Might Catastrophic Cooling be Triggered by Greenhouse Warming
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This is the unedited version of an article published under the title "Greenhouse and Icehouse" in the Whole Earth Review 73:106-111 (1991). Copyright 1991 by William H. Calvin. Personal copying permitted; inquire otherwise ([email protected]). Might Catastrophic Cooling be Triggered by Greenhouse Warming? William H. Calvin A recurrent nightmare for some scientists is to imagine Europe suddenly deprived of its customary wintertime bonus of tropical heat, traditionally delivered courtesy of the North Atlantic Current. As it happens, that shutoff scenario doesn't require a catastrophe-prone imagination: it's already happened many times in the past. What's new is how the global warming might, paradoxically, trigger yet another abrupt episode of continental cooling. If you are flying from Paris to Seattle (or London to Los Angeles), you look down on the North Atlantic Current sweeping up from the warm tropics to the vicinity of Iceland (well, at least you see the clouds that it encourages, drifting toward Europe). This current (Fig. 1), with only a minor contribution from the Gulf Stream, is what keeps Europe wet and warm. After all, judging from its northerly latitudes and the associated sunshine, Europe really ought to be like Canada. Or perhaps Siberia. While Canada is a very nice place, its agriculture only supports about 4 percent as many people as Europe's climate sustains (France alone has twice Canada's population). Such is the difference made by the North Atlantic Current. The day before my most recent transatlantic overview, while browsing the library at La Cit‚ des Sciences et de l'Industrie, I came upon the article by Mikolajewicz et al. in the 14 June 1990 issue of Nature. And it ruined my last night in Paris. Even though the authors do not discuss abrupt climate shifts of the past or their mechanisms, the results of their simulations of ocean circulation are disturbing to anyone who has been following the news about ice age climates. The predicted changes in the North Atlantic Ocean in the next few decades of greenhouse warming of the atmosphere are like those that others, have suggested were responsible for the last major episode of cold and arid climate in Europe (eastern North America was affected to a lesser extent). If global warming can trigger abrupt cooling, that's far more alarming than the now-familiar predictions of slowly rising sea level and slowly thinning ozone. Minor fluctuations in North Atlantic climate a thousand years ago were responsible for why Iceland was not named Greenland and vice versa (by the time that the coast of Greenland was settled by explorers from Iceland, things had warmed somewhat). Then came the Little Ice Age which wiped out the non-Inuit settlements on Greenland. Yet neither such changes of the last millennium, nor the occasional multiyear drought, is what is meant by abrupt climate change. The most recent abrupt episode was the Younger Dryas: in the midst of the rising CO2 and the general warming trend that melted the ice sheets of the last glaciation, there was a "cold spike" that lasted about 800 years. It caused European forests to die within a decade or two; they were replaced with Arctic-adapted plants such as Dryas. It caused Scotland's glaciers to form once again. Because this happened when northern hemisphere summer sunshine was near its astronomical maximum, it served to alert climatologists that there was more to ice advance than just the familiar Milankovitch cycles (it has since been discovered that southern hemisphere ice sheets also fail to follow the predictions; rather, they often advance at the same times that northern hemisphere glaciers do). A good time for rapidly melting all that ice in the northern hemisphere is, just as Milankovitch predicted, when the earth's axial tilt is maximal and the earth's closest approach to the sun occurs in June -- but there appears to be something else going on that can occasionally override this general pacemaker of the ice cycles. The Younger Dryas cooling started 11,500 years ago; it lasted until 10,700 years ago, when it ended even more suddenly than it began. Thanks to the year-by-year detail in the ice cores of Greenland studied by Dansgaard et al., we know that rainfall returned over a 20 year period and, as Europe's land surface warmed up, the formerly severe winter storms diminished dramatically in that same two-decade-long period. As can be seen in the top of Figure 2, cooling episodes are just as rapid (though often with associated hot-and-cold "whiplash" chattering). Once triggered, mode- switching climatic "leaps" evidently operate on a far faster time scale than 20,000-to-100,000 year Milankovitch cycles, faster even than the century- long time scale of the predicted greenhouse warming. What triggers such an abrupt change in climate? Disturbing the Waterfall What the sleep-depriving ocean current simulations2 implied is that, in response to greenhouse warming, northern Atlantic surface salinity will decrease, deep water production will drastically decrease in the ocean just south of Iceland, and surface temperatures will drop. Unless you know about ice age climates and salt economies, that combination might not seem noteworthy. Those three factors are, alas, the conditions earlier proposed by Broecker and coworkers3 as having encouraged the onset of the Younger Dryas. Their theory is akin to traditional ones for how extra rainfall reverses the salt circulation in estuaries (and in the Mediterranean during the last Pluvial), just scaled up to an entire ocean. One says "encourage" rather than "cause" because cause-and-effect reasoning can be tricky, given that nonlinear systems often chase their tails. That is a particularly apt description of the wintertime North Atlantic Current (Fig. 1): it even does a vertical U-turn. Northbound, it rises to the surface near Iceland and releases a bonus of heat to the Europe-bound winds from Canada; this contribution is equal to 30 percent of what sunshine provides to the northern Atlantic4. Then the current -- now so cold and hypersaline that it is denser than any layer of underlying water -- plunges from the surface to the abyss. Once the dense water has sunk under its own weight to the sea floor, it flows south (just as Benjamin Thompson -- Count Rumford -- predicted back in 1800) -- and so attracts even more warm currents north to replace it. It is unfortunate that there isn't a giant northern Atlantic whirlpool or waterfall for television crews to focus upon, as commentators somberly warn of its key role in Europe's viability. Suffice it to say that this "deep water production" is equal in magnitude to 20 times the combined flow of all the rivers of the world4 -- and that three-quarters of it might disappear in only 35 years, according to those new greenhouse simulations2. That's why I had no appetite for dinner, that last night in Paris. In the boolean logic demonstration at La Cit‚ (as I remembered in the middle of the night), hydraulic currents of colored water in glass columns switch themselves on and off. So can the North Atlantic Current: despite its enormous momentum, this salty stream stops flowing, ceasing to transport tropical heat north for airborne transfer east to Europe. It did that during the last major glaciation, and again during the Younger Dryas; one supposes that another such shutdown will be associated with the beginning of the next ice age (and our present interglacial period has already lasted as long as the previous one, between 128,000 and 118,000 years ago). What might derail the conveyor? The obvious way to break the loop of warm water chasing hypersaline water, and thus turn off the virtual waterfall, is to flood the surface of the northern Atlantic with a deluge of freshwater. Say, from a lake of glacial meltwater impounded behind a dam that breaks. Or, more gradually, just from changes in rainfall patterns, presumably a major factor in the freshening seen in those greenhouse simulations (which didn't take such nonlinear ice dynamics into account). Shutdown can also be achieved by smothering: floating ice blanketing the northern Atlantic might prevent the wind-driven evaporation that makes the surface waters so hypersaline and heavy every winter. Once interrupted by one means or another, the loop that warms Europe might take some time to get started again, awaiting climate fluctuations that carry ocean currents into initial conditions for a diving loop. Tail-chasing loops can be harder to initiate than to maintain. Of Floods and Firehoses Broecker et al. have recently examined whether the Younger Dryas was associated with the partial drainage of a giant midcontinental lake of meltwater, flooding the North Atlantic via the St. Lawrence River's outlet. Some of those worried about repeat performances of the Younger Dryas have likely welcomed the Lake Agassiz theory with a sense of relief, since massive amounts of meltwater are no longer available for release from the Canadian coastline (or, for that matter, from the Scandinavian). But the coral-reef record of late glacial sea level change (from which century-by- century meltwater additions can be inferred) does not indicate that there was massive flooding during the Younger Dryas,,. I would also note that, especially during lowered sea level, the St. Lawrence River largely emerges southwest of the Grand Banks of Newfoundland, mixing with Atlantic waters at 45degN, hardly a major site of deep water production. One would expect the northern Atlantic to be more sensitive to freshwater flooding. Above 60degN, the vertical U-turn is presumably shaped by the sea floor rising 3000 meters from the abyss to the shallow continental shelf south and west of Iceland; fresh flows from Greenland's fjords might shut down a particularly active region of the waterfall. The enormous fjord system on Greenland's east coast can presumably produce strong freshwater flows channeled south between Iceland and Greenland (Fig. 1). This "concentrated" freshwater flow out of Denmark Strait might act something like a firehose, quenching the salty waterfall in a sensitive spot. Whether by such focal freshwater or the more diffuse dilutions (which also act by causing winter pack ice to extend further south, capping evaporation), repeat performances of the Younger Dryas shutdown might still be possible, as there are massive amounts of meltwater available from a greenhouse Greenland. The last time that I flew from Seattle to Copenhagen, the iceberg factory in Greenland's long east coast fjords at 70degN looked quite active. There were a number of meltwater lakes somewhat inland, on the shoulders of the ice sheet. Often such a lake drains because a crevasse opens up beneath it, but the sudden deluge into the depths may serve to lubricate the ice's attachment to underlying rock. This may, in turn, promote a glacial surge into the tidal waters, thereby amplifying the melting. Furthermore, each fjord (and Greenland has more than its share) is capable of temporarily housing a meltwater lake. This happens when a glacial surge comes in somewhere along one side of the narrow channel; a few years ago, the entire southern arm of Yakutat Bay in Alaska was dammed up in this manner, trapping many marine mammals in the freshening waters. After enough backup, such ice dams eventually break, releasing months-to-years of meltwater within days. So one expects a lot of month-to-month variation in freshwater flows near a deglaciating Greenland, something that would be smoothed out if the meltwater had to first traverse such long overland paths as the Mississippi River or the St. Lawrence River. The Careful Handling of Instabilities Whatever the fate of any such theories for the Younger Dryas per se, "relief" is likely wishful thinking: it must be remembered that the Younger Dryas was only the most recent of more than a dozen abrupt climate changes in the northern Atlantic region. As Broecker noted, "The records of the last 150,000 years... scream at us that the earth's climate system is highly sensitive to nudges... By adding infrared-absorbing gases to the atmosphere, we are effectively playing Russian roulette with our climate." In addition to all the nudges from fjord floods, the northern Atlantic may also have an underlying instability, a tendency to flip. Given such predispositions, we must not let the complexities of triggering one particular episode obscure the more general problem: Understanding what attracts warm tropical waters to the northern Atlantic (which surely involves the salt circulation), what could trigger another abrupt loss of this warming (which surely includes the freshening of surface waters in the northern Atlantic), and what serves to stabilize the loop. The dynamics of such "latch-up," and the occasional "chattering" between the resulting modes when conditions are marginal, will likely be addressed by catastrophe theorists; models that utilize average melt rates, and so smooth out the major floods, may be inadequate, failing to discover shutdown and whiplash scenarios. We certainly need some assessment, and soon, of just how close we currently are to the switchover conditions. Meanwhile, those with practical experience in dealing with nonlinear systems would undoubtedly offer this cautionary rule of thumb: Avoid sudden changes. In the early days of airplane design, bumpy air (of the kind one encounters over the Atlantic) or sudden maneuvers could put an airplane into a tail spin. Sometimes the plane was shaken apart. The obvious advice to minimize unpleasant surprises: Take it slowly, unless you thoroughly understand the system (or, as with paper airplanes, can afford to engage in destructive testing). Taking it slowly is not what we have been doing, given the speed with which CO2 and greenhouse gases are increasing. Or the rapidity with which our tropical forests are being eliminated. Whether it qualifies as a nudge or a kick remains to be estimated by the theorists. We simply cannot now say exactly when the icehouse cometh, just that it will probably happen much more abruptly than Milankovitch-based thinking has envisaged. Certainly our scientific understanding of the mode-switching processes in atmosphere and ocean is far short of what we will need to stabilize the North Atlantic Current. And from the current level of resources available to researchers, you'd think that it was Antarctica that was threatened rather than Europe and the east coast of North America: I didn't see a fleet of oceanographic research ships down there studying the North Atlantic Current year-around, nor have I heard of a half-dozen supercomputer-equipped theoretical groups modeling the dynamics of the Current, nor is there a high-powered planning group evaluating technological responses, should we discover that the winter waterfall is weakening. There is no major effort in reproductive physiology to find new ways of stopping the population explosion: the increases in greenhouse gases are often secondary to more people, and it remains to be seen if current plans to clean up emissions over the coming decades will even compensate for what the worldwide population increase will add in the meantime. The 500 million people in Europe who depend on that bonus from the North Atlantic Current (perhaps 700 million: the Younger Dryas climate changes reached at least as far east as the Ukraine) have a considerable interest in preventing such unpleasant surprises as were experienced by the hunters and gatherers living in Europe 11,500 years ago. The thousand-fold population increase since then causes Europe to be particularly vulnerable to climatic shocks that arrive with little warning; the two-decade-long excursion of the proxy climate indicators (Fig. 2) should be interpreted to mean that significant changes could occur in several years. Essentially, a drought would start, get worse -- and then it would be too late for stockpiling. Regional Cooling, Worldwide Challenge But non-Europeans are vulnerable too, and not just those along the eastern shores of North America (and elsewhere around the world where repercussions of the Younger Dryas have been detected). Abrupt and widespread agricultural shortfalls in densely-populated technological societies tend to suggest lebensraum-style global conflict. Affected populations will initially switch (as they have during brief droughts of the past) to themselves eating the feed grains that now produce meat at 20 percent efficiency -- but remember how poorly an "economic response" worked for Ireland in the 19th-century famine. Another cold spike need not endure for 800 years to exhaust stockpiles and people's patience. Just imagine any country affected by the North Atlantic Current contemplating starvation -- while possessing the military technology needed to take over another country (which will undoubtedly be described by the aggressors as "irresponsibly squandering its agricultural potential while others starve"). From the Younger Dryas, one sees that regional cooling can occur in the process of global warming, that the transition can be quite abrupt, and that the duration can be far longer than the usual drought, plague, or war. Preventing another shutdown of the North Atlantic Current seems the only sensible strategy, as the climate's transition is likely to be too precipitous for peaceful economic rearrangements and population relocations. Overhauling our technology that contributes to greenhouse warming is an obvious first step, and now an even more urgent one. But we shall need to specifically address the icehouse as well, with a level of basic science that will serve to quickly suggest a variety of possible technological responses. To suppose that a climatic cooling cancels greenhouse warming, in the familiar way we fix a scalding shower by adjusting the cold water tap, is to indulge in a fool's paradise. The distribution is all important. Even if the hot water heater has been readjusted to produce warmer water, you can still get an inescapable blast of icy water -- if the hot water supply is abruptly diverted to the washing machine. We now see how a gradual greenhouse could paradoxically promote an abrupt icehouse. Whatever the generalities applicable to the long term, we need the science and technology to first survive the short term, to somehow maneuver around the whiplash conditions that might shake our civilization apart. ### The author is a neurophysiologist at the University of Washington, NJ-15, Seattle, Washington 98195, USA. Internet e-mail: [email protected] 3050 words, plus two illustrations without legends References 1. Broecker, W.S., Nature 328, 123-126 (1987). 2. Mikolajewicz, U., Santer, B. D., & Maier-Reimer, E. Nature 345, 589-593 (1990). 3. Broecker, W.S., Peteet, D.M., & Rind, D., Nature 315, 21-26 (1985). 4. Broecker, W.S., & Denton, G.H., Scientific American 262(1), 48-56 (1990). 5. Levenson, T., Ice Time (Harper and Row, 1989). 6. Imbrie, J., & Imbrie, K.P., Ice Ages (Harvard University Press, 1986). 7. Dansgaard, W., White, J. W. C., & Johnsen, S. J., Nature 339, 532- 534 (1989). 8. 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