Consider the work of Helmke et al. (2002), which we discussed in our Editorial of 30 Jan 2002.  After constructing a half-million-year history of late-Pleistocene climate from data derived from a deep-sea sediment core obtained from a well-studied ice-rafted debris belt in the Northeast Atlantic, they identified three distinct levels of climate variability: high, during periods of ice sheet growth and decay; medium, during glacial maxima; and low, during what Helmke et al. call "peak interglaciations," or periods of greatest warmth, as had earlier been found by Oppo et al. (1998) and McManus et al. (1999), which findings are indicative of the fact that earth's climate becomes not less stable, but ever more stable, as temperatures rise higher and higher.

So why is this the case?  Schmittner et al. (2002) suggest that "reduced calving of icebergs into the North Atlantic after a widespread ice sheet surge constitutes a trigger for the rapid glacial warming events," which typically occur a few hundred years later.  This triggering mechanism grows out of their model-predicted finding that the stability of the thermohaline circulation during glacial periods is much reduced from that which prevails during interglacials, such as the one that has prevailed for the past 10,000 years.

Further support for this latter concept is provided by the modeling work by Ganopolski and Rahmstorf (2001, 2002) and Alley and Rahmstorf (2002), which suggests that the North Atlantic branch of the global thermohaline circulation possesses two potential modes of operation during glacial times: a cold stable mode and a warm marginally unstable mode, the latter of which typically lasts for but a few hundred years.  The cold stable mode is characterized by deep-water formation south of Iceland; while the warm unstable mode is characterized by deep-water formation in the Nordic Seas and shares many characteristics with the circulatory mode of the current interglacial, although it is not quite as strong.

All else being equal, the cold stable mode of the ocean's thermohaline circulation would be expected to persist throughout an entire glacial period.  However, as Ganopolski, Rahmstorf and Alley (GRA) note, a weak real-world forcing with a periodicity on the order of 1500 years produces small cyclical variations in freshwater input to high northern latitudes at approximately the same periodicity; and these perturbations, when in the declining phase, often, but not always, initiate a transition to the warm unstable mode of thermohaline circulation, which includes a shift in the location of most deep-water formation from south of Iceland to the Nordic Seas.  This new mode of circulation (warm unstable, which is accompanied by rapidly warming air temperatures) then persists for a few hundred years before reverting back (because of its inherent instability) to the cold stable mode of circulation (and its accompanying colder air temperatures).

An interesting aspect of this regularly-recurring rapid-warming followed by slower-cooling scenario is that the cyclical perturbation that leads to the change in the ocean's mode of thermohaline circulation is directly responsible for only a small fraction of the change in deep-water formation that is required to trigger the rapid warming events.  The rest, according to Ganopolski and Rahmstorf (2002), comes from the background noise in the climate system, which "triggers the events and thus amplifies the weak cycle into major climatic shifts with global reverberations," as per the concept of stochastic resonance.

These observations suggest that a very weak radiative perturbation may well have the ability to produce large changes in earth's climate under glacial conditions; and one such forcing factor that presents itself to our minds is solar variability.  This thought has also presented itself to GRA.  Ganopolski and Rahmstorf (2001), for example, state that the low-amplitude cycle in freshwater forcing responsible for the large-amplitude cyclical changes in glacial climate could be "ultimately due to solar variability," while Alley and Rahmstorf (2002) say that "a possible cause could be a weak periodic variation in the output of the sun."  Bond et al. (2001) have actually committed themselves to this conclusion, particularly as it applies to the reduced-amplitude climatic oscillations of the Holocene, for which period of time they have assembled a vast array of compelling evidence that essentially proves the robustness of the sun-climate connection.

Another interesting thing about the Holocene, according to Ganopolski and Rahmstorf (2002), is that its climate, as described by their model, "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."  Also, as they correctly report, "there is no [real-world] evidence for regime switches during the Holocene," the one possible exception being the abrupt century-long cold event that occurred a little over 8,000 years ago (Alley, 2000), which might possibly have been unrelated to any external forcing (Hall and Stouffer, 2001).

This is also the conclusion of many other climate scientists, such as Bard (2002), who describes glacial-period episodes of rapid climate change where "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."  Likewise, Clark et al. (2002) say that palaeoclimate data and model results "indicate that the stability of the thermohaline circulation depends on the mean climate state," which is such that rapid warmings only occur during glacial times.  On a much smaller scale, Manrique and Fernandez-Cancio (2000) have additionally noted that climate variability during the Little Ice Age was much greater than that which prevailed during the Modern Warm Period.

When all is said and done, therefore, this is the lesson to be learned: Holocene climate is not susceptible to catastrophic increases in temperature.  Hence, when you hear someone say CO2-induced global warming will be so great and occur so fast that many species of plants and animals will not be able to migrate poleward in latitude or upward in elevation rapidly enough to avoid extinction, believe it not.  Over all the interglacials of the past, there has never been such a warming, which suggests that none is looming on the horizon now.

References
Alley, R.B.  2000.  Ice-core evidence of abrupt climate changes.  Proceedings of the National Academy of Sciences USA 97: 1331-1334.

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

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

Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G.  2001.  Persistent solar influence on North Atlantic climate during the Holocene.  Science 294: 2130-2136.

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.

Hall, A. and Stouffer, R.J.  2001.  An abrupt climate event in a coupled ocean-atmosphere simulation without external forcing.  Nature 409: 171-174.

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.

Manrique, E. and Fernandez-Cancio, A.  2000.  Extreme climatic events in dendroclimatic reconstructions from Spain.  Climatic Change 44: 123-138. Climatic Change 44: 123-138.

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.

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.