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Feedback Factors (Thermohaline Circulation) -- Summary
Driscoll and Haug (1998) and Haug and Tiedemann (1998) analyzed a number of proxy climate indicators and potential feedback mechanisms in an attempt to understand what triggered the approximate 100,000-year glacial cycles that began nearly three million years ago.  Their analyses led them to postulate that the closure of the Central American Seaway by the Isthmus of Panama increased the strength of the Gulf Stream that transported warm and saline surface waters to high northern latitudes, which resulted in increased evaporation from the North Atlantic Ocean and provided the augmented moisture source needed for ice sheet growth over Europe and Asia.  Simultaneously, however, the augmented moisture source also led to the delivery of more freshwater to the North Atlantic via river discharge, reducing the strength of deep water formation there and leading to a weakening of the thermohaline circulation, including the Gulf Stream.  The two papers then describe how this dynamic tension could be tweaked by the characteristics of earth's orbit about the sun to produce the 100,000-year glacial cycles that have occurred since the rise of the Isthmus of Panama.

These observations suggest to us that the intensification of the planet's hydrologic cycle that would likely occur in response to a greenhouse-gas-induced warming of the globe - particularly in high northern latitudes, as predicted by most general circulation models of the atmosphere - would produce the same chain of events.  It would: (1) enhance freshwater delivery to the North Atlantic Ocean via increased fluvial system discharge, (2) reduce the strength of deep water formation there, (3) weaken the thermohaline circulation, and thereby (4) stop the warming of high northern latitudes.

Further support for this scenario is provided by the study of Barber et al. (1999), who analyzed many different palaeoclimatic data sets in an attempt to explain the dramatic cooling event that occurred approximately 8200 years ago: a temperature drop of 4-8C in central Greenland and drops of 1.5-3C at marine and terrestrial sites around the northeastern North Atlantic Ocean.  The evidence they studied argued strongly for the catastrophic release of huge amounts of freshwater into the Labrador Sea via the final outburst drainages of glacial Lakes Agassiz and Ojibway.  Barber et al. suggest this freshwater input reduced the formation rates of Labrador Sea Intermediate Water and North Atlantic Deep Water enough to significantly slow the thermohaline circulation, which therefore dramatically reduced northward heat transport to this part of the world via the Gulf Stream and led to the observed dramatic cooling event.  Analogously, we believe that a less dramatic influx of freshwater, such as would be produced by the gradual warming of the globe that has been observed over the past century or so, would do much the same thing, only in a more gradual and less dramatic fashion. The end result, however, would be the same: the temperature of the region would be capped, so to speak, putting a lid on global warming, as we have also noted in our Editorial of 15 July 1999.

In view of these several observations - and the thoughts of the scientists who have studied them - we conclude that the surface of the planet is so configured at the present time as to resist the occurrence of inordinately high global air temperatures.

References
Barber, D.C., Dyke, A., Hillaire-Marcel, C., Jennings, A.E., Andrews, J.T., Kerwin, M.W., Bilodeau, G., McNeely, R., Southon, J., Morehead, M.D. and Gagnon, J.-M.  1999.  Forcing of the cold event of 8,200 years ago by catastrophic drainage of Laurentide lakes.  Nature 400: 344-348.

Driscoll, N.W. and Haug, G.H.  1998.  A short circuit in thermohaline circulation: A cause for Northern Hemisphere glaciation?  Science 282: 436-438.

Haug, G.H. and Tiedemann, R.  1998.  Effect of the formation of the Isthmus of Panama on Atlantic Ocean thermohaline circulation.  Nature 393: 673-676.