In our Journal Review of the paper of Rind et al. (2001), we discuss this subject and report how the conclusions of the NAS Committee fly in the face of recent modeling work of the Goddard Institute for Space Studies.  Based on sensitivity analyses of the response of the thermohaline circulation to North Atlantic surface water "freshening due to the [predicted] warming climate of the next century," the Goddard group concluded that one of the two major driving forces of the thermohaline circulation, i.e., North Atlantic deep water formation, "decreases linearly with the volume of fresh water added," and that it does so "without any obvious threshold effects," additionally noting that "the effect is not rapid."

Although no one ever chided us about the fact that we were merely pitting one human construct against another in that Journal Review, the approach we used is not very satisfying; for one could well ask why either of the conflicting model-based theories should be believed, particularly when we do not accept certain of the other climatic projections of the Goddard modeling group.  Hence, it is always better to use real-world data to dispatch unfounded climate claims such as those of the NAS Committee; so we now will do a little of that, using as a springboard the recently-published data-driven study of Helmke et al. (2002).

In this newly-published analysis, the authors reconstructed the evolution of late Pleistocene climate variability on millennial timescales from a half-million-year-long deep-sea sediment core obtained from a well-studied ice-rafted debris belt in the Northeast Atlantic.  This exercise led them to identify three distinct levels of climate variability, the most dramatic of which typically occurred during times of either ice sheet growth or ice sheet decay.  Medium climate variability, on the other hand, was the norm during glacial maxima, while the most stable of all climates were those associated with what Helmke et al. call "peak interglaciations" or periods of greatest warmth.  Similar data-driven conclusions had previously been reached by Oppo et al. (1998) and McManus et al. (1999), whose findings we have also reviewed.

Clearly, these several real-world observations suggest just the opposite of what is claimed in the report of the Committee on Abrupt Climate Change, as the data of all three studies indicate that earth's climate becomes not less stable, but ever more stable as temperatures rise to their interglacial maximums.  And it is worth pointing out, in this regard, that the temperatures of all four of the interglacials that preceded the one in which we currently live were warmer than the present one, and by an average temperature in excess of 2°C, as determined from the real-world data of Petit et al. (1999).  Hence, even if the earth were to continue its century-long recovery from the global chill of the historically-recent Little Ice Age, that warming will not only not lead to the type of climate instability suggested by the NAS Committee, but rather to a state of enhanced climate stability that will likely be noted for its "inevitable non-surprises."

An interesting refinement to these observations provided by Helmke et al. is that past periods of maximum climate variability coincided with times when global sea level was approximately 40% of its value during the last glacial maximum, which they say is indicative of "threshold behavior."  This finding, too, agrees with previous studies, suggesting, in the words of the authors, "that a minimum amount of continental ice is required to sustain prominent climate fluctuations on millennial time scales."  Hence, it is very clear, as they state in their final conclusions, that large-amplitude climate variations - such as those discussed by the NAS Committee on Abrupt Climate Change - "are restricted to times when continental ice volume exceeds a threshold, equivalent to sea level at 40% of its value during the last glacial maximum," which is clearly not the situation we are faced with today.

In conclusion, although there does indeed appear to be a threshold at which abrupt climate changes can - and typically do - occur, that threshold occurs at a much lower temperature than that of the present, as opposed to the slightly higher temperature suggested by the NAS Committee.  Indeed, rather than being a cause of large and rapid changes in climate, global warmth is a proven protection against such adverse climatic transitions.

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

Helmke, J.P., Schulz, M. and Bauch, H.A.  2002.  Sediment-color record from the northeast Atlantic reveals patterns of millennial-scale climate variability during the past 500,000 years.  Quaternary Research 57: 49-57.

McManus, J.F., Oppo, D.W. and Cullen, J.L.  1999.  A 0.5-million-year record of millennial-scale climate variability in the North Atlantic.  Science 283: 971-974.

Oppo, D.W., McManus, J.F. and Cullen, J.L.  1998.  Abrupt climate events 500,000 to 340,000 years ago: Evidence from subpolar North Atlantic sediments.  Science 279: 1335-1338.

Petit, J.R., Jouzel, J., Raynaud, D., Barkov, N.I., Barnola, J.-M., Basile, I., Bender, M., Chappellaz, J., Davis, M., Delaygue, G., Delmotte, M., Kotlyakov, V.M., Legrand, M., Lipenkov, V.Y., Lorius, C., Pepin, L., Ritz, C., Saltzman, E. and Stievenard, M.  1999.  Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica.  Nature 399: 429-436.

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.