Plenty of environmentalists think eating beef is a bad idea. For one thing, it’s inefficient: it takes up to 16 pounds of grain, according to some estimates, to produce one pound of meat, and the grain would feed a lot more people. For another, it’s bad for the atmosphere: the bacteria that help cattle digest all that grain generate huge volumes of methane, a heat-trapping greenhouse gas more potent than carbon dioxide (for the record, and contrary to popular wisdom, the methane escapes from the cow’s mouth in belches, not from the other end).
David Wilkinson, an ecologist at Liverpool John Moores University, in the UK, was well aware of this beef with beef, and a thought occurred to him. During the Mesozoic era, from 250 million to 65 million years ago, giant, long-tailed, long-necked, plant-eating sauropod dinosaurs (Brontosaurus, Diplodocus and Brachiosaurus, to name just three) would likely have generated methane too, and lots of it. “We asked ourselves just how much,” Wilkinson said in an interview, “and spent lots of time fiddling around to try and come up with an estimate.”
The answer he and his colleagues came up with, as described in a new paper just published in Current Biology: a whole lot. The Mesozoic was up to 18°F warmer than the Earth is today, with no ice at the poles and sea level much higher — and dinosaur gas may have part of the reason. “It wouldn’t have all been down to sauropods,” Wilkinson said, “but if our numbers are even vaguely realistic, they could have contributed.”
Those numbers are huge. In the modern world, about 550 million metric tons of methane enter the atmosphere each year, with more than half of that coming from human sources, including domesticated livestock, landfills and leakage from natural-gas drilling. In the Mesozoic, it turns out, sauropod emissions alone could have added up to a whopping 520 million tons. “That really surprised me,” Wilkinson said. “I might have guessed it was marginally important, but I wasn’t expecting a number that big.”
Admittedly, the calculation was based on a number of assumptions. “To start with,” Wilkinson said, “we had to estimate the population density of sauropods. Several people have attempted to put numbers to this, and it’s clearly very difficult.” In the end, they tried to be reasonably conservative, going with a figure of a few large animals per square kilometer.
Then they had to calculate how much methane each sauropod would have emitted. The scientists used equations derived from the methane emissions per body weight of modern animals, but no animal comes even close to the 100 tons of the biggest sauropods (an elephant weighs in at a mere 10). “We had to project way, way past what the equations actually say,” Wilkinson explained, “which is something I tell my students you really shouldn’t do. We wouldn’t have, if there were a better alternative.”
As for which end of the dinosaurs the gas escaped from, Wilkinson is agnostic. “I don’t have a particularly strong opinion,” he said. “It could be either end.” For so-called fore-gut fermenters, such as cows, the methane comes out in belches; for hind-gut fermenters, including antelopes, it escapes out the rear. The latter have lower emissions, in general, so to be conservative, Wilkinson said, “we used the equation for hind-gut fermenters. So yes, you can say we assumed the dinosaurs farted.”
Once again, Wilkinson emphasized, the methane figures he and his colleagues came up with are merely an estimate. If any of the assumptions going into the model are incorrect, the number would change. They tried to be conservative at every step, however, so the most likely changes would be in the upward direction. “But even if the sauropods’ contribution was half as big as we think,” Wilkinson said, “it would still have been important.”