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Deep Water Formation -- Summary
What initiated the Northern Hemispheric glacial-interglacial cycles that first appeared (in rudimentary form) in the Northern Hemisphere between two and three million years ago?  In a pair of papers that attempt to answer this question, Driscol and Haug (1998) and Haug and Tiedemann (1998) suggest that the initiating event was the closure of the Central American Seaway by the Isthmus of Panama.  This closure, they say, would have enhanced the Gulf Stream transport of warm saline surface water to high northern latitudes, which would have led to increased evaporation and provided the moisture source needed for ice sheet growth over Europe and Asia.  Equally as important, the enhanced hydrologic cycle driven by the extra atmospheric moisture would have increased freshwater delivery to the Arctic via enhanced river discharge, which would have led to a reduction in the sinking of cold saline surface water in the North Atlantic, weakening the strength of the worldwide thermohaline circulation and reducing the transport of heat to high northern latitudes.  Together, these several phenomena would have dramatically cooled that portion of the planet and initiated Northern Hemispheric glaciation, which would have then proceeded in cyclical fashion as has historically been described in terms of variations in earth's orbital parameters.

A number of studies have provided strong supporting evidence for the deep water aspect of this theory.  Ruhlemann et al. (1999) derived a high-resolution record of sea surface temperature for the past 29,000 years for the western tropical North Atlantic from a sediment core obtained southeast of Grenada.  Comparing it with other temperature records from around the world and a high-resolution record of the strength of the thermohaline circulation in the vicinity of the Bermuda rise, they found that during the dramatic cooling events of 16,900 to 15,400 years ago and 12,900 to 11,600 years ago, there was a significant slowdown of North Atlantic Deep Water formation, which is believed to have been caused by the injection of large volumes of freshwater into the northern North Atlantic that resulted in decreased sea surface salinity there and, hence, diminished deep convective overturning of the local ocean waters that ultimately led to diminished northward transport of heat by the Gulf Stream.  Likewise, in an analysis of a large number of data sets related to the dramatic cooling event of 8200 years ago, Barber et al. (1999) concluded that the catastrophic release of a massive amount of freshwater into the Labrador Sea from the final outburst drainage of glacial lakes Agassiz and Ojibway reduced the formation rates of Labrador Sea Intermediate Water and North Atlantic Deep Water sufficiently to reduce the global thermohaline circulation and significantly curtail northward heat transport to this region of the world, thereby producing the near-contemporaneous dramatic cooling event that is evident in the proxy temperature records of that period.

Bringing this subject into a more modern context, Broecker et al. (1999) additionally consider the earth's other major source of oceanic deep water formation, i.e., the Southern Ocean around Antarctica.  They note that over the past 800 years, the North Atlantic and Southern Oceans have been supplying roughly equal quantities of new deep water to the global thermohaline circulation, but that over the last several decades the contribution of the Southern Ocean has apparently decreased to only a third of its prior 800-year average.  Broecker et al. suggest that the Little Ice Age may have been a consequence of the preceding several-hundred-year more-intense-than-present deep water formation in the Southern Ocean, helped by a concomitant reduced production of deep water in the North Atlantic, which further suggests that the slowdown in the rate of Southern Ocean deep water formation over the past several decades may be the cause of the contemporaneous warming that has been experienced over this period, particularly in the Northern Hemnisphere.

Further support for this idea comes from the work of Marchitto et al. (1998), who studied cadmium/calcium ratios in benthic foraminifera shells contained in sediment cores retrieved from the Bahama Banks region of the Northwest Providence Channel that connects the North Atlantic Basin to the Florida Straits.  The evidence they obtained suggests that "periods of enhanced intermediate-water production alternate with periods of enhanced deep-water formation on both orbital and millennial timescales."  And like Bacon (1998), they also found that "analogous dynamics operate in the modern North Atlantic on much shorter (decadal) timescales."

This complex nested cyclical behavior of the global thermohaline circulation has an equally complex nested cyclical impact on climate that must clearly be considered in interpreting the meaning of various climatic trends.  Earth's current temperature trend, in and of itself, for example, tells little about what is causing it.  In order to identify the driving force or forces responsible for the warming trend of the past century or so, we need to know more about what influences deep water formation in both Northern and Southern Hemispheric source regions.  In particular, in the words of Broecker et al., we must "gain a better understanding of the Little Ice Age and its demise" if we are ever to have any confidence in predictions of future climate change.

It is very possible, for example, that if current thinking about the control of deep water formation is correct, any global warming that might occur in the future would intensify earth's hydrologic cycle and deliver more freshwater to the critical deep water source region of the North Atlantic, leading to reduced deep water formation there and a slowdown of the thermohaline circulation that would stall the global warming that initiated the whole process.  Could it be that this linkage constitutes 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?  Until such questions as this can be confidently answered, it would be unwise, if not downright foolish, to develop energy policies relative to anthropogenic CO2 emissions that are based on presumed, but unproven, climatic considerations.

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

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.

Broecker, W.S., Sutherland, S. and Peng, T.-H.  1999.  A possible 20th-century slowdown of Southern Ocean deep water formation.  Science 286: 1132-1135.

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.

Marchitto Jr., T.M., Curry, W.B. and Oppo, D.W.  1998.  Millennial-scale changes in North Atlantic circulation since the last glaciation.  Nature 393: 557-561.

Ruhlemann, C., Mulitza, S., Muller, P.J., Wefer, G. and Zahn, R.  1999.  Warming of the tropical Atlantic Ocean and slowdown of thermohaline circulation during the last deglaciation.  Nature 402: 511-514.