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A Natural Thermostat to Keep a Lid on Global Warming?
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.

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

What was done
The authors of these two papers 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 in the Northern Hemisphere nearly three million years ago.

What was learned
Analyses of the data led the authors of both papers to postulate that the closure of the Central American Seaway by the Isthmus of Panama resulted in an enhanced Gulf Stream that transported warm, saline surface waters to high northern latitudes.  The warmer surface waters may have led to increased evaporation, providing the moisture source needed for ice sheet growth over Europe and Asia.  Further analyses of proxy moisture data from Siberia indicated that humidity increased there about the same time that the Isthmus of Panama arose.  The enhanced moisture, according to Driscoll and Haug, would have increased freshwater delivery to the Arctic via river discharge.  The result was an increase in sea ice and a weakening of the thermohaline circulation, which likely would have reduced heat transport to the higher latitudes.  The authors of both papers make a good case for the theory that these mechanisms, with a little help from changes in the obliquity of earth's orbit about the sun, resulted in conditions that have favored the 100,000-year Northern Hemispheric glacial cycles that have been particularly prominent over the past two million years.

What it means
This study highlights the important role of the North Atlantic thermohaline circulation in orchestrating the cyclical behavior of the Northern Hemisphere's glacial/interglacial oscillations.  It also suggests, in our view, that the intensification of the earth's hydrologic cycle that would likely occur in response to any further warming of the globe would enhance fresh water delivery to the arctic via fluvial system discharge.  And as "the ocean's thermohaline circulation is rather sensitive to even small inputs of fresh water," in the words of Driscoll and Haug, it is possible that this consequence could reduce the formation of North Atlantic Deep Water, thereby reducing the efficiency of the thermohaline circulation and creating an impetus for climatic cooling that could negate any further tendency for global warming.  For earth's present geological configuration, this set of linked phenomena may well constitute a "natural thermostat" that limits global warming to no more than what has consistently been observed throughout the many interglacials that have punctuated the primarily glacial climate of the past two million years [see Climatic History: (Overview - The Last 2,000,000 Years)].

Reviewed 1 November 1998