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Lessons from the Ice Ages?

What can be learned from all of this? The first lesson from the Ice Ages is that the rate of change is all-important, as we have seen in the last section. Sudden warming can melts glaciers and produce a freshwater layer in the oceans, re-enforced by a warm-water layer. This makes for stable stratification in the high-latitude ocean. In turn, this changes circulation and the associated heat transport in ways that are hard to predict.

One type of change, which emerges from computer model experiments, is that the sinking of cold water in the Norwegian Sea can be stopped by rapid warming or by meltwater input. Since much of the heat now being delivered to Norwegian shores depends on bringing in warm water from the south and cooling it, there is a question whether the privileged position of Norway (and Sweden and Finland) would be threatened by such a stoppage. Right now, strawberries and apples grow in Norway, which shares roughly the same latitudes with ice-covered Greenland. Would snow and ice expand if the warm water from the south is no longer pulled in to be cooled and exported as frigid deep water?

Sudden, strong warming may activate other instabilities in the system, even unstable ice masses. While the ice masses that once covered Canada and Scandinavia (and the Barents Sea) are gone, there is plenty of ice based below sea level on Antarctica still. We don't know whether it is likely to be mobilized and at what threshold, and what might be the effects. Guesses vary greatly, and some are more alarmed than others. Rapid warming also tends to make large areas unsuitable for the vegetation adapted to their particular environment. It could lead to die-off, especially of stressed trees, from attack by insects. Dead wood is a good source of methane when it undergoes bacterial decay, and methane is a powerful greenhouse gas, as we have seen.

Finally, as seen in the ice core data from Antarctica, carbon dioxide goes up each time the climate changes rapidly in one way or the other. This effect has only been noted recently, and, again, it suggests that rapid change does not make things happen faster, it makes things happen differently. Presumably, many of the usual checks and balances available to Earth, with which it tends to stabilize existing conditions are suspended when change is very rapid. This is readily understood if we consider that many of the processes useful for negative feedback (that is, stabilizing feedback) take some time to become effective.

Many of these processes have to do with the mixing of the ocean. It takes time to mix the oceans to great depth (about a millennium) and even to moderate depth of a few hundred meters (a few hundred years). If one would take advantage of the great stabilizing flywheel that is the ocean, with respect to gases in the atmosphere or to climate in general, one needs to give the ocean time to respond.

Another aspect is that of reversibility. Rapid change can move a system quickly into a condition where positive feedback takes over and keeps things going in the same direction, no matter what the forcing process is doing. This was clearly the case in the two abrupt warming events, at the onset of the Boelling-Alleroed, and at the end of the Younger Dryas. Some, as yet unknown, process took over, and the warming became irreversible until it had run its course.

What can we expect from the climate sciences regarding these matters? In some respects, a lot. In other respects, rather little. We can expect a better understanding of the processes governing climate change, both from careful global observation of the changing climate and from experiments with ever-improved computer models, which show us where we need more information. However, we cannot expect a precise understanding of the phenomenon of abrupt climate change, as we see it in the record of the polar ice caps, for example. Threshold effects are rarely predictable. Well-known examples in the earth sciences are earthquakes, volcanic eruptions, El Niño events, toxic algal blooms, and hurricanes. These things happen unannounced or in any case with but little warning. Abrupt climate change, as exemplified in the deglaciation period, differ in scale but not in principle.

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