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Rapid Climate Change (Thermohaline Circulation) -- Summary
Many people fear -- or at least claim they do -- that CO2-induced global warming will lead to enhanced precipitation in high northern latitudes, which will lead to augmented freshwater runoff to the North Atlantic Ocean, which will reduce local sea surface salinity, which will lead to a precipitous decline in North Atlantic Deep Water formation, which will produce a swift reduction in the global ocean's thermohaline circulation, which will shut down the Gulf Stream, which will bring cold times to Europe [see Stocker (2000)].  Are their worries justified?

To explore this hypothesis, Saenko et al. (2003) used a "super model" comprised of an ocean general circulation model, a dynamic-thermodynamic sea-ice model and an energy-moisture-balance atmospheric model to examine the effect of mid-latitude meridional moisture transport in the Southern Hemisphere on the Meridional Overturning Circulation (MOC) of the Atlantic Ocean.  This exercise revealed, in their words, "that the Atlantic MOC, northward oceanic heat transport, and the associated air-sea heat flux anomalies are all proportional to the southward moisture transport from subtropical to subpolar regions in the Southern Hemisphere."  Consequently, although it has often been claimed that in a warmer world an intensified hydrological cycle would weaken the MOC of the Atlantic Ocean by transporting more moisture northward [see our Editorial of 10 July 2002], Saenko et al. say their results suggest "that the intensified hydrological cycle could also tend to stabilize the MOC by transporting more moisture southward."  The bottom line, as they thus remark, is that the several mechanisms that have been proposed for controlling deep water formation in the North Atlantic "remain controversial."

In an even more explicit study of the subject, Rind et al. (2001) employed different versions of the Goddard Institute for Space Studies coupled atmosphere-ocean climate model to perform multiple virtual experiments with sustained and gradual St. Lawrence freshwater inputs to the North Atlantic Ocean that are similar to what is predicted for future CO2-induced global warming.  This exercise is particularly relevant in light of the U.S. National Academy of Sciences' Committee on Abrupt Climate Change (2001) report on the subject, wherein it is claimed there is a heightened potential for large and rapid temperature transitions to occur in response to continued anthropogenic CO2 emissions.

The National Academy report talks of "large, abrupt climate changes" of "as much as 10C change in 10 years," stating that these changes "can occur when gradual causes push the earth system across a threshold."  They also claim that "human activities could trigger abrupt climate change," stating that "warming and the associated changes in the hydrological cycle constitute a threshold for the THC [thermohaline circulation]" of the world's oceans.  "Once reduced, the THC is more susceptible to perturbations," the Academy Committee claims," also stating that "very close to a threshold, the evolution of the THC loses predictability altogether."

So what did the study of Rind et al. reveal?  First of all, it indicated that the sensitivity of their model to freshwater input through the St. Lawrence was "similar to that associated with freshening due to the warming climate of the next century," clearly indicating their study's applicability to the CO2-climate problem.  Second, although thresholds for changes in the THC figure highly in the speculative scenarios of the NAS Committee on Abrupt Climate Change, the Rind et al. team finds absolutely no evidence for them.  They note, for example, that "North Atlantic Deep Water [NADW] production decreases linearly with the volume of fresh water added through the St. Lawrance" and that it does so "without any obvious threshold effects."  Third, they find no evidence for great rapidity in the freshening-induced reductions in NADW production.  "The effect is not rapid with realistic freshwater inputs," they say.  In fact, they estimate that, in the extreme, "NADW cessation would take some 350 years to occur."  Fourth, the authors note that other studies, such as that of Schiller et al. (1997), have reached pretty much the same conclusions.

As time has progressed, still other studies have tended to refute the thinking of the NAS Committee on Abrupt Climate Change.  Seidov and Haupt (2003) performed a number of sensitivity experiments with the ocean model of the Geophysical Fluid Dynamics Laboratory.  These experiments showed, in their words, that "Atlantic-Pacific sea surface salinity asymmetry is one of the most critical elements for maintaining the global ocean conveyor," and, hence, that "high-latitudinal freshwater impacts, as a mechanism of altering global thermohaline circulation may be less effective than inter-basin freshwater communications."

Also weighing in on the subject are Munk and Wunsch (1998), Wunsch (2000) and Wunsch (2002), who conclude, on the basis of fundamental theoretical considerations, that the THC is sustained primarily by the work of the wind and secondarily by tidal forcing.  So basic are these considerations, in fact, that Wunsch (2000) categorically states "there cannot be a primarily convectively driven circulation of any significance."

In further explaining this fact, Wunsch (2002) notes that "both in models and the real ocean, surface buoyancy boundary conditions strongly influence the transport of heat and salt," acknowledging the matters upon which both the NAS Committee and Seidov and Haupt have focused their attention; but he emphasizes that "these boundary conditions do not actually drive the circulation," noting again that "for past or future climates, the quantity of first-order importance is the nature of the wind field."

Although Munk and Wunsch's thoughts may seem a bit esoteric, the energy requirements of their more basic view of the subject have been observationally verified by the work of Egbert and Ray (2000), while other supporting evidence has been supplied by Berger and von Rad (2002).  Consequently, since these data suggest that the mass flux of the THC is primarily a creature of wind and tide, there is little reason to entertain the view that continued anthropogenic CO2 emissions will lead to a dramatic shutdown of the THC.

References
Berger, W.H. and von Rad, U.  2002.  Decadal to millennial cyclicity in varves and turbidites from the Arabian Sea: hypothesis of tidal origin.  Global and Planetary Change 34: 313-325.

Committee on Abrupt Climate Change (Richard B. Alley, Chair).  2001.  Abrupt Climate Change: Inevitable Surprises. National Academy Press, Washington, DC.

Egbert, G.D. and Ray, R.D.  2000.  Significant dissipation of tidal energy in the deep ocean inferred from satellite altimeter data.  Nature 405: 775-778.

Munk, W.H. and Wunsch, C.  1998.  Abyssal recipes II: Energetics of tidal and wind mixing.  Deep-Sea Research 45: 1977-2010.

Rind, D., deMenocal, P., Russell, G., Sheth, S., Collins, D. Schmidt, G. and Teller, J.  2001.  Effects of glacial meltwater in the GISS coupled atmosphere-ocean model.  1.  North Atlantic Deep Water response.  Journal of Geophysical Research 106: 27,335-27,353.

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.

Saenko, O.A., Weaver, A.J. and Schmittner, A.  2003.  Atlantic deep circulation controlled by freshening in the Southern Ocean.  Geophysical Research Letters 30: 10.1029/2003GL017681.

Schiller, A., Mikolajewicz, U. and Voss, R.  1997.   The stability of the thermohaline circulation in a coupled ocean-atmosphere general circulation model.  Climate Dynamics 13: 325-348.

Seidov D. and Haupt, B.J.  2003.  Freshwater teleconnections and ocean thermohaline circulation.  Geophysical Research Letters 30: 10.1029/2002GL016564.

Stocker, T.F.  2000.  Past and future reorganizations in the climate system.  Quaternary Science Reviews 19: 301-319.

Wunsch, C.  2000.  Moon, tides and climate.  Nature 405: 743-744.

Wunsch, C.  2002.  What is the thermohaline circulation?  Science 298: 1179-1181.