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Decadal-Scale Changes in Ocean Circulation
Reference
Bacon, S. 1998.  Decadal variability in the outflow from the Nordic seas to the deep Atlantic Ocean.  Nature 394: 871-874.

What Was Done
Hydrographic data were used to create a 40-year history of deep water outflow from the Nordic seas to the Atlantic Ocean.  This record was then compared with a corresponding record of polar air temperature.

What was learned
Over the past four decades, the deep water current approximately doubled and then returned to flows characteristic of the mid- to late-1950s.  These changes were correlated with the mean air temperature of the three winters that preceded each outflow value.  It was found that high outflows were associated with low air temperatures, and vice versa.

What it means
The data suggest that the Arctic and North Atlantic atmospheric and oceanic circulations are linked and that rising polar air temperatures may further weaken the Atlantic thermohaline circulation.  This tendency has been theorized to advance us toward a potentially rapid transition to an ice-age-type climate.  In the words of the author, however, "we cannot yet tell whether, or how much of, the variability reported here is normal or unusual."  But it does give one pause to consider the warning flag that has been raised by Wallace Broecker in this regard, i.e., the possibility that an initial CO2-induced greenhouse warming could in short course shut down the global deep-sea "conveyor belt" and rapidly propel the planet into a new ice age.  Then again, we have experienced temperatures significantly warmer than those of today during more than one period of the current interglacial without such dramatic changes occurring.  Hence, it is clear that we still have much to learn in this important area of study.

Reviewed 1 October 1998