THEGREENGROK    Planetary Watch

Day After Tomorrow Not Any Time Soon

by Bill Chameides | June 3rd, 2009
posted by Erica Rowell (Editor)

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The oceans and winds play a big role in the Earth’s climate. In the past a change in the saltiness of ocean waters may have triggered a rapid cooling. Could such a scenario happen again?

Model simulations predict a global warming-induced ice age is not in the cards this century. Cool … or should I say, “Neat”?

It is admittedly a bizarre scenario: global warming causing the Northern Hemisphere to suddenly freeze over. But bizarre or not, many scientists believe it’s a scenario that has happened before and could happen again. (You might recall it was the basis for the box office thriller The Day After Tomorrow, whose depictions of Dennis Quaid running for his life to prevent advancing ice from swallowing him up were just a tad over the top.)

It’s All About the Ocean’s Overturning Circulation

Switching gears for a moment, let me recap a post from a couple of weeks ago. In it I discussed the paper by Amy S. Bower and Susan Lozier and colleagues on the ocean’s overturning circulation, which brings warm water from the tropics poleward via the Gulf Stream. These waters make their way to the northern reaches of the North Atlantic Ocean near Greenland, Norway, and Labrador, where they sink and then flow southward. Eventually, the waters rise back to the surface and reform the Gulf Stream closing the loop.

Because the circulation involves an exchange of surface and deep waters, it is called an overturning circulation. (In their paper, researchers Bower and Lozier and colleagues found that the deep-water part of the circulation is more complex than previously thought.)

How This Relates to Ice Ages

The flow of that warm water in the Gulf Stream moves heat from the topics to the northern latitudes, helping moderate temperatures in the Northern Hemisphere and especially over western Europe. It is one of the reasons why London rarely freezes in winter even though its latitude is about 700 miles north of Chicago’s where sub-freezing temperatures are normal.

Some climate scientists believe that the Gulf Stream is critical to Europe’s climate and perhaps to much of the Northern Hemisphere’s. If the Gulf Stream stopped flowing, temperatures in these areas would fall precipitously, perhaps prompting ice age conditions.

OK, you say, but why would the Gulf Stream stop?

In a warming world, glaciers and sea ice melt, adding fresh water to the oceans. If enough of that meltwater enters the North Atlantic fast enough, it will dilute the salt water making it less dense, preventing it from sinking, and, the theory goes, stopping the overturning circulation.

And Now the Global Warming Hook

It sounds pretty far out, but evidence suggests this rapid switch to ice age conditions in the Northern Hemisphere has happened before. One such episode is believed to have occurred about 12,000 years ago during the so-called Younger-Dryas. While the world was thawing out from the previous ice age and temperatures where on the rise, suddenly snap — cold temperatures returned for a couple of thousand years before the warm temperatures that characterize our current climate took hold.

What happened? There is reason to believe that that switch from warming to cold temperatures was caused by a change in the rate of overturning circulation triggered by the sudden flow of meltwater from the Northern Hemispheric ice sheets into the North Atlantic.

Scientists have worried about a repeat scenario, not as a result of the end of an ice age but as the result of warming temperatures from greenhouse gas pollution. And there are reasons to take the possibility seriously. Glaciers and sea ice are melting, and the waters in the North Atlantic right in the regions where sinking occurs are getting less salty — we call it freshening.

The question has been: is the freshening happening fast enough to stop the overturning?

And that’s where a new paper by Aixue Hu of the National Center for Atmospheric Research and colleagues published in Geophysical Research Letters comes in.

They used a global climate system model that combines atmospheric and ocean circulations to assess the effects of melting glaciers and sea ice on climate in the 21st century.

On the negative side, their calculations indicated that even adding moderate amounts of Greenland meltwater could lead to larger than predicted sea-level rise along the northeastern coast of North America. Sea-level rise in the Northeast is already expected to be larger than the global average because any ocean warming will weaken the cold, dense layer in the deep Atlantic resulting from overturning circulation. As the deep water warms, it will expand, raising sea level.

On the positive side, they found only a modest weakening in the circulation from melting ice in the North Atlantic. The result was a small diminution in the warming in the high northern latitudes, but no cooling temperatures, and definitely no ice age — all in all a good thing.

There are many things to worry about these days. Fortunately, it appears that finding ourselves in Dennis Quaid’s shoes running from an ice-age apocalypse is not one of them.

filed under: climate change, faculty, oceans, Planetary Watch, science, Younger-Dryas
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