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A new European ice age?

naturalSCIENCE: Cover Image
Image: NASA/GSFC. The Gulf Stream and North Atlantic Currents. Colors show differences in phytoplankton concentration (blue is cool plankton-rich water, red is warm plankton-poor water).

November 1, 1997: According to a recent paper in Nature, rising atmospheric carbon dioxide concentrations may cause sharp cooling in Western Europe. The study, authored by University of Bern researchers Thomas Stocker and Andreas Schmittner, shows by means of simplified climate models that unabated anthropogenic carbon dioxide emissions may disrupt the thermohaline circulation which transports a billion megawatts of heat (equivalent to the total energy output of a million nuclear power plants) from the Gulf of Mexico via the Gulf Stream to the North Atlantic and Europe.

Warm, salty surface water moves northward along the coast of North America and eastward to the European coast, where it loses heat to westerly onshore winds. As the surface water cools, it becomes more dense, sinks to a depth of about 2 km and flows at a rate of about 10 km per day back across the Atlantic and then southward.

This thermohaline circulation, or ocean overturning, is driven by comparatively small differences in sea water density, which depend on a balance between sea water cooling and salinity fluctuations arising from freshwater input (precipitation and river run-off) in the North Atlantic. A shift in this balance is foreseeable as models of global warming predict heavier precipitation at higher latitudes. The increase could reduce salinity in the North Atlantic enough to weaken or halt the thermohaline circulation.

Stocker and Schmittner's study concludes that the severity of a disruption in thermohaline circulation brought on by global warming will depend on the rate of warming and, hence, on the rate of greenhouse gas emissions. With a well-tested, coupled atmosphere-ocean climate model they show that, at present-day rates of carbon dioxide emission, thermohaline circulation will cease altogether by the time that atmospheric carbon dioxide concentration has doubled (to 750 p.p.m.v.). However, if carbon dioxide concentration increases more slowly, circulation will merely weaken with a doubling of atmospheric carbon dioxide concentration. The limits of climate models prevent exact predictions as it is impossible to include in the model all possible parameters and feedbacks. A particular source of uncertainty is change in precipitation, which is the deciding factor in thermohaline circulation shut-down.

The consequences of a shut-down in the Atlantic Ocean thermohaline circulation are uncertain. Paleoclimatological evidence shows that disruption of the thermohaline circulation has happened before. Deep ocean sediments dating from the last Ice Age (~11,000 year ago), suggest that the fresh water run-off from melting ice masses decreased sea water density sufficiently to cause a breakdown of ocean overturning. What followed was a European winter 10 degrees Celsius below normal and cold spells that lasted for hundreds of years.

Without offsetting effects of global warming, comparable cooling could be expected today with a shut-down in thermohaline circulation. For London, this could mean a climate similar to that now experienced by the Arctic island of Spitzbergen. However, studies that take account of global warming indicate that a halt to Atlantic Ocean overturning would cause only a small region of air cooling somewhere south of Greenland. But whatever the immediate climatic consequences, a slowdown or cessation of Atlantic overturning would reduce oceanic uptake of carbon dioxide. This, in turn, would impact the climate by exacerbating thermohaline instability and raising atmospheric carbon dioxide concentrations.

Despite the uncertainty surrounding climate models and their ability to predict future scenarios accurately, they indicate a range of possible outcomes to present climatological trends and are thus worthy of careful attention. As is evident from palaeoclimatic records, our climate system can be pushed from its present equilibrium state into one much less favourable. In December 1997, delegates from 160 countries will be meeting in Kyoto, Japan for a conference on greenhouse gas emissions. It remains to be seen whether the right course of action will be taken to limit not only the volume of greenhouse gas, but also the speed of change in emission rates.


REFERENCES and LINKS

(1) Stocker, Thomas F. and Andreas Schmittner. 1997. Influence of carbon dioxide emission rates on the stability of the thermohaline circulation. Nature 388:862-865.

(2) Rahmstorf, Stefan. 1997. Risk of sea-change in the Atlantic. Nature 388:825-826.

(3) Appenzeller, Christof. Draft Fact-sheet Thermohaline Circulation. University of Bern, Switzerland.
Provides easy to understand answers to commonly asked questions about thermohaline circulation.

(4) Schiller, Andreas. 1997. The Stability of the Thermohaline Circulation in a Coupled Ocean-Atmosphere General Circulation Model. CSIRO Australia.



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