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Abrupt Climate Change:
Not on the Human-Induced Agenda

Volume 6, Number 14: 2 April 2003

Large, abrupt, and widespread climate changes with major impacts have occurred repeatedly in the past, when the Earth system was forced across thresholds. Thus begins the abstract of a major new review of the subject of rapid climate change (Alley et al., 2003), which was written by the authors of an earlier National Research Council (NRC) report dealing with the same topic (Alley et al., 2002).

The new review article and the NRC report that inspired it contain both reasonable and illogical proposals. On the sensible side of the ledger is Alley et al.'s (2003) suggestion that policy-makers should consider "improving monitoring systems, and taking actions designed to enhance the adaptability and resilience of ecosystems and economies." Since no one can accurately predict the future trajectory of world climate -- and many cannot even agree what has happened in the past, most notably with respect to the temperature history of the planet -- it should be clear that reliable meteorological monitoring systems are definitely needed. And since we cannot yet actually do anything about either weather or climate, adaptation to what nature brings our way is the only viable option for making the best of whatever climate surprises might possibly confront us in the future.

On the logically-deficient side of the ledger is the authors' suggestion that "human forcing of climate change is increasing the probability of large, abrupt events." It is our contention that not only is this suggestion incorrect, it is fully 180 degrees out of phase with reality.

Consider, for example, the study of Helmke et al. (2002), who developed a history of late Pleistocene climate variability from an analysis of a deep-sea sediment core retrieved from a well-studied ice-rafted debris belt in the northeast Atlantic Ocean. This exercise led them to detect and quantify three distinct levels of climate variability that have been operative over the past half-million years. Their findings? Maximum climate variability occurred during times of either ice sheet growth or ice sheet decay, medium climate variability was the norm during glacial maxima, while minimum climate variability was observed during what Helmke et al. call "peak interglaciations," which are essentially periods of greatest warmth.

Similar conclusions were reached by Oppo et al. (1998) and McManus et al. (1999), also as a result of analyzing real-world data. What is more, Alley et al.'s main model-based scenario of possible human-induced abrupt climate change -- which the NRC authors link to the exceeding of some threshold value of the ocean's thermohaline circulation, which they say could be caused by "warming and associated changes in the hydrological cycle" -- has been challenged by another model study. Based on a set of sensitivity analyses of the response of the ocean's thermohaline circulation to the freshening of North Atlantic surface water that could be caused by the predicted warming of the 21st century, Rind et al. (2001) 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," noting additionally that "the effect is not rapid."

Clearly, if we are dispassionate in the application of logic, and if we really want to do something to reduce the likelihood of abrupt climate change, real-world data pertaining to the planet's palaeoclimatic history (as well as some climate model work) tell us we should be attempting to prevent global cooling. And, in fact, that is precisely what we are doing via our burning of massive quantities of coal, gas and oil.

So how is this grand -- but unplanned -- endeavor progressing? Actually, not very well, for the planet's temperature is running well below "normal." Based on the Antarctic ice-core study of Petit et al. (1999), the present interglacial is more than 2C cooler than all four of the interglacials that preceded it. Although earth's temperature may have risen half a degree C over the last century or so, we can take little credit for that development, as the bulk of the warming was likely a result of the most recent natural upswing of the probably-solar-induced millennial-scale climatic oscillation that is a persistent feature of both glacial and interglacial epochs alike, the two prior upswings of which resulted in the establishment of the Medieval Warm Period and the antecedent Roman Warm Period. In addition, the planet is still considerably cooler than it was throughout the great "climatic optimum" of the mid-Holocene, which itself defined much of the mean temperature of the current interglacial that falls far short of the corresponding mean temperature of the prior four interglacials.

Where does all of this leave us? On the one hand, logic and real-world data suggest that the potentially catastrophic abrupt climate changes discussed by Alley et al. would best be avoided by either maintaining the climatic status quo or by warming. On the other hand, the predictions of manifestly imperfect climate models -- which sometimes predict nearly opposite outcomes, as in the case of the response of the ocean's thermohaline circulation to global warming -- are used by climate alarmists and a host of politicians to mandate measures to resist warming.

These two positions are about as different from each other as night and day. Hence, it should not be too difficult for a reasonably rational person to decide which is the more correct. The question of paramount importance for each of us, therefore, is this: Am I a reasonably rational person?

Sherwood, Keith and Craig Idso

Alley, R.B., Marotzke, J., Nordhaus, W.D., Overpeck, J.T., Peteet, D.M., Pielke Jr., R.A., Pierrehumbert, R.T., Rhines, P.B., Stocker, T.F., Talley, L.D. and Wallace, J.M. 2002. Abrupt Climate Change: Inevitable Surprises. National Research Council, National Academy Press, Washington, DC.

Alley, R.B., Marotzke, J., Nordhaus, W.D., Overpeck, J.T., Peteet, D.M., Pielke Jr., R.A., Pierrehumbert, R.T., Rhines, P.B., Stocker, T.F., Talley, L.D. and Wallace, J.M. 2003. Abrupt climate change. Science 299: 2005-2010.

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.