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Rapid Climate Changes
Bard, E. 2002. Climate shock: Abrupt changes over millennial time scales. Physics Today 55(12): 32-38.

What was done
The author - a professor at College de France, where he holds the chaire d'evolution du climat et de l'ocean, and works at CEREGE in Aix-en-Provence, France - reviews what is known about abrupt climate changes over millennial time scales as deduced from both models and observations.

What was learned
With respect to what has been learned from models (but not necessarily confirmed), Bard says "it is now recognized that the ocean-atmosphere system exhibits several stable regimes under equivalent external forcings" and that "the transition from one state to another occurs very rapidly when certain climatic parameters attain threshold values." Specifically, he notes that in the models "a slight increase in the freshwater flux above the modern level F produces a decrease in the NADW [North Atlantic Deep Water] convection and a moderate cooling in the North Atlantic," but that "the system flips to another state once the flux reaches a threshold value F + deltaF," which state has no deep convection and "is characterized by surface temperatures up to 6C lower in and around the North Atlantic."

With respect to what has been learned from observations, Bard concentrates on the region of the North Atlantic, describing glacial-period millennial-scale episodes of dramatic warming called Dansgaard-Oeschger events (with temperature increases "of more than 10C"), which are evident in Greenland ice core records, and episodes of "drastic cooling" called Heinrich events (with temperature drops "of up to about 5C"), which are evident in sea surface temperature records derived from the study of North Atlantic deep-sea sediment cores. In the Greenland record, he reports that "the temperature warms abruptly to reach a maximum and then slowly decreases for a few centuries before reaching a threshold, after which it drops back to the cold values that prevailed before the warm event."

What it means
Although much is made of the role of models in studying "the complex interplay between Dansgaard-Oeschger warm phases and Heinrich cold events," Bard correctly reports that "at present, models coupling the atmosphere, ocean, and ice sheets are still unable to correctly simulate that variability on all scales in both time and space," which suggests we do not fully understand the dynamics of these rapid climate changes. Indeed, he forcefully notes that "all the studies so far carried out fail to answer the crucial question: How close are we to the next bifurcation [which could cause a rapid change-of-state in earth's climate system]?" In this regard, he also notes that "an intense debate continues in the modeling community about the reality of such instabilities under warm conditions [our italics]," which is a particularly important point, seeing that all dramatic warming and cooling events have been observed in either full glacial or transitional periods between glacials and interglacials.

This latter real-world fact clearly suggests we are unlikely to experience any dramatic warming or cooling surprises, as long as the earth does not beginning drifting towards glacial conditions, which is but another reason to not be concerned about the ongoing rise in the air's CO2 content. Indeed, it suggests that more CO2 in the atmosphere and its potential for modest warming are actually to be preferred as a preventive measure or "insurance policy" against unexpected abrupt climate changes. Interglacial warmth seems to inoculate the planet against climatic instabilities, allowing only the mild millennial-scale climatic oscillation that alternately brings the earth slightly warmer and cooler conditions typical of the Medieval Warm Period and Little Ice Age. Hence, and in light of the fact that the four preceding interglacials were able to tolerate temperatures fully 2C warmer than those of the current interglacial (Petit et al., 1999), without any adverse climatic consequences, humanity would probably be wise to not surrender the atmospheric CO2 insurance policy we worked so hard to put in place over the course of the Industrial Revolution.

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.

Reviewed 22 January 2003