CiRCULATiOn Changes And A ThermohaliNe Shutdown

one possibility that is raised from time to time is that global warming will bring about a slowdown or cessation of the meridional overturning circulation, sometimes called a thermohaline shutdown.11 Is such a thing likely? First recall that in chapter 4 we divided the ocean circulation up into a quasi-horizontal circulation that is primarily wind driven (the gyres) and an overturning circulation. With warming, the winds may change in

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Figure 7.7. Schema of a C02-temperature scenario. Carbon dioxide levels increase from 1900 to 2100 (period B) before leveling off (period C) because of controls on emissions. Temperature increases rapidly in period B, then more slowly in period C. At the end of period C (the year 2300 in the figure), anthropogenic emissions go to zero, and the level of C02 slowly diminishes through periods D and E back to levels close to, but probably a little above, the preindustrial period. In period D, temperature stays roughly constant for centuries before it too eventually falls back to near pre-industrial levels in period E. Many plausible scenarios can be adapted from this plot by changing 2100 and 2300 to other dates and calibrating the y-axis.

detail but almost certainly will not change in their basic structure. Thus, we can confidently predict that the gyres and their western boundary currents will remain qualitatively unchanged in the decades ahead, although certainly there might be shifts in latitude of a few degrees if the surface winds were to change by that amount.

The overturning circulation could, conceivably, change by a larger amount. Recall that one controlling factor in the intensity of the overturning circulation is the meridional buoyancy gradient at the surface of the ocean. Convection at high latitudes in the North Atlantic occurs because the surface waters there are sufficiently dense that they sink in convective plumes, and an overturning circulation ensues. Two things could happen to bring this process to a halt. First, if there were to be increased rainfall or a release of freshwater from the melting of glaciers at high latitudes, the surface waters of the ocean will freshen and become lighter, and thus become less likely to sink. Second, if the surface waters were to become warmer, then they would become lighter, and this too could lead to a cessation in the overturning circulation. Note that this latter effect will only occur when global warming has proceeded far enough to make a sufficient difference to the surface density at high latitudes. However, we would not expect it to be a permanent effect because ultimately the overturning circulation is maintained by a difference in density between high latitudes and low latitudes, and if the primary effect of global warming is simply to raise temperatures worldwide, these differences are eventually likely to be similar to what they are today. If there are permanent changes in precipitation patterns, however, the circulation could become permanently weaker if the high-latitude ocean stays fresher.

We cannot tell with any certainty whether a shutdown in the overturning circulation will occur, but recent experiments with comprehensive climate models suggest that, at least over the next several decades, a complete shutdown is quite unlikely. However, a number of climate models do show a weakening of the overturning circulation by a significant fraction, mainly caused by increased high-latitude rainfall, and then an eventual recovery. The reductions vary considerably from model to model, from no reduction to a reduction of about 50%, with an average of about 25% reduction. The worst-case scenario would be a melting of part or all of the Greenland ice sheets, bringing a surge of freshwater to the North Atlantic that might well cause overturning circulation to halt, but that is unlikely in the near or medium future.

The consequences of a severe slowdown in the overturning circulation could be quite drastic because it would mean that the ocean would transport less heat to high latitudes and, ironically, this could cause temperatures at high latitudes to fall under global warming. This change would, ultimately, most likely be a self-correcting consequence because a fall in temperature at high latitudes would create conditions conducive to the overturning circulation turning back on. In fact, however, the reductions in the overturning circulation that models predict for the twenty-first century are sufficiently mild that even in and around the North Atlantic ocean, including northwestern Europe and Greenland, where the effects are likely to most noticeable, the overall increase in temperature caused by global warming outweighs the cooling caused by the reduced ocean heat transport.

If global warming were to continue into the twenty-second century and beyond and produce a warming of several degrees celsius, then all bets are off. It is possible that the deep meridional overturning circulation would significantly weaken and possibly even fall to zero for several decades, and possibly centuries, until the deep ocean temperature warms and the meridional circulation re-establishes itself in the new climate equilibrium. The timescales for the overturning circulation to turn off and back on again are measured in decades and centuries, and if there really were no significant meridional overturning circulation for such a period, the consequences could be severe indeed, with possible wholesale changes in climate at high latitudes. It should be said that most climate models do not predict a shutdown in the foreseeable future, but the consequences could be severe if a shutdown were to happen. Global warming makes society confront possibilities that are unlikely but, if they do happen, would produce severe consequences.

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