Scientists are beginning to suspect that a change in global wind patterns helped end the last major ice age.
In a new review paper published this week in the journal Science a team of researchers looked to a global shift in winds to explain why Earth suffered such an extreme warming despite evidence only existing to explain a northern hemisphere warming.
“This paper pulls together several recent studies to explain how warming triggered in the north moves to the south, ending an ice age,” said study co-author Bob Anderson, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory. “Finally, we have a clear picture of the global teleconnections in Earth’s climate system that are active across many time scales. These same linkages that brought the earth out of the last ice age are active today, and they will almost certainly play a role in future climate change as well.”
Ice Age 101
Every 100,000 years or so Earth enters an ice age as it’s orientation toward the sun shifts. The last ice age reached its peak 20-18,000 years ago and sent massive ice sheets stretching from the North Pole all the way down to cover areas of America as far as New York City, the Alps and the Himalayas.
Canada was almost completely covered by ice as well as large swathes of northern North America. The Scandinavian ice sheet reached down as far as the British Isles, Germany, Poland and extended as far as the Taimyr Peninsula in western Siberia, Russia.
How the North beat the South
However the ice age was sent into retreat, at least initially, by an orbital shift of the planet which caused more sunlight to fall across the northern hemisphere. The massive ice sheets melted and sent ice bergs wandering throughout the North Atlantic Ocean, which in turn freshened the waters which shut down the Gulf Stream current that was bringing warm water north from the equator. Sea ice spread across the North Atlantic, Greenland and northern Europe suffered deep winters the planet’s wind belts underwent a profound shift.
The tropical trade winds shifted south, bringing with the change significant dry spells across much of Asia and rain to normally arid regions of Brazil. The shift in winds also brought hot air and warm seawater south which caused the beginning of warming in the southern hemisphere.
18,000 years ago mountain glaciers in South America and New Zealand started to melt. The researchers believe that the southern hemisphere westerly winds also shifted south, which would have brought warm air and sea water to the mid latitudes and started the warming of the glaciers. By 16,000 years ago the glaciers were in full retreat.
“It’s the great global warming of all time,” said the study’s lead author, George Denton, a glaciologist at the University of Maine, and an adjunct scientist at Lamont. “We’re trying to answer the puzzle: why does the Earth, when it appears so firmly in the grip of an ice age, start to warm?”
Carbon Dioxide’s Involvement
The westerlies which had by now turned in to southerlies were causing heavy mixing in the Souther Ocean around Antarctica, with large amounts of carbon dioxide being dissolved from the water into the air. Ice core records show that between 18,000 and 11,000 years ago atmospheric carbon dioxide levels rose from 185 parts per million to 265 parts per million, in comparison to today’s levels of 393 parts per million after a sustained industrial era.
The scientists behind the review paper believe that the rise in carbon dioxide came just as the planet’s orientation was shifting and possibly helped prevent Earth from falling into another ice age.
Scientists have long believed that carbon dioxide played some major role in the last ice age but have had significant trouble explaining how, especially considering that glaciers in Patagonia and New Zealand had started melting prior to the rise in carbon dioxide levels. The shifting southern hemisphere westerlies explain the rapid warming that these glaciers suffered.
Richard Alley, a glaciologist at Penn State University who was not involved in the study, called the hypothesis comprehensive and convincing. “This subject has long intrigued policy-makers, and although the issue in no way changes the very strong evidence that adding CO2 to the atmosphere has a warming effect, the inability of the scientific community to provide a complete explanation of the natural CO2 changes across ice-age cycles may have led some people to more broadly question climate science. Testing this hypothesis will be very interesting, to see whether it successfully “predicts” the observed timing of CO2 and temperature changes in the south.”