BOULDER, Colo., Dec. 15 (UPI) -- Of all the warnings made about the ramifications of climate change, the most counterintuitive is that the global warming phenomenon could lead to a new Ice Age.
The culprit in this curious conclusion is the predicted impacts of atmospheric carbon dioxide and warming on what is called the thermohaline circulation of the world's oceans.
The process, also known as THC, often is compared to a huge conveyor belt of waters in the world's oceans, driven by an engine that resides in the North Atlantic. The word thermohaline describes the interaction of temperature -- thermo -- and salt concentrations -- haline -- of ocean water.
To oversimplify, the conveyor belt carries an enormous amount of warm, shallow water up the coast of Africa to the vicinity of Iceland, where winter cooling increases the density of the salty water, allowing it to sink to the bottom. The deep, cold water flows back to the south, past South America and along about the 40th degree southern parallel past Australia, then up the Pacific to near Alaska, where it warms, rises to the surface, and returns eventually as warm shallow water to Iceland via the Indian Ocean.
The dynamics are more complex, of course, but this appears to be the general pattern. There is a huge volume of water involved -- equivalent to 100 Amazon rivers -- and the journey is slow, taking perhaps 1,000 years for a complete circuit.
Thomas Stocker, professor of climate and environmental physics at the University of Bern, Switzerland, and colleagues wrote a 2001 paper on the issue for the American Geophysical Union.
"The thermohaline circulation strongly influences the climate on regional-to-hemispheric scales," they said. "Changes in both the ocean THC and the atmospheric storm tracks would seriously affect the climate in northwestern Europe."
The dynamics of this circulation is one of the main reasons Europe's climate is moderate and agriculturally productive, despite being 10 degrees to 15 degrees farther north than the most populous areas of North America and Asia.
If the temperature warms and additional fresh water enters the North Atlantic from Greenland and the Arctic, these factors could alter the buoyancy relationships, preventing the warm surface water from sinking, possibly slowing or shutting down the system.
"A modeled increase to 750 parts per million by volume C02 (carbon dioxide) with 100 years (corresponding approximately to a continuation of today's growth rate) leads to a permanent shutdown of the thermohaline circulation," the researchers concluded.
The rate of increase is important, however. If the C02 accumulates more slowly, the circulation slows, but does not stop altogether.
Wallace Broecker, a geologist with Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y., thinks THC changes have led to rather abrupt changes in Earth's climate in the past -- changes that occurred within decades, rather than centuries or millennia.
A weakening of THC, he said, "might led to a jump to one of the Earth's climate system-alternate modes. Because these jumps were heralded by a several-decades-long series of flickers, such an event would perhaps stress our ability to feed the 9 billion or so people likely to be present on the planet a century from now."
One possible result, among many, from the breakdown of the warm water circulation is a renewed Ice Age in the northern hemisphere.
The fresh water needed to trigger these events could come from a melting of Greenland glaciers and Arctic ice.
Peter Gent, head of the National Center for Atmospheric Research's oceanography group in Boulder, said there are two primary things that could trigger a thermohaline response.
"One is the influx of fresh water to the surface, and the other is that the surface warms up some," he told United Press International. "Both of those tend to make the surface water less dense ... If the surface water is less dense, the depth of deep convection is reduced, which leads to a weakening of thermohaline circulation."
The first of these conditions seems well under way. "The trends in the sea ice are well documented," Gent said. "The sea ice in the Arctic is shrinking in area and reducing in thickness."
Despite this evidence however, Gent said he does not see a thermohaline catastrophe on the horizon.
"There have been ice ages in the past, but I'm not sure why they happen," he explained. "We have absolutely no evidence that we could go into a cold patch at any time in the near future. We have quite a lot of evidence for the opposite."
Gent added he thinks there will likely be a gradual response by THC, perhaps resulting in some moderation of the warming expected in Europe.
"In my view," he said, "the most likely thing is that Europe would not warm quite as quickly as otherwise."
One problem with the predictions is THC cannot be observed directly. It only can be inferred from theory and models.
"From the models we run, there are varying strengths of responses," Gent noted. "The honest answer is that we expect a slowing down, but exactly the magnitude or the rate we are uncertain of at the moment."
Dan Whipple covers the environment for UPI Science News. E-mail firstname.lastname@example.org