Giant dinosaurs that roamed the Earth millions of years ago may have warmed the planet with the gas they produced from eating leafy plants, British scientists said on Monday.

Much like modern cows that emit a significant amount of methane in their digestive process, the 20,000 kilogram (44,000 pound) sauropods contributed the same way, and likely more, to the warming climate, said the study in the US journal Current Biology.

The climate during the Mesozoic Era, which spanned 250 million years ago to about 65 million years ago, was believed to be hotter than it is today.

With bulky bodies and long necks that allowed sauropods like the Brontosaurus to graze on grasses or high in the treetops, these creatures were plentiful 150 million years ago, ranging from a few individuals to a few dozen per square kilometer.

In all they may have emitted a total of 520 million tons (520 Tg) per year of global methane emissions, a level that is comparable to the total given off today by animals and industrial activities, the study authors said.

Currently 80 million metric tons of methane are produced annually by global livestock, making up about 28 percent of global methane emissions from human-related activities according to the US Environmental Protection Agency.

"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," said Dave Wilkinson of Liverpool John Moores University.

"Indeed, our calculations suggest that these dinosaurs could have produced more methane than all modern sources -- both natural and man-made -- put together."

About 150 years ago, before modern industry took hold, methane emissions were about 200 Tg per year, less than half that of the Mesozoic Era.