W. S. Broecker, "What If the Conveyor Were to Shut Down? Reflections on a Possible Outcome of the Great Global Experiment," GSA Today 9(1):1-7 (January 1999). See also http://www.geosociety.org/pubs/gsatoday/gsat9901.htm.
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What If the Conveyor Were to Shut Down?
Reflections on a Possible Outcome of the Great Global Experiment
W. S. Broecker
Lamont-Doherty Earth Observatory, Palisades, NY 10964
Suggestions that the ongoing greenhouse buildup might induce a shutdown of the ocean's thermohaline circulation raise the questions as to how Earth's climate would change if such an event were to occur. The answer preferred by the popular press is that conditions akin to those that characterized the Younger Dryas--the last kiloyear cold snap--would return. But this extreme scenario is an unlikely one, for models suggest that in order to force a conveyor shutdown, Earth would have to undergo a 4 to 5°C greenhouse warming. Hence, the conditions at the onset of the shutdown would be very different from those that preceded the Younger Dryas. Thus, it is unlikely that new climate conditions would be nearly so severe. Unfortunately, because no atmospheric model to date has been able to create the observed large and abrupt changes in climate state of Earth's atmosphere, we lack even the crudest road map. However, as was the case for each of the abrupt changes recorded in Greenland's ice, if the conveyor were to shut down, climate would likely flicker for several decades before locking into its new state. The consequences to agricultural production of these flickers would likely be profound.
Past shutdowns of the Atlantic Ocean's conveyor circulation appear to have played a key role in triggering the large and abrupt global climate changes that punctuated the last period of glaciation including the millennial duration Younger Dryas (Broecker and Denton, 1990). Modeling studies suggest that the ongoing greenhouse warming and consequent strengthening of the hydrologic cycle might trigger yet another such shutdown (Manabe and Stouffer, 1993; Stocker and Schmittner, 1997). To most science writers, this result has been construed as implying that conditions similar to those that prevailed during the Younger Dryas cold event would return. Were this analogy correct, then indeed a shutdown of the conveyor would have awesome consequences. Iceland would become one large ice cap. Ireland's climate would be transformed to that of Spitzbergen. Winters in Scandinavia would become so cold that tundra would replace its forests. The Baltic Sea would be permanently ice covered, as would much of the ocean between Greenland and Scandinavia. Further, the impacts of such a mode change would not be limited to the northern Atlantic basin; rather, they would extend to all parts of the globe (see Fig. 1). Rainfall patterns would dramatically shift. Temperatures would fall. The atmosphere would become dustier. Finally, the transition to this new state would be completed in decades, and very likely during this transition period, climate would flicker.
But is it realistic to believe that a shutdown of the conveyor a century or so from now would produce the conditions that characterized the last glacial period? The answer is very likely "no," for several reasons. The first has to do with the fact that during the Younger Dryas, Canada and Scandinavia still had sizable ice caps. The second is that the abrupt part of the warming at the close of the Younger Dryas brought climate only about halfway to its interglacial state (Severinghaus et al., 1998). The other half of the transition was more gradual, reflecting perhaps the post-Younger Dryas retreat of the residual ice caps in Canada and Scandinavia. Finally, modeling studies (Manabe and Stouffer, 1993; Stocker and Schmittner, 1997) that forecast a greenhouse-induced conveyor shutdown do so only after a substantial global warming (4 to 5°C) has occurred. Hence, the global climate conditions prevailing at the time of the shut-down would be substantially warmer than those that existed just before the onset of the Younger Dryas. For these reasons, the analogy to the conditions that prevailed during the Younger Dryas surely constitutes a worst case scenario.
Figure 1. Map showing locations where abrupt climate changes (i.e., Dansgaard-Oeschger events) have been documented in records kept in marine sediments or polar ice (red and blue dots). Yellow dots show those locations where the last of these events (i.e., Younger Dryas) is recorded by major advances of mountain glaciers. While for most of the globe, these events are in phase, in parts of the Southern Ocean and of the Antarctic ice cap, they are clearly antiphased. This switch in phasing at high southern latitudes appears to reflect a seesawing of deep-ocean ventilation between the northern Atlantic and the perimeter of the Antarctic continent.
If the climate change from Younger Dryas to present is not an apt analogy to that which would accompany a conveyor shutdown, then how might we go about estimating the consequences of such an event? As noted by some readers of my papers that warned of a possible green-house- induced conveyor shutdown (Broecker, 1997a, 1997b), I stopped short of presenting a specific scenario, for I was fully aware of the pitfalls associated with any such attempt.
ALLERØD-YOUNGER DRYAS ANALOGY
A less imperfect analogy to what might happen if the conveyor were to shut down is the climate change that accompanied the abrupt transition from the near interglacial conditions that prevailed during the Allerød to the cold conditions that prevailed during the Younger Dryas (see Table 1). The reasons are as follows. First, this transition represents a shutdown rather than a start-up of the conveyor. Second, the melting of the Northern Hemisphere's residual ice caps nearly halted during the Younger Dryas. Hence this analogy is flawed neither by the influence of changing ice cap size nor by that of changing sea level. But it is flawed in that the base state (i.e., the Late Allerød climate) was different from today's and even more different from that which would prevail at the time of a greenhouse-induced conveyor shutdown. Nevertheless, it is worthwhile to compare the climate of the late Allerød with that of the Younger Dryas.
The contrast between climate conditions during the warm Allerød and cold Younger Dryas is recorded in four major ways (see Fig. 2): (1) pollen and beetle remains in lake and bog sediments tell us about differences in continental temperature, (2) moraines formed during the Younger Dryas record advances of mountain glaciers, (3) planktonic foraminifera shells in marine sediments document decreases in surface ocean temperature, and (4) the oxygen isotope records kept in ice and lacustrine calcium carbonate record shifts in hydrological conditions. These records send a consistent message. Conditions during the Allerød were nearly as warm as those that characterized the Holocene. As clearly shown by pollen records, the beginning of the Bolling-Allerød marked a worldwide transition from glacial to interglacial conditions. The lapse back to cold conditions during the Younger Dryas, while documented at many localities throughout the world, has a puzzling signature. It is clearly recorded by the descent of mountain snowlines in the American Rockies (Gosse et al., 1995), in the Swiss Alps (Ivy-Ochs et al., 1996), in the tropical Andes (Van der Hammen and Hooghiemstra, 1995; Clapperton et al., 1997), and in the New Zealand Alps (Denton and Hendy, 1994). The oxygen isotope records in Swiss (Eicher and Siegenthaler, 1983) and Polish (Goslar et al., 1995) lakes, tropical mountain glaciers (Thompson et al., 1995) and in the Greenland ice sheet (Dansgaard et al., 1993) make clear that the hydrologic cycle in the region surrounding the northern Atlantic operated quite differently during the cold episodes (late glacial and Younger Dryas) than during the warm episodes (Allerød and Holocene). That these differences in the hydrologic cycle extended well beyond the region around the northern Atlantic is suggested by the substantially lower rate of global methane production during the Younger Dryas as recorded in ice cores from Antarctica and Greenland (Chappellaz et al., 1993; Brook et al., 1996). As the methane content of the atmosphere is set by the areal extent and temperature of the world's wetlands, these systems must on the average have been drier and colder. The dust record preserved in Greenland ice implies that storminess in the Asian deserts from which the dust has been shown to originate (Biscaye et al., 1997) must have been more intense during the Younger Dryas than during the Allerød.
Finally, the benthic oxygen proxy for the deep Santa Barbara basin (Behl and Kennett, 1996) for the Arabian Sea (Schulz et al., 1998) and for the Cariaco Trench (Hughen et al., 1996, 1998) suggests major alternation in thermocline ventilation between these times. In contrast, the Younger Dryas is weakly expressed in many pollen records, giving rise to numerous claims that it didn't cause significant climate change outside northern Europe. Even in Switzerland, where the snowline descent and 18O change are large and thoroughly documented, the Younger Dryas pollen change is muted. One interpretation for this seeming dichotomy is that while its impacts were global, the Younger Dryas was not simply a return to glacial state. Rather, it lacks an analog and represents yet another mode of operation of the Earth system.
One other aspect of the Allerød- Younger Dryas oscillation must be mentioned. Ice cores from the polar plateau in Antarctica reveal that the millennial-duration climate changes that punctuated the last glacial period were antiphased with respect to those elsewhere in the world (Blunier et al., 1998). During the Allerød, the ongoing warming of the polar plateau came to a halt. Then, at approximately the time of the onset of the Younger Dryas, the warming commenced once again at an even steeper rate than that in progress before the Allerød pause. Based on reconstructions of the radiocarbon content of surface ocean carbon, Hughen et al. (1996) clearly demonstrated that at the onset of the Younger Dryas, the Atlantic's conveyor circulation must have shut down, allowing newly produced 14C to be backlogged in the atmosphere and upper ocean. Then, 200 years later, the backlogging ceased and the excess 14C in the atmosphere and upper ocean was gradually drained back down. I suggested that this drain-down was caused by the inception of a new mode of deep water formation in the Southern Ocean, and that this new mode delivered extra heat to the Antarctic continent, reinitiating the stalled warming (Broecker, 1998).
When the difference in base conditions between those that prevailed during the Allerød and those that would prevail when the greenhouse warming has become sufficiently intense to threaten a conveyor shutdown is taken into account, then the picture looks quite different. As shown by the simplistic scenario presented in Figure 3, while conditions in the northern Atlantic basin would likely become cooler than now, for the rest of the world this change might only ameliorate part of the accrued greenhouse warming. But of course, even if the temperature change could be adequately assessed, we would still lack information regarding those aspects of the climate change which would matter the most (rainfall patterns, soil moisture, storminess, dustiness, etc.). One must keep in mind that as the physics of mode changes is so poorly understood, diagrams such as that in Figure 3 are unlikely to portray what would happen if the Earth system were to undergo a mode switch. The consequences of such a change defy prediction.
The last point to be made is that the Allerød to Younger Dryas transition was punctuated by flickers (see Fig. 4). Electrical conductivity measurements on the GISP2 ice core (Taylor et al., 1993a, 1993b) show that the onset of the Younger Dryas was marked by a period of increased dust fall onto the Greenland ice cap which lasted for about 5 years. This brief dust episode was followed by a several-year-long respite. Then came a second and a third episode each followed by respites. Finally, about 45 years after the onset of
Figure 2. Records demonstrating the profound change in climatic conditions that occurred in the northern Atlantic basin between the Bølling-Allerød (BA) warm interval and the Younger Dryas (YD) cold interval. Left: temperature record based on beetle carapaces (Atkinson et al., 1987). Center: oxygen isotope records from Greenland ice (Dansgaard et al., 1993) and from Swiss lake CaCO3 (Eicher and Siegenthaler, 1983). Right: abundance of the cold-loving planktonic foraminifera species N. pachyderma (left coiling) in the Norwegian Sea (Lehman and Keigwin, 1992).
Figure 3. Simplistic scenario of possible impact on Earth temperatures of a greenhouse-induced conveyor shutdown based on an analogy to the Allerød to Younger Dryas transition, but taking into account that Earth temperatures just prior to a greenhouse-induced shutdown would be several degrees warmer than those that prevailed during the Allerød. While this change would likely cause temperatures around the northern Atlantic basin to drop below their present values, for the rest of the world, it would merely alleviate some part of accrued greenhouse warming. While seemingly comforting, this analogy says nothing regarding all-important changes in the hydrologic cycle, which would surely accompany such a mode change.
WHAT TRIGGERS THERMOHALINE REORGANIZATIONS
The trigger for the precipitous Younger Dryas cooling as first proposed by Rooth (1982) was likely the large pulse of fresh water released into the northern Atlantic as a result of the sudden switch in the outlet of proglacial Lake Agassiz from the Mississippi to the St. Lawrence drainage. This switch was triggered by the retreat of the Laurentian ice cap, which formed the northern shoreline of the lake. When the ice dam gave way, the lake surface dropped in a series of steps by about 100 m (Teller and Thorliefson, 1983). The water released flooded eastward into the northern Atlantic and presumably reduced the salinity of surface waters there to the point where deep water could no longer form. Radiocarbon dating places the timing of the drop in lake level resulting from this switch at about 11000 14C yr ago (that is, within the dating uncertainty of the time of the onset of the Younger Dryas). Confirmation comes from the record kept in Gulf of Mexico sediments, which reveals that a reduction in the input of low 18O meltwater from the Mississippi occurred at close to this time (Broecker et al., 1989). I published a full account of this scenario as a popularized article entitled "The Biggest Chill" in Natural History (Broecker, 1987). Unbeknownst to me, the editors added the following subtitle: "When ocean currents shifted, Europe suddenly got cold."Then they went on to say, "Could it happen again?"At the time, this statement greatly annoyed me because I had carefully avoided any mention of the future in the article itself. But now in retrospect, perhaps I should forgive them.
During the course of the 50 000-yr-duration glacial period, 20 climate shifts similar to that marking the beginning of the Younger Dryas occurred. It is highly unlikely that each was driven by a sudden influx of ponded meltwater. Rather, there must have been another cause. One possibility is that these shifts were driven by a salt oscillator (Broecker et al., 1990). During times when the conveyor was off, the northern Atlantic region was extremely cold, and fresh water accumulated in the ice caps of Canada and Scandinavia rather than running off to the sea. This allowed the salinity of surface waters in the Atlantic Ocean to rise. When the density of waters in the northern Atlantic became large enough, conveyor circulation was reinitiated. Once in action, the heat released from the conveyor's upper limb caused the ice caps to recede, releasing fresh water to the Atlantic. Surface water salinities were then driven back down to that level where deep water could no longer form, causing the conveyor to shut down. Viewed in this context, one would conclude that during the Allerød, warm ice cap melting drove down the salinity of the northern Atlantic until the shutdown threshold was reached. Likely the surge of water stored in Lake Agassiz merely pushed the system over the brink; i.e., in the absence of such a surge, the system might well have reached this threshold due to the progressive reduction in salinity caused by the ice cap melting. Similarly, greenhouse-driven polar warming and strengthening of the hydrologic cycle during the coming 100 or so years may push the system over the brink once again, bringing the conveyor to a halt.
As has been emphasized by many authors (see Rahmstorf, 1996), regardless of the impetus, once the conveyor is shut down, a fresh water lid forms in the northern Atlantic, temporarily locking ocean circulation into one of its alternate modes of operation.
MODELS TO THE RESCUE?
But wouldn't predictions based on conveyor shutdowns carried out in linked ocean-atmosphere climate models be more informative than analogies to past changes? I would contend that to date no model is up to the task. No one understands what is required to cool Greenland by 16°C and the tropics by 4±1°C, to lower mountain snowlines by 900 m, to create an ice sheet covering much of North America, to reduce the atmosphere's CO2 content by 30%, or to raise the dust rain in many parts of Earth by an order of magnitude. If these changes were not documented in the climate record, they would never enter the minds of the climate dynamics community. Models that purportedly simulate glacial climates do so only because key boundary conditions are prescribed (the size and elevation of the ice sheets, sea ice extent, sea surface temperatures, atmospheric CO2 content, etc.).
In addition, some of these models have sensitivities whose magnitude many would challenge. What the paleoclimatic record tells us is that Earth's climate system is capable of jumping from one mode of operation to another. These modes are self-sustaining and involve major differences in mean global temperature, in rainfall pattern, and in atmospheric dustiness. In my estimation, we lack even a first-order explanation as to how the various elements of the Earth system interact to generate these alternate modes. One intriguing proposal implies that excess atmospheric dust lowers the mean residence time of water molecules in the
Figure 4. Electrical conductivity of the Summit, Greenland, GISP ice core measured by scraping a pair of electrodes along a fresh ice surface (Taylor et al., 1993a, 1993b). Periods of high dust fall had low conductivity because CaCO3 in the dust neutralizes proton-bearing acids carried by snow. As annual layers are clearly seen, there is no question regarding duration of each episode.
The fact that we are unable to provide
satisfactory estimates of the probability
that a conveyor shutdown will occur or of
its consequences is certainly reason to be
extremely prudent with regard to CO2
emissions. The record of events that transpired
during the last glacial period sends
us the clear warning that by adding greenhouse
gases to the atmosphere, we are
poking an angry beast (Fig. 5).
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