown University.

That compares to one million metric tons of greenhouse gas emissions recorded by the British Ministry of Defence in the same year. The US spends roughly twelve times as much as the UK on defence.

Pentagon emissions were higher than those of Portugal, which the international research project Global Carbon Atlas ranks 57th for its CO2 emissions, and Sweden, which ranks 65th worldwide.

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Using and moving troops and weapons accounted for around 70 per cent of the Pentagon’s energy consumption, mostly due to the burning of jet and diesel fuel, said Neta Crawford, author of the Cost of War study.

The defence department accounts for roughly 80 per cent of all US government energy consumption and is the single largest producer of greenhouse gas emissions in the world, the report states.

The Pentagon called climate change “a national security issue” in a January report to Congress and has launched multiple initiatives to prepare for its impact, but a request from Reuters for comments on the latest study went unanswered.

Ms Crawford said the benefits from reducing fossil fuel use would extend beyond simply reducing greenhouse gas emissions: it would also reduce the dependence of troops in the field on oil, which makes them more vulnerable to attack, and allow the US government to “decrease its military spending and reorient the economy to more economically productive activities”.

Global temperatures are on course for a 3-5C rise this century, well in excess of a global target of limiting the increase to 2C or less.

Crawford said the Pentagon had reduced its fuel consumption significantly since 2009, including by making its vehicles more efficient and moving to cleaner sources of energy at bases.

Turning carbon dixide back into coal

It could reduce them further by cutting fuel-heavy missions to the Persian Gulf to protect access to oil, which were no longer a top priority as renewable energy gained ground, she said.

“Many missions could actually be re-thought, and it would make the world safer,” she said.