Within this context, it is important to know what the proponents of the Protocol consider a dangerous climate impact worthy of immediate action.  Taking the Intergovernmental Panel on Climate Change as their guide, O'Neill and Oppenheimer (2002) say it is an impact that either imposes a risk upon unique and threatened ecosystems or engenders a risk of some large-scale discontinuity in earth's climate system.  On this basis, they claim there are three warming-related risks that are serious enough to implement the Kyoto Protocol with all due haste.  These risks are the potentials for (1) the infliction of extreme damage to earth's coral reefs, (2) the disintegration of the West Antarctic Ice Sheet, and (3) the virtual shutdown of the marine thermohaline circulation.

With respect to the third of these risks - we dealt with the first and second ones in our Editorials of 26 June 2002 and 3 July 2002 - O'Neill and Oppenheimer (hereafter, O & O) claim there is strong evidence that the thermohaline circulation or THC has shut down many times in the past "in association with abrupt regional and perhaps global climate changes," citing Broecker (1997).  They also say that "most coupled atmosphere-ocean model experiments show weakening of the THC during this century in response to increasing concentrations of greenhouse gases, with some projecting a shutdown if the trends continue."  Hence, they conclude that "to avert shutdown of the THC, we define a limit at 3°C warming over 100 years, based on Stocker and Schmittner (1997)."

With respect to O & O's claim that the THC abruptly shut down several times in the past - with which we have no argument - there is an important auxiliary fact they fail to mention; and that is that these shutdowns typically occurred during very cold glacial or transition periods, but rarely, if ever, during warm interglacials.  Part of their failure to report this fact may be related to their reliance on somewhat outdated publications, such as Broecker (1997), who referenced a minor (and readily explained) exception to this rule that occurred approximately 8200 years ago, and Stocker and Schmittner (1997), who have recently reported much different modeling results than they did half a decade ago.

In the case of the dramatic regional cooling of 1.5-3°C that is known to have occurred at marine and terrestrial sites around the northeastern North Atlantic some 8200 years ago, Barber et al. (1999) have made a strong case for the likelihood that it was caused by the catastrophic release of a huge amount of freshwater into the Labrador Sea as a result of the final outburst drainage of glacial lakes Agassiz and Ojibway, which significantly reduced the strength of the THC and retarded the transport of heat to the northeast North Atlantic.  Consequently, this event could validly be classified as a "holdover" phenomenon from the prior glacial-to-interglacial transition period.

In a model study of a more gradual increase in freshwater input through the St. Lawrence River system "similar to that associated with freshening due to the [predicted] warming climate of the next century," Rind et al. (2001) found that "North Atlantic Deep Water production decreases linearly with the volume of fresh water added through the St. Lawrence" and that it does so "without any obvious threshold effects." Under such circumstances it would be expected that the gradual slowing of the THC would gradually reduce the northward transport of heat in the North Atlantic, leading to a gradual reduction in freshwater input to the North Atlantic through the St. Lawrence and other rivers that would ultimately lead to a gradual intensification of the THC, and etc., thereby producing a climatic oscillation of much more modest amplitude than the abrupt changes of which O & O are so concerned.

Similar conclusions have been reached by a number of other investigators as well.  The modeling work of Ganopolski and Rahmstorf (2001, 2002) and Alley and Rahmstorf (2002), for example, suggests that the North Atlantic branch of the THC possesses two potential modes of operation during glacial times, between which it oscillates in response to weak (and probably solar-induced) forcings that produce small cyclical variations in freshwater input to high northern latitudes that are amplified by stochastic resonance and therefore produce rapid warmings followed by slower coolings that are a full order of magnitude greater than similar oscillations that occur during interglacials.  Throughout these latter much warmer periods, such as the current Holocene, however, Ganopolski and Rahmstorf (2002) report that climate "is not susceptible to regime switches by stochastic resonance with plausible parameter choices and even unrealistically large noise amplitudes, and neither is it in conceptual models."  Furthermore, they correctly report that real-world observations reveal "there is no evidence for regime switches during the Holocene," as previously noted by Stocker (2000) and suggested by the modeling work of Latif et al. (2000) and Gent (2001).

Schmittner et al. (2002) come to pretty much the same conclusion, i.e., that the stability of the THC during glacial periods is much reduced from what it is during interglacials; and in a review of the current status of our knowledge of abrupt climate change and its relationship to changes in the THC, Clark et al. (2002) conclude that essentially all of the rapid warmings and associated slower coolings of which we have record "were characteristic of the last glaciation," as opposed to the Holocene.  They thus state that "the palaeoclimate data and the model results also indicate that the stability of the thermohaline circulation depends on the mean climate state."  And in this regard, cold is incredibly robust, producing large and rapid changes in climate, while warm is much more subdued, producing more gentle variations of which periodic swings between Medieval Warm Period and Little Ice Age conditions are typical.

These several observations thus suggest that, if anything, additional warmth may actually provide an insurance policy against radical reorganizations of the THC, as well as the abrupt climate changes that accompany them.  Hence, this last of the three most dangerous impacts of global warming identified by O & O - like its predecessors - turns out to be pretty much of a non-issue.

Reference
Alley, R.B.S. and Rahmstorf, S.  2002.  Stochastic resonance in glacial climate.  EOS, Transactions, American Geophysical Union 83: 129, 135.

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.  1997.  Thermohaline circulation, the Achilles heel of our climate system: Will man-made CO2 upset the current balance?  Science 278: 1582-1588.

Clark, P.U., Pisias, N.G., Stocker, T.F. and Weaver, A.J.  2002.  The role of the thermohaline circulation in abrupt climate change.  Nature 415: 863-869.

Ganopolski A. and Rahmstorf, S.  2001.  Rapid changes of glacial climate simulated in a coupled climate model.  Nature 409: 153-158.

Ganopolski, A. and Rahmstorf, S.  2002.  Abrupt glacial climate changes due to stochastic resonance.  Physical Review Letters 88: 038501.

Gent, P.R.  2001.  Will the North Atlantic Ocean thermohaline circulation weaken during the 21st century?  Geophysical Research Letters 28: 1023-1026.

Latif, M., Roeckner, E., Mikolajewicz, U. and Voss, R.  2000.  Tropical stabilization of the thermohaline circulation in a greenhouse warming simulation.  Journal of Climate 13: 1809-1813.

O'Neill, B.C. and Oppenheimer, M.  2002.  Dangerous climate impacts and the Kyoto Protocol.  Science 296: 1971-1972.

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. I. North Atlantic Deep Water response.  Journal of Geophysical Research 106: 27,335-27,353.

Schmittner, A., Yoshimori, M. and Weaver, A.J.  2002.  Instability of glacial climate in a model of the ocean-atmosphere-cryosphere system.  Science 295: 1489-1493.

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

Stocker, T.F. and Schmittner, A.  1997.  Influence of CO2 emission rates on the stability of the thermohaline circulation.  Nature 388: 862-865.