The United States creates more planet-warming greenhouse gas emissions through its defense operations alone than industrialized countries such as Sweden and Portugal, researchers said on Wednesday.
The Pentagon, which oversees the U.S. military, released about 59 million metric tons of carbon dioxide and other greenhouse gases in 2017, according to the first study to compile such comprehensive data, published by Brown University.
The Pentagon's emissions were "in any one year ... greater than many smaller countries' greenhouse gas emissions," the study said.
If it were a country, its emissions would make it the world's 55th largest contributor, said Neta Crawford, the study's author and a political scientist at Boston University.
"There is a lot of room here to reduce emissions," Crawford said.
Request for comments to the Pentagon went unanswered.
Using and moving troops and weapons accounted for about 70% of its energy consumption, mostly due to the burning of jet and diesel fuel, Crawford said.
It dwarfed yearly emissions by Sweden, which the international research project the Global Carbon Atlas ranks 65th worldwide for its CO2 emissions.
Pentagon emissions were higher than those of Portugal, ranked 57th by the Global Carbon Atlas, said Crawford.
China is the world's largest emitter of carbon dioxide, the main gas responsible for climate change, followed by the United 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.
Global temperatures are on course for a 3-5 degrees Celsius (5.4-9.0 degrees Fahrenheit) rise this century, far overshooting a global target of limiting the increase to 2C or less, the U.N. World Meteorological Organization said in November.
Four degrees Celsius of warming would increase more than five times the influence of climate on conflict, according to a study published in Nature magazine on Wednesday.
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.
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.
