What was done
The authors used different versions of the Goddard Institute for Space Studies (GISS) coupled atmosphere-ocean climate model to perform multiple experiments with sustained and gradual St. Lawrence freshwater inputs to the North Atlantic Ocean that are similar to what might be expected for global warming scenarios such as those typically predicted for the future.  This exercise is particularly relevant in light of the recent online release of a National Academy of Sciences report (Committee on Abrupt Climate Change, 2001) that claims a heightened potential for large and rapid temperature transitions to occur in response to the global warming it attributes to anthropogenic CO2 emissions.

The National Academy report talks of "large, abrupt climate changes" of "as much as 10°C change in 10 years," stating that these changes "can occur when gradual causes push the earth system across a threshold."  They further conclude 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 saying that "very close to a threshold, the evolution of the THC loses predictability altogether."

What was learned
First of all, the authors note that the sensitivity of their model to freshwater input through the St. Lawrence "is similar to that associated with freshening due to the warming climate of the next century" - which freshening comes primarily from the increased northern-latitude precipitation that accompanies global warming when tropical temperatures rise and more moisture is advected poleward - clearly indicating their study's applicability to the CO2-climate problem.  Second, although "thresholds" for changes in the THC figure highly in the highly speculative scenarios of the NAS Committee on Abrupt Climate Change, this newest contribution from the Goddard group and associates finds absolutely no evidence for such leverage points in their current study.  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.

What it means
In light of the fact that this and other climate model studies fail to produce anything like what the NAS Committee on Abrupt Climate Change claims could so readily happen, we wonder what their purpose was in producing such a baseless report.  The only rational suggestion we have seen so far comes from Virtual Climate Alert No. 45 of the Greening Earth Society, where a member of the NAS Committee is quoted as saying "the intent of the report was to draw attention to a common field of interest so that they could raise awareness enough to generate some funding support."  Guess that about says it all!

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

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.