Sauropods, known for their enormous size and long necks, had methane-producing microbes in their guts to aid digestion by fermenting plant food, as cows do today, they said.
"A simple mathematical model suggests that the microbes living in sauropod dinosaurs may have produced enough methane to have an important effect on the Mesozoic climate" 150 million years ago, Dave Wilkinson of Liverpool John Moores University said.
Wilkinson and study co-author Graeme Ruxton from the University of St. Andrews detailed their research in the journal Current Biology.
"Indeed, our calculations suggest that these dinosaurs could have produced more methane than all modern sources -- both natural and man-made -- put together," Wilkinson said.
Studies of methane production in a range of modern animals has provided equations that predict methane production from animals of different sizes.
A medium-sized sauropod weighed approximately 40,000 pounds, researchers said, and global methane emissions from sauropods could have been 520 million tons per year, comparable to total modern methane emissions.
By comparison, modern ruminant animals, including cows, goats, giraffes and others, produce methane emissions of 50 million to 100 million tons per year, they said.