Abrupt changes in the Earth's climate, such as the human-induced temperature spike of the past century, are anomalies in the longer cycles of planetary warming and cooling. Like any anomaly, however, these abrupt changes tend to repeat themselves over a long enough time scale.
One such event began around 12,900 years ago, during the decline of the last ice age. Known among scientists as the Younger Dryas stadial, or more colloquially as the "Big Freeze," this geologically brief event saw a rapid return to colder conditions for roughly 1,300 years, followed by a continuation of the previous warming trend.
What caused this "mini-ice age"? The question is still debated among scientists, though a common theory holds that it was brought about by the collapse of the North American Ice Sheet. However, new evidence from Dartmouth College supports an even more dramatic root cause: A meteor, large enough to destabilize the ice sheet and send the world into a thousand-year climate tailspin, may have struck Quebec just before the Younger Dryas.
"It's a compelling theory," said Mukul Sharma, an isotope geochemist at Dartmouth and co-author of the study. "What is clear is that an impact did take place, and that it closely preceded the Younger Dryas. What the mechanism was, what specifically happened to the climate -- these are trickier questions we still need to answer."
Previous research had identified the presence of glassy spherules -- pellets of molten rock, potentially produced through a meteor's impact -- found in the Younger Dryas boundary layers at sites in New Jersey and Pennsylvania. Using electron microscopes, Sharma and his colleagues inspected cross-sections of the spherules to determine their geological origins. Their mineral profile, they found, tied them to a region of southern Quebec.
"We have for the first time narrowed down the region where a Younger Dryas impact did take place," said Sharma. "It appears that the impact would have taken place near the southernmost margin of the retreating ice sheet."
Early humans adapted to changing conditions
While the causes of the Big Freeze are up for debate, its impacts on plants, animals and early human life are better understood. In many northern latitudes, where the effects of the Younger Dryas were most pronounced, forests gave way to hardier tundra. Accompanying droughts hastened desertification in parts of Africa and Asia, seeding the atmosphere with dust. Many large animal species that made up the diet of paleo-Indian cultures went extinct in North America.
In one way, at least, the Younger Dryas may have hastened the development of human civilization. Around this time, the Natufian people of the Levant abandoned their previously nomadic lifestyles in favor of a sedentary existence and possibly cereal cultivation, paving the way for the agricultural societies that would dominate for the next 12,000 years.
Some researchers have theorized that this switch was catalyzed by environmental stressors. While cereals had long been a staple of the Natufian diet, the droughts brought on by the Younger Dryas would have challenged the plant, replacing it with scrub brush. Understanding that their principal food source was under threat, the Natufians began planting cereals and clearing brush, establishing a rudimentary form of farming.
Following the Younger Dryas, the planet continued its interrupted warming trend, allowing the first true agricultural societies to take root in the Fertile Crescent.
Reprinted from ClimateWire with permission from Environment & Energy Publishing, LLC. 202-628-6500.E&E Publishing is the leading source for comprehensive, daily coverage of environmental and energy issues. Click here to start a free trial to E&E's information services.
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