Background
The author notes that "the possibility of abrupt climate change occurring because of the ongoing global warming and its oceanic effects is attracting great attention," but he says that the concept is riddled with "assumptions, assertions and inferences," including the idea that abrupt climate changes "can be traced back to major changes (extending to "shutdown") of the North Atlantic meridional overturning circulation and perhaps even failure of the Gulf Stream." Indeed, he reports that "an extensive literature has collected around this hypothesis and it has been the focus of numerous modeling efforts as well as being presented as 'fact' to the public."

What was done
Wunsch re-examines several of the "assumptions, assertions and inferences" to which he refers and finds them wanting in many respects.

What was learned
With respect to the hypothesis that "fresh water injection controls the North Atlantic circulation," the Massachusetts Institute of Technology researcher states that "existing climate models, which are the main tool that have been used to study the hypothesis, do not have the resolution, either vertical or horizontal, to properly compute the behavior of fresh water and its interaction with the underlying ocean and overlying atmosphere." He also notes that some models use an "inappropriate surface boundary condition for salinity, leading to serious questions about the physical reality of the resulting flows (Huang, 1993)," and that most models "have almost always been run with fixed diffusion coefficients," noting that "depending upon exactly how the mixing coefficients are modified, fresh water additions can actually increase [our italics] the North Atlantic mass circulation (Nilsson et al., 2003)." Finally, he notes that "the prime mover of the ocean circulation, including its mixing coefficients as well as providing the major direct input of energy (Wunsch and Ferrari, 2004), is the wind."

What it means
Wunsch concludes that modeling studies "remain primarily as indicators of processes that can be operating, but with no evidence that they dominate." He notes, for example, that "given the comparatively small contribution of the ocean to the high-latitude meridional flux of heat, it seems an unlikely primary stimulus of major climate shifts beyond the North Atlantic basin." Instead, Wunsch focus on the wind, noting that "wind fields are capable of great volatility and very rapid global-scale teleconnections, and they are efficient generators of oceanic circulation change." However, he foresees no great changes of this type in the immediate future, noting that the only time they have occurred in the past was when there were "two enormous mountain ranges of high albedo [the Laurentide and Fennoscandian ice sheets], nearly bracketing Greenland." When these features have been present, Wunsch says that Dansgaard-Oeschger abrupt climate change events have been observed, but that "when they are absent, Dansgaard-Oeschger events are also absent," as they are now and should continue to be until the next great ice age rolls around.

References
Huang, R.X. 1993. Real freshwater flux as a natural boundary condition for the salinity balance and thermohaline circulation forced by evaporation and precipitation. Journal of Physical Oceanography 23: 2428-2446.

Nilsson, J., Brostrom, G. and Walin, G. 2003. The thermohaline circulation and vertical mixing: does weaker density stratification give stronger overturning? Journal of Physical Oceanography 33: 2781-2795.

Wunsch, C. and Ferrari, R. 2004. Vertical mixing, energy, and the general circulation of the oceans. Annual Review of Fluid Mechanics 36: 281-314.