WASHINGTON - The United States Department of Defence (DOD) is the largest institutional user of petroleum, and correspondingly the largest producer of greenhouse gases (GHG) - which drive global warming - by a single organisation in the world, according to a study by the Watson Institute of International and Public Affairs at Brown University in Boston.

If it were a country, the United States military would be the world's 55th largest greenhouse gas emitter, with emissions larger than Portugal, Sweden or Denmark, says the paper authored by Professor Neta Crawford, Chair of Political Science at Boston University.

But if it could rethink military missions, it could reduce that, the paper suggests.

The largest sources of military GHG emissions are buildings and fuel, says the paper titled Pentagon Fuel Use, Climate Change, and the Costs of War.

The DOD maintains over 560,000 buildings at approximately 500 domestic and overseas military installations, which account for about 40 per cent of its greenhouse gas emissions, Dr Crawford has calculated. The rest is operations, which use up fuel at a staggering rate.

America's armed forces comprise more than two million people, 11 nuclear aircraft carriers, and thousands of ships and aircraft, operating globally.

"DOD emissions for all military operations from 2001 to 2017 are estimated to be about 766 million metric tonnes of CO2e," the paper says. CO2e means carbon dioxide equivalent; that is, all GHGs are calculated in terms of their equivalent in carbon dioxide (CO2).

Wars were a big contributor of the DOD's CO2 emissions from 2001.

"Of these military operations, it is estimated that total war-related emissions including for the "overseas contingency operations" in the major war zones of Afghanistan, Pakistan, Iraq and Syria, are more than 400 million metric tons of CO2e," the paper says.

The approximately 60,000 Humvees remaining in the US Army's fleet get between four to eight miles per gallon of diesel. In comparison, even a heavy-duty diesel pickup truck gets 14 to 15 miles per gallon.

In January 2017, two B-2B bombers and 15 aerial refuelling tankers flew more than 12,000 miles (20,000km) from Whiteman Air Force Base in Missouri to bomb Islamic State in Iraq and Syria (ISIS) targets in Libya, killing about 80 suspected militants.

Not counting the tankers' emissions, the B-2s emitted about 1,000 metric tonnes of greenhouse gases, the report says.

Ironically, a lot of the US military's operations are to protect the flow of global oil supplies - an issue once again occupying minds as Iran and the US face off, with Washington waging a mostly economic "maximum pressure" campaign to coerce Teheran into talking about giving up its nuclear ambitions, but also more recently sending an aircraft carrier group and more troops to the region.

Security strategists have long been concerned about vulnerabilities associated with dependency on Persian Gulf oil.

And US officials have said they want to reduce dependence on fuel oil, which has seen the military investing in solar energy and other renewable power sources.

The Pentagon has increased its use of renewable energy since 2009.

But all its efforts offset less than one per cent of its GHG emissions, the paper says.

And ironically, climate change - notwithstanding the Donald Trump White House downplaying it - is acknowledged by the DOD as a major security risk.

Yet the military "seems unaware of how much its efforts to protect access to Persian Gulf oil, its other military operations including war, and consumption at installations are a major driver of greenhouse gas emissions, and therefore ultimately of climate change", the paper contends.

"The Pentagon focuses their efforts on adapting to climate change and preparing for climate caused insecurity, even as they continue to ensure that Americans continue to have relatively inexpensive access to imported oil. And it spews a huge amount of greenhouse gases doing so," it says.

One potential solution may be to recognise that Persian Gulf oil is no longer vital to American interests, which would remove the need to deploy forces to protect its flow.

"With dramatic growth in renewable energy and diminishing US dependence on foreign oil, it is possible for Congress and the President to rethink our nation's military missions and reduce the amount of energy the armed forces use to protect access to Middle East oil," Dr Crawford wrote this week in (June 12) in the online Livescience blog.