Dinosaurs may have helped warm ancient Earth via their own natural gaseous emissions, a new study says.
Like modern-day ruminants, giant plant-eating dinosaurs likely had microbes in their guts that gave off large amounts of methane—a potent greenhouse gas even more effective at trapping heat than carbon dioxide. (Read about the greenhouse gas effect.)
Today cows, goats, sheep, giraffes, and other ruminants contribute to global warming by releasing as much as 50 million to 100 million metric tons of methane per year—a significant chunk of the 500 million to 600 million metric tons emitted annually, mostly due to human activity, according to the World Meteorological Organization.
The cud-chewing animals have large forestomachs packed with microbes that break down coarse plant material. The main byproduct of the process is methane—and it's got to go somewhere.
"Methane can come out of either end of an animal. For example, with cows it's mainly the front," said study co-author Dave Wilkinson, an ecologist at Liverpool John Moores University in England.
As for how these approximately 20-ton beasts—the largest of all known dinosaurs—expelled their methane, Wilkinson said, "we don't have any strong view on what happened with sauropods."
Sauropods as Huge Methane Sources
To estimate how much methane sauropods emitted, the scientists guessed that there were roughly ten sauropods per square kilometer (0.4 square kilometer) of vegetated land.
The team's analyses of modern ruminants suggest a sauropod might give off about 4.2 pounds (1.9 kilograms) of methane daily. A U.S. cow, by comparison, might give off a daily average of 0.4 to 0.7 pound (0.2 to 0.3 kilogram).
Assuming there were 29 million square miles (75 million square kilometers) of vegetated land when sauropods lived, their global methane production might have reached a whopping 520 million metric tons annually.
"When it first occurred to us to wonder what the methane output of sauropods was, I would have guessed rather less than our maths eventually predicted," said Wilkinson, whose study appears May 8 in the journal Current Biology.
"Clearly there are large uncertainties attached to our estimates, but they suggest that the amount of methane given off by sauropods may have been approximately equal to all modern global methane sources, both natural and manmade."
(Read blog post: "The Life of Sauropod Dinosaurs.")
Methane Study Based on Educated Guesses
Wilkinson and colleagues admit they made a number of educated guesses in their analyses, and digestive physiologist Marcus Clauss agrees.
Clauss—who wasn't part of the study—noted, for example, that the team's calculations of methane emissions based on body weight were made from measurements of modern reptiles and mammals, not birds, which are the closest living relatives to dinosaurs.
The problem is that it's still unknown how much methane plant-eating birds release.
"If we would find that herbivorous birds produce less methane than similar-size herbivorous mammals, then ... the whole calculation might have to be redone," said Clauss, of the University of Zurich.
Regardless of the calculations, fossil finds make it clear that sauropods lived in a much warmer world than we do. "People sometimes describe it as a super-greenhouse," study co-author Wilkinson said.
To better link dinosaur emissions to the warming, the University of Zurich's Clauss suggests that climate modelers and paleontologists begin looking for signs of an uptick in methane during the sauropod heyday.
After all, study co-author Wilkinson added, then as today, "life can play a major role in shaping the physics and chemistry of its environment."