A paper in Nature Geoscience published early this month was much derided by the usual suspects in the pseudoskeptic community. Contrary to what many critics of “Methane emissions from extinct megafauna” claim, the research does not lead to the conclusion that humans are solely responsible for a global cooling event known as the Younger Dryas, which saw a brief reversal in the warming trend that brought the last ice age to an end. But it does remind us of just how interconnected are all the elements of the planetary ecosystem, and how dangerous it is to tinker with one of them.
The authors, Felisa A. Smith of the University of New Mexico, Scott M. Elliott of Los Alamos National Laboratory and S. Kathleen Lyons of the Smithsonian Institution, tried to estimate how much methane, a potent greenhouse gas, was being produced by the herbivorous mammoths, camels and giant sloths that used to inhabit the Americas before humans came on the scene about 13,000 years ago.
Then they looked at atmospheric methane levels covering the time around the extinction of all those species, as recorded in ice cores. And they concluded that “that the loss of megafauna could explain 12.5 to 100% of the atmospheric decrease in methane observed at the onset of the Younger Dryas.”
There’s a large margin for error, here. The authors estimate that the now-extinct herbivores were responsible for about 9.6 teragrams of methane, but that number could be as low as 2.3 and as high as 25.5. Still, it makes a fair bit of sense. If, as has been speculated, the megafauna of the period in the Americas enjoyed higher densities than modern African counterparts, then their disappearance would have significant consequences for atmospheric methane levels.
This is not the same as saying humans caused the Younger Dryas. It’s not even saying that falling methane levels, brought on by whatever ultimate cause, cooled the planet. As noted in the paper, the Younger Dryas is a singular event, and one that might have been the result of a rare combination of factors. Here’s what can be said, according to Smith et al:
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.
We are not the first to suggest that human-mediated activities influenced the planet prior to the industrial age. Although still controversial, the megafaunal extinction is the earliest catastrophic event attributed to human activities.
As far as policy implications go, the paper is relatively modest:
…we propose that the onset of the ‘Anthropocene’ should be recalibrated to 13,400 years before present, coincident with the first large-scale migrations of humans into the Americas.
Which is perhaps a little premature, given just how controversial is the notion that humans killed off the American megafauna. As I wrote a couple of years ago, just because we’ve screwed up the climate doesn’t mean we’re responsible for everything. But this kind of research should give us pause. It could be that prehistoric humans contributed to a unique change in the Earth’s climate, by indirectly fiddling with levels of a single greenhouse gas. Today, by comparison, we’re directly and single-handedly responsible for changes in a several GHGs, and making those changes at an unprecedented pace.