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Good News: No Sign of Slowing Ocean Conveyor Belt

by Bill Chameides | March 29th, 2010
posted by Erica Rowell (Editor)

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Greenland’s glaciers are melting — faster today than a decade ago, studies show. Could such melting steep the oceans with enough fresh water to trigger an ice age? It’s possible. But soon? A new paper offers insights. (NASA/Wallops)

New observations suggest that The Day After Tomorrow is at worst a long way off.

Among the concerns about global warming is the specter of rapid climate change — the possibility that slowly warming temperatures could suddenly trigger a swift and major shift in the climate system with catastrophic consequences. In the 1990s some climate scientists warned of the possibility of one such scenario: an ice age caused by the shutdown of the so-called ocean conveyor belt.

Ocean Conveyor Belt and Climate

One of the critical jobs of the climate system is to move heat from the tropics, where much of the energy from the Sun is deposited, to the cool polar regions. The atmosphere does much of this redistribution, but the ocean is also a key player.

As noted in an earlier post, “without the oceanic and atmospheric mechanism, the temperature difference between the poles and the equator would be an astounding 200 degrees Fahrenheit rather than the 55-degree difference that exists today.”

The ocean plays a big role in distributing temperatures around the globe. The thermohaline circulation, also known as the great ocean conveyor, describes how heat (thermo-) and salt content (-haline), which affects density, work together to affect climate. (Courtesy of Windows to the Universe, University Corporation for Atmospheric Research [UCAR])

The ocean’s heat redistribution in the Northern Hemisphere is largely accomplished by the so-called ocean conveyor belt (in more technically inclined company we refer to this as the Atlantic meridional overturning circulation or AMOC or simply MOC). It works like this:

  • Warm topical waters in the Atlantic Ocean flow northward along the Gulf Stream.
  • Along the way the waters cool (providing a source of heat to northern climes) and evaporate (making the remaining waters of the Gulf Stream saltier).
  • As water becomes cooler and saltier, it gets denser.
  • Near Greenland and the Norwegian and Labrador seas, the dense water, now heavier than that below, sinks.
  • The cold, sinking water eventually closes the conveyor belt by flowing southward and resurfacing.

Sudden Ice Age Onset?

Could global warming change all that?

Some have speculated yes, arguing that melting glaciers could dilute that salty water in the Gulf Stream’s northern reaches, preventing it from getting dense enough to sink. (See here and here.)

Without sinking in the north, the waters moving northward in the Gulf Stream have nowhere to go and so the flow of the Gulf Stream would have to eventually stop. And without the Gulf Stream and its transport of heat northward, the Northern Hemisphere could get quite cold — perhaps cold enough to bring ice-age conditions back to our latitudes.

Global warming causing an ice age? It sounds rather oxymoronic and perhaps as far-fetched a prediction as the disaster scenario depicted in 2004’s blockbuster The Day After Tomorrow. But there is at least an ounce of reality to the scenario.

A Refresher: Ice Age in Not Too Distant Past Set Off by an Influx of Fresh Water

There is some evidence to suggest that such sudden deep freezes may have happened in the past, for example during the so-called Younger Dryas.

Scientists believe that about 12,000 years ago, as the last ice age was on the wane, a huge amount of meltwater released from a colossal glacial lake in the middle of North America was dumped into the northern Atlantic via the St. Lawrence Seaway. This sudden injection of fresh water, it is theorized, shut down the ocean conveyor belt and thrust the Northern Hemisphere back into ice-age conditions over a couple of thousand years.

But that was then and this is now. Could the present-day melting of glaciers in Greenland do the same thing?

Well, observations indicate that the waters of the Labrador and Norwegian Seas are becoming less salty.

But the rate at which fresh water is being added is considerably slower that that huge injection way back in the Younger-Dryas. And model calculations suggest that the current change is not rapid enough to cause an actual shutdown of the ocean conveyor belt.

Support for this contention has now been provided by a new paper in Geophysical Research Letters by Josh Willis of the California Institute of Technology’s Jet Propulsion Laboratory.

Using a combination of satellite observations of sea-surface heights and measurements of ocean temperature, salinity, and currents, Willis found “no significant trend in overturning strength between 2002 and 2009.” And, if anything, the strength of the ocean conveyor has actually increased since 1993.

Willis concludes that “substantial slowing of the AMOC did not occur during the past 7 years and is unlikely to have occurred in the past 2 decades.”

So you can probably cross “sudden ice age” off the top of your worry-list for the immediate future anyway. And feel free to chuckle at the sight of Dennis Quaid trying to outrun the advancing ice in The Day After Tomorrow.

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