Around 12,000 years ago, the Earth spun into The Big Freeze, a (geologically) brief cold snap known as the Younger Dryas event. Glaciers returned to parts of the Northern Hemisphere and humans who were around then probably shivered quite a bit.

The Clovis people in North American, the first paleo-Indian inhabitants that made distinctive bone and ivory tools, took a population nosedive. What caused The Big Freeze?

The prevailing theory is a shutdown of the ocean conveyor belt caused by a rapid influx of fresh water from the melting of an immense glacial lake — Lake Agassiz — the used to cover much of Canada. Comets are also speculated culprits. But a New Mexico team of researchers is looking into an unusual contributing factor — a steep decline in large animal flatulence.

It’s no secret that humans probably caused the extinction of megafauna species in the New World, which used to be richer than modern day Africa. Within a thousand years of human arrival, species like mammoth, camelids (the ancestors of camels) and giant ground sloths were destined as museum pieces.

In a paper published in Nature Geoscience this month, Methane Emissions from Extinct Megafauna, Felisa Smith of the biology department at the University of New Mexico and colleagues, attribute a steep drop in atmosphere methane levels to the disappearance of these behemoths.

If domestic livestock contributes about 20 percent of global methane emissions today, then mammoths and the like must have had powerful bowels. Since Pleistocene methane levels were considerably lower than today, these megaherbivores might have had a larger influence on global methane levels.

The researchers went about estimating how much gas an 8-ton herbivore would emit. They examined 114 herbivores that went extinct at the end of the Pleistocene epoch, adjusting for body size and other traits to compute overall methane production from these beasts.

Their calculation suggested that the Pleistocene extinction dropped methane levels by about 9.6 teragrams (or megatons), making the mass extinction responsible for somewhere between a 12.5 to 100 percent of overall methane decline. That’s a big range, but the researchers go on to point out a unique attribute of global methane decline at that time. It happened quite quickly, two to four times faster than any other time interval, “which suggests that novel mechanisms may be responsible.”

Novel, indeed. The overall drop in methane, a potent greenhouse gas, during that epoch equated to temperatures 16 to 22 degrees Fahrenheit cooler. The researchers write:

The attribution and magnitude of the Younger Dryas temperature shift, however, remain unclear. Nevertheless, our calculations suggest that decreased methane emissions caused by the extinction of the New World megafauna could have played a role in the Younger Dryas cooling event.

They go on to say that the megafauna extinction may have been “the earliest catastrophic event attributed to human activities.” It may even be the first time humans altered the climate.

-Alison Hawkes

Tags: Clovis, megafauna, paleoclimate, Pleistocene, Younger Dryas