Aerial of the Pentagon, the Department of Defense headquarters in Arlington, Virginia, near Washington DC, with I-395 freeway on the left, and the Air Force Memorial up middle.

The U.S. Department of Defense is the largest institutional consumer of fossil fuels in the world, new research shows, creating more planet-warming greenhouse gas emissions than industrialized countries such as Portugal or Sweden.

The Pentagon, which oversees the U.S. military, released around 1.2 billion metric tons of greenhouse gases between 2001 and 2017, according to research by Brown University. The study, published Wednesday, is the first of its kind to compile such comprehensive data.

The Pentagon's emissions were "in any one year… greater than many smaller countries total greenhouse gas emissions," researchers of the study said.

The findings showed that if the Pentagon was listed as a country, its emissions would make it the world's 55th largest contributor of greenhouse gases.

The Pentagon was not immediately available to comment when contacted by CNBC on Thursday.