Did megafauna extinction cool the planet?
The rapid decline of mammoths and other megafauna after humans spread
across the New World may explain a bone-chilling plunge in global
temperatures some 12,800 years ago, researchers reported Sunday.
Tasmanian Megafauna. Source: google |
The 100-odd species of grass-eating giants that once crowded the
North American landscape released huge quantities of methane — from both
ends of their digestive tracks.
As a heat-trapping greenhouse gas, methane is 30 times more potent
than carbon dioxide (CO2).
It was not enough to trigger runaway global warming. But when all
that gaseous output suddenly tapered off, it caused or at least
contributed to a prolonged freeze known as the Younger Dryas cold event,
they argue.
If so, the “Anthropocene epoch” — the era of major human impacts on
Earth’s climate system — began not with the industrial revolution in the
1800s, but the large-scale influx of two-legged predators to the
Americas more than 13,000 years earlier.
Calculations by a trio or researchers led by Felisa Smith of the
University of New Mexico, published in Nature, show how all the pieces
of this previously unsolved puzzle might fit together.
Extrapolating from data on cows and other modern-day ruminants, the
scientists estimated the total methane output of pre-historic megafauna
at nearly 10 trillion grams per year.
At the same time, ice-core samples reveal that an abrupt drop in
atmospheric methane levels of 180 parts per billion by volume (ppbv)
coincides with both, the virtual extinction of these gas-gushing
herbivores and the onset of the deep chill that followed.
Greenland ice cores from other periods show that a reduction in
methane levels of 20 ppbv corresponds to a reduction in temperature of
roughly 1.0 degrees Celsius (1.8 degrees Fahrenheit). That would add up
to a decrease of 9.0 to 12.0 C (16 to 22 F), a near-perfect match with
the Younger Dryas cold snap. “We find that the loss of megafauna could
explain 12.5 to 100 percent of the atmospheric decrease in methane
observed,” the researchers said.
The theory is bolstered by the fact that the plunge in concentration
was two-to-four times faster than the five other largest methane drops
during the last 500,000 years, suggesting a unique confluence of
factors. “The megafaunal extinction is the earliest catastrophic event
attributed to human activity,” the study concluded.
“We thus propose that the onset of the ‘Anthropocene’ [epoch] should
be recalibrated to 13,400 years before present, coincident with the
first large-scale migrations of humans into the Americas.”
AFP |