Answer to what ended the last ice age may be blowing in the winds, paper says

Jun 25, 2010
Sea ice in the North Atlantic may have reorganized Earth's winds, spreading warmth from the northern hemisphere to south at the end of the last ice age. Credit: NASA.

Scientists still puzzle over how Earth emerged from its last ice age, an event that ushered in a warmer climate and the birth of human civilization. In the geological blink of an eye, ice sheets in the northern hemisphere began to collapse and warming spread quickly to the south. Most scientists say that the trigger, at least initially, was an orbital shift that caused more sunlight to fall across Earth's northern half. But how did the south catch up so fast?

In a review paper published this week in the journal Science, a team of researchers look to a global shift in winds for the answer. They propose a chain of events that began with the melting of the large northern hemisphere ice sheets about 20,000 years ago. The melting ice sheets reconfigured the planet's wind belts, pushing warm air and seawater south, and pulling from the deep ocean into the atmosphere, allowing the planet to heat even further. Their hypothesis makes use of preserved in cave formations, polar ice cores and deep-sea sediments to describe how Earth finally thawed out.

"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 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."

Earth regularly goes into an ice age every 100,000 years or so, as its orientation toward the sun shifts in what are called Milankovitch cycles. At the peak of the last ice age, about 20,000 years ago, with New York City and large parts of Europe and Asia buried under thick sheets of ice, Earth's orbit shifted. More summer sunlight began falling on the northern hemisphere, melting those massive ice sheets and sending icebergs and fresh water into the North Atlantic Ocean.

It may sound counterintuitive, but the paper says freshening of the North Atlantic triggered a series of cold spells in Greenland and northern Europe by shutting down the Gulf Stream current, which usually carries warm water north from the equator. Sea ice spread across the North Atlantic, bringing bitter cold winters to Europe and profoundly reshaping the planet's wind belts.

With the North Atlantic in a deep freeze, the tropical trade winds shifted south, bringing dry spells across much of Asia and rain to normally arid regions of Brazil. The displaced winds moved not only rain further south, but hot air and warm seawater, heating up the southern hemisphere.

The southern hemisphere westerly winds also shifted south, bringing warm air and seawater to the mid-latitudes. By about 18,000 years ago, mountain glaciers in South America and New Zealand started to melt as the displaced winds blew warm air their way. By 16,000 years ago, the glaciers had beaten a spectacular retreat. This shift in westerly winds would also amplify the warming in both hemispheres by resetting the planet's thermostat, as Anderson has proposed in an earlier study in Science. The displaced westerlies caused heavy mixing in the Southern Ocean around Antarctica, pumping dissolved carbon dioxide 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. (Today levels are 393 parts per million, after a sustained rise during the industrial era.) The infusion of carbon dioxide came just as the planet's orientation was shifting, and summer sunlight to the was declining, at about 11,000 years ago. The boost in carbon dioxide may have prevented Earth from falling into another ice age, the scientists say.

"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?"

Scientists have long suspected that carbon dioxide played a major role in the last ice age but have had trouble explaining the early warming in the southern hemisphere, where glaciers in Patagonia and New Zealand were melting before carbon dioxide levels rose significantly. Some scientists suggest that a change in ocean currents, triggered by the freshening of the North Atlantic, caused this early warming. But computer models using ocean circulation to explain the rapid warming in the south have been unable to recreate the large temperature jumps seen in the paleoclimate record. Now, with the evidence for shifting southern hemisphere westerlies, the rapid warming is readily explained.

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 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."

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User comments : 11

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GSwift7
1 / 5 (8) Jun 25, 2010
"Finally, we have a clear picture of the global teleconnections in Earth's climate system"

"every 100,000 years or so, as its orientation toward the sun shifts in what are called Milankovitch cycles"

"More summer sunlight began falling on the northern hemisphere, melting those massive ice sheets"

"These same linkages that brought the earth out of the last ice age are active today"

Really? I don't see any massive ice sheets covering the northern hemisphere, and I don't think it's been 100,000 years since the last Milankovitch cycle.

"Testing this hypothesis will be very interesting, to see whether it successfully "predicts" the observed timing of CO2 and temperature changes in the south"

Now THAT part makes sense.
CarolinaScotsman
2 / 5 (4) Jun 25, 2010
Really? I don't see any massive ice sheets covering the northern hemisphere, and I don't think it's been 100,000 years since the last Milankovitch cycle.

We're still in the warming phase from the last one. Something many alarmists seem to either forget or ignore.
Zarky
1 / 5 (4) Jun 25, 2010
What a load of rubbish

The problem/the initiator of Ice Ages is a hot ocean.... once this cools the cycle is reversed.

Unless you can postulate a SIMPLE cycle, rather than fantasising a complex series of events, your explanations hold no credence.

The key to all this is salinity
and the fresh water to salt water ratio... it naturally oscillates which is reflected in glacial and interglacial periods.
furlong64
Jun 25, 2010
This comment has been removed by a moderator.
emergent
1 / 5 (5) Jun 26, 2010
"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?"

wrong question, the planet is always warming and at apex points it abruptly changes. volcanism
Paradox
1 / 5 (2) Jun 26, 2010
"Most scientists say that the trigger, at least initially, was an orbital shift that caused more sunlight to fall across Earth's northern half."

And just who would these be?
alivation
5 / 5 (6) Jun 26, 2010
omg ... all you arrogant armchair experts. wtf would you all know ? You do a little reading or maybe took a paper on weather or climate at uni and you become the experts, pushing your own little agendas without any real research or understanding of the latest evidence to back it up. What a lot of egotistical tossers you are. Get some humility and gain some respect for those that work full time in the field. They may not be right but there is far more chance they are than you are.
robbscholl
Jun 26, 2010
This comment has been removed by a moderator.
MichaelExe
not rated yet Jun 26, 2010
There's been more than one ice age, they come in cycles, like the rest of climate change.
http://en.wikiped...h_cycles
http://upload.wik...tion.jpg

It also seems to me that, in general, the earth had been cooling since a few million years ago, until about maybe 0.5 or 0.4 million years ago, while also gaining greater variance between its highs and lows.
http://upload.wik...ange.png

The current temperatures aren't out of the norm of the expected amounts. We've still yet to exceed the last peak (excluding our current peak), which was about 130 000 years ago, or 3 peaks ago, ~325 000 years ago. Looking at the two following graphs, it seems like the current peak could have hit its maximum about 10 000 years ago.
http://upload.wik...data.svg]http://upload.wik...data.svg[/url]
http://upload.wik...data.svg]http://upload.wik...data.svg[/url]
Nik_2213
not rated yet Jun 26, 2010
"I don't see any massive ice sheets covering the northern hemisphere"

Well, there's Greenland, of course, but those ice-sheets have gone underground as permafrost-- And that seems to be thawing...
furlong64
5 / 5 (2) Jun 26, 2010
alivation and robbscholl, thanks for your sane comments! Unfortunately, the folk here will likely continue posting as though they know the simple truth that eludes the experts. It's the only thing they're good at in their wretched lives.
rwinners
not rated yet Jun 26, 2010
"Earth regularly goes into an ice age every 100,000 years or so, as its orientation toward the sun shifts in what are called Milankovitch cycles."
Well, until it doesn't anymore. Sun cycles are only one factor in the earth's periodic climate shifts and probably not the only major one.
crackerhead
not rated yet Jun 30, 2010
none of this has even been tested yet is reported as fact no wonder we arm chair enthusiast are led astray!

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