Mariana Ruiz Villareal
Calculations of dinosaur biomass suggest that plant-eating sauropods like the ones pictured here in an artist's conception could have contributed enough methane to warm Earth's climate 150 million years ago.
By Alan Boyle
Some scientific findings are just too good to leave alone, even if you don't know if they can ever be confirmed: Such is the case for a study saying that plant-eating dinosaurs could have emitted enough digestive methane to warm Earth's climate 150 million years ago.
"It is known that the time of these dinosaurs was warmer than now," said David Wilkinson, an environmental scientist at Liverpool John Moores University who's the lead author of a paper on the subject appearing in the journal Current Biology. "This is explained usually by an enhanced greenhouse effect, mainly carbon dioxide. If we are correct, then methane from sauropods may have been a contributor to this greenhouse effect."
Methane is a more potent greenhouse gas than carbon dioxide, and modern-day livestock are thought to be responsible for about a quarter of the methane released in the United States. Some say that the belches and flatulence of cattle, pigs and sheep are a significant contributor to the warming effect caused by greenhouse-gas emissions. So why wouldn't it have been the same in the age of giant plant-eating dinosaurs, when global biomass density was at least several times what it is today?
"All vertebrates that feed on leaves, etc., use microbes to help digest these, and usually give off methane," Wilkinson told me in an email. "This includes both mammals and reptiles. ... Although details vary within groups, everything around today does this, so the assumption is [that] larger herbivorous dinosaurs did as well."
He and his colleagues ran the numbers, using what they saw as conservative estimates for the total amount of dinosaur biomass and methane production rates per kilogram of body mass. They came up with a figure of 520 million tons of methane emitted per year, which is more than total modern-day methane emissions from all sources, natural and industrial. The current estimate for total methane emission is around 500 million tons a year, with 50 to 100 milllion tons of that coming from ruminant animals such as cows and goats, Wilkinson said.
"Our work certainly suggests biology and climate were involved in a feedback loop," he said.
Biologists have found that most of the modern-day methane emissions from livestock come from belching rather than flatulence. Was it the same for dinosaurs? "We have no particular view which end of the sauropod the methane came out," Wilkinson told me. "Could be either or both."
Chemical analysis of ancient marine sediments has found that greenhouse-gas levels went through a huge rise 201 million years ago, around the time of a mass extinction that set the stage for the rise of the dinosaurs. Scientists suspect that the atmospheric methane levels at that time were pumped up by a massive release of methane from the seafloor.?Such evidence suggests that plant-eating dinosaurs weren't responsible for starting the upswing in Mesozoic methane. But did they help preserve the methane-rich atmosphere and toasty temperatures until they were killed off by an asteroid strike?
Wilkinson noted that his paper was titled "Could Methane Produced by Sauropod Dinosaurs Have Helped Drive Mesozoic Climate Warmth?" ??not "Did Methane Produced by Dinosaurs Help Drive Climate Warmth."
"What our simple calculations show is that, yes, it could. It's a real possibility. But we don't show that it did happen," he said. "That would require much more work, and indeed it may be impossible to completely prove this without a time machine."
Extra credit: A dozen years ago, the BBC quoted a Chinese news report that quoted an unnamed French scientist as saying the dinosaurs were wiped out 65 million years ago not by an asteroid, but by their own flatulence. This hypothesis proposed that the methane emissions from the giant beasts became so great that the climate changed, the vegetation withered and the dinosaurs all starved. But that's just too silly to consider. Or is it?
More about methane:
In addition to Wilkinson, authors of the Current Biology paper include the University of London's Euan G. Nisbet and the University of Glasgow's Graeme D. Ruxton.
Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.
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