Ocean Currents and Climate

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On our watery planet the climate is governed largely by the oceans. But for a long time, discussions of climate change did not take the oceans fully into account, simply because very little was known about them. It seemed that the tremendous masses of water could vary only over a geological timescale. Ideas began to change at the start of the 1960s. Studies of clay extracted from the seabed, supported by new theoretical ideas, suggested that ocean current patterns might shift within mere thousands of years. Other studies began to build a picture of the complex and surprisingly fragile circulation of the world-ocean. In the 1980s, evidence from Greenland ice cores, supported by crude computer models, showed that the North Atlantic circulation could switch radically within a century or two (or maybe less). If global warming triggered such a switch, it would wreak dire harm.


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Unpleasant Surprises? (1980-1988)

Another handle on the problem was provided by the Deep Sea Drilling Program (which was followed in 1985 by an international Ocean Drilling Program using the American JOIDES Resolution, converted from an oil-drilling ship). In expeditions across the seven seas, year after year workers pulled up thick cylinders of clay and ooze, totaling many kilometers. These were stored in "libraries" which any scientist could exploit for climate studies alongside many other topics.

Studies of fossil shells in the cores gave clues about ocean waters in the past, with a striking conclusion. It now seemed beyond doubt that there had been shifts in the North Atlantic particularly around the end of the last ice age some 11,000 years ago--a time geologists on land had long known as the "Younger Dryas" climate shift. The entire pattern of ocean circulation had evidently changed within a couple of thousand years, or perhaps only a few hundred. (1)

That resonated with studies by Willi Dansgaard, Hans Oeschger, and others using cores drilled out of the Greenland ice sheet in the early 1980s. Certain periods such as the Younger Dryas had seen very abrupt cooling around the North Atlantic, episodes so striking that they got a name of their own, the "Dansgaard-Oeschger events." Meanwhile a study of changes in microscopic deep-sea fossil species showed that the cooling had extended clear to the ocean floor. Such studies using microbiology were not given much credence at the time, however. A bit more convincing was a 1983 report, using the geochemistry of isotopes in fossils, with complex evidence pointing to "a dramatic change in ocean circulation" in the last glacial period. The deep waters of the North Atlantic had apparently grown cold and still. Scientists were being gradually pushed to think about dramatic transitions in the circulation of the North Atlantic, or even the entire world-ocean. (2)

Oeschger was particularly struck by a rapid rise in the atmospheric concentration of CO2 at the end of the last ice age, which others had recently discovered in ice and deep-sea cores. The vexing problem of how the gas got in and out of the atmosphere had intrigued him ever since 1958, when he had worked with Revelle's group at Scripps just as they were discovering that greenhouse warming was plausible. Oeschger also understood that a feedback that released more and more of the gas might accelerate the end of an ice age.

The main reservoir of CO2 was the oceans, so that was the first place to think about. In 1982 Broecker visited Oeschger's group in Bern, Switzerland, and explained current ideas about the North Atlantic circulation. Broecker also shared an intriguing new idea: the ocean's uptake of CO2 during an ice age depended on biochemical changes involving the growth and death of plankton. Oeschger reflected that Broecker's biochemical mechanism would take thousands of years to operate, too slow for the rapid changes found in ice cores. Perhaps, he thought, there had been a transition in the ocean from "a relatively stagnant state" to a state where more rapid mixing brought nutrients to the surface which changed the biochemistry.

The stagnant state might have been caused (as some had earlier speculated) by fresh water flowing in as continental ice sheets melted. That would have diluted the surface salt water until it would not sink, halting the circulation. Many questions remained, Oeschger conceded. But major circulation changes might well have been involved--perhaps triggered by some little perturbation. (3)

Oeschger was one of the first to worry that a switch between ocean circulation modes might be set off by the greenhouse gases that humanity was adding to the atmosphere. But as a colleague recalled, "his early warnings were often greeted with disbelief." Oeschger tried to find collaborators to write a paper on circulation modes for submission to a top scientific journal like Nature or Science, but he met only skepticism and gave up the effort. Two colleagues at Bern did publish a paper in Nature suggesting that "ocean circulation changes were the essential cause" of the rapid CO2 variations seen in ice cores, giving Oeschger credit for the idea, but like Broecker they concentrated on biochemical changes rather than the circulation as such. Oeschger continued to bring the idea up at scientific meetings. Broecker heard him, and his interest was stimulated. (4)

As we have seen, Broecker had been thinking for decades about possible ocean instabilities. The reports of big, rapid CO2 variations in Greenland ice cores stimulated him to put this interest into conjunction with his oceanographic interests, since nothing but a major change in the oceans could cause such a swift and global shift in the atmosphere. In fact, scientists later realized that the rapid variations seen in the ice cores had been misinterpreted. They did not reflect changes in atmospheric CO2, but only changes in the ice's acidity due to dust layers (something had indeed changed swiftly, but not necessarily the CO2 level). No matter: the error had served a good purpose, pushing Broecker to a novel and momentous calculation. Broecker recalled that one day as he sat in Bern, listening to a lecture by Oeschger describing the abrupt variations in his data, "an idea hit my brain.... As quick as that, my studies in oceanography and paleoclimatology merged." (5)

In 1985, Broecker and two colleagues published a paper in Nature titled, "Does the Ocean-atmosphere System Have More than One Stable Mode of Operation?" Crediting Oeschger as the first to suggest that the apparent CO2 changes in Greenland ice cores represented a jump between "two modes of ocean-atmosphere-biosphere-cryosphere operation," the paper continued that "it is tempting to speculate" that Oeschger's two modes corresponded to different states of the North Atlantic circulation.

Broecker and his collaborators now identified the key: it was what he later described as a "great conveyor belt" of sea water carrying heat northward. Although the GEOSECS survey of radioactive tracers had laid out the gross properties of the circulation a decade earlier, it was only now, as Broecker and others worked through the numbers in enough detail to make crude computational models, that they fully grasped what was happening. They saw that the vast mass of water that gradually creeps northward near the surface of the Atlantic is as important in carrying heat as the familiar and visible Gulf Stream. "It was an easy calculation," recalled Broecker, "and I was astounded by the amount of heat that it had." The energy carried to the neighborhood of Iceland was "staggering," Broecker explained--nearly a third as much as the Sun sheds upon the entire North Atlantic. If something shut down the conveyor belt, climate would change across much of the Northern Hemisphere. (6)

In one sense this was no discovery, but only an extension of an idea that could be traced back to Chamberlin at the start of the century. Few scientific "discoveries" are wholly new ideas. An idea becomes a discovery when it begins to look real. Broecker made that happen by providing solid numbers and plausible mechanisms. Chamberlin had speculated that the circulation could shut down if the North Atlantic surface water became less salty. Now the effect had been calculated. And Broecker pointed out geological evidence that this had actually happened, at the start of Younger Dryas times. Just then a vast lake dammed up behind the melting North American ice sheet had suddenly drained, releasing a colossal surge of fresh water into the ocean.

Broecker's portentous idea was typical of many ideas in geophysics for the way it drew upon several different areas of data and theory. His own career (as may be seen elsewhere in these essays) had rambled through a variety of fields. Ever since the days when he had trudged around fossil lake basins in Nevada for his doctoral thesis, Broecker had been interested in sudden climate shifts. The idea had remained in his mind while he studied the Atlantic Ocean's circulation as revealed by radioactive tracers, the geo-biochemistry of surface sea water as reflected in deep-sea cores, the timing of sea level changes as measured in coral reefs in New Guinea, and numerous other seemingly unrelated topics. "It's like doing a picture puzzle," he remarked. "You get stuck on one, and then it just sits there. And then along comes an idea, and you say, 'Oh my God, that's a piece that fits right there.'" The trick was to keep many pieces on the table, which meant keeping several different lines of research going at the same time. (7) When one piece fitted into another an unexpected picture could appear, like the possibility of a sudden shutdown of the North Atlantic conveyor belt.

The paper by Broecker and his collaborators made a stir among scientists, less for its new ideas than for putting forth in a plausible and dramatic way hypotheses that until then had been hazy and unappreciated. "Until now," the authors wrote, "our thinking about past and future climate changes has been dominated by the assumption that the response to any gradual forcing will be smooth." Even the most elaborate computer models of climate had shown only gradual transitions--but by their very structure that was all they could be expected to show. In the real world, when you push on something steadily it may remain in place for a while, then move with a jerk.

The numerical ocean models of the 1980s were inadequate to explore such a jerk. Even the fastest computers could still scarcely handle the immense number of calculations that even a quite simple model required. Modelers normally began with a static ocean and ran it through a few simulated decades (or if they could get enough computer time, a century or so) of "spin-up" to watch the currents establish themselves. The models did not get through even a single complete cycle of the globe-spanning circulation that interacted with climate change. As a real-world check, scientists also needed to get a much closer look at the details of the fossil climate record. "Unless we intensify research in these areas," Broecker warned, "the major impacts of CO2 will occur before we are prepared fully to deal with them." (8)

In 1987, Broecker followed up with an even more provocative Nature paper titled, "Unpleasant surprises in the greenhouse?" Here he emphasized the risk that the current buildup of greenhouse gases might set off a catastrophe. "We play Russian roulette with climate," he exclaimed. (9) Meanwhile he issued the same warning in testimony to Congressional committees, in discussions with journalists and in a magazine article. (10)

A few scientists and the science writers who listened to them began to warn that the ocean circulation might shut down without much warning, making temperatures plunge drastically all around the North Atlantic. London and Berlin are in the same latitude as Labrador, they pointed out, and would be as barren if not for the prevailing winds that pick up heat from the ocean and carry it westward. Only the more attentive members of the public heard the warning, and they could easily ignore it as just another science fiction speculation. [Later, others pointed out that Europe's climate would never get as bad as Labrador's. England is warmed by the prevailing Westerly winds that pick up the ocean's heat, part of which is simply retained heat from the summer, whereas Labrador is downwind from tundra that freezes in winter. Subsequent computer modeling suggested that a shutdown of the ocean circulation conveyor would bring significant but not drastic cooling, in North America as well as Europe.] (11)

Among the scientists and others who did pay attention to climate change, prospects of a North Atlantic shutdown became one of the most persistent concerns. A few worried that the North Atlantic region was precisely where most of the data on abrupt climate change came from, and where most of the people who thought about it lived. They wondered whether other trigger mechanisms elsewhere remained to be discovered.

"Does the ocean-atmosphere system have more than one stable mode of operation?" Broecker's question was already on the mind of computer modelers concerned with future climate change. Even before Broecker published his ideas, Kirk Bryan and others had been working up numerical simulations that included changes in ocean salinity as well as wind patterns. What they found was troubling. A 1985 study suggested that if the level of atmospheric CO2 jumped fourfold, the ocean's thermohaline conveyor belt circulation could cease altogether. (12) Another study found that even small perturbations could give rise to radically different modes of ocean circulation. In particular, a spurt of fresh water suddenly released from a melting continental ice sheet--the kind of event that some thought might have triggered the Younger Dryas--could switch the circulation pattern in as little as a century. (13)

These studies were no more than suggestive, for the models of the mid 1980s were still extremely limited. The planet might be represented in the computer as, to take one example, three equal continents and three equal oceans, extending from pole to pole like the segments of a grapefruit, with the oceans all of uniform depth and the continents without mountains. To keep computation time within reason, Bryan had to hold the cloudiness constant, although he knew clouds would interact with climate change in crucial feedbacks. Along with all that, as Bryan remarked, "uncertainties abound concerning the interaction of the ocean circulation and the carbon cycle." (14)

Syukuro Manabe and Ron Stouffer developed a coupled atmosphere-ocean model with more realistic geography. As they were varying the CO2 to see how that might change climate, they made an inadvertent discovery. If they started two computer runs with the same CO2 level and other overall physical parameters (the "boundary conditions"), but with different random "initial conditions" for the first day's weather, they could wind up with two radically different but stable states. In one state, the thermohaline conveyor belt was operating. In the other, it wasn't. The model was still packed with unrealistic simplifications, of course. Yet it seemed at least an "intriguing possibility," as they put it, that global warming might shut down the North Atlantic circulation within the next century or so, with grave implications for regional climates. (15)

Most groups still had too little computer power and too little understanding to manage full-scale models of both ocean and atmospheric circulation and to link them together. They continued to treat the oceans as a passive "swamp," which exchanged moisture with the air but did little else. That forced the model atmosphere to handle all the heat transport from the tropics to the poles, whereas in the real world ocean currents do a good share of the work. And it entirely missed how heat might sink into the ocean deeps. (16)

After 1988

Coupled ocean-atmosphere computer models improved rapidly through the 1990s, and gradually took a central role in thinking about climate change. A variety of studies showed it was quite possible that global warming would indeed shut down the North Atlantic circulation, although the process would probably take more than a century. (17) Confidence in the validity of models increased as some reproduced the striking El Niño oscillations quite well. Still more encouraging, computer specialists managed to reproduce not only the current state of the atmosphere and oceans but also, using the same models without artificial adjustments, the radically different climate that had prevailed at the height of the last ice age.

Despite these triumphs, much remained to be done before anyone could form a clear picture of how the oceans connected to long-term climate change. Perhaps the most vexing of the many difficulties was figuring in the large amount of CO2 that the ocean's plankton absorbed from the atmosphere. The plankton population depended on the sea surface temperature, and still more on nutrients brought in by rivers, by wind-borne dust, and by the upwelling of ocean currents--all of which could change as climate changed. The plankton's biochemical behavior meanwhile would affect the chemical balance of sea water, which was also crucial for CO2 uptake or release. Scientists would have to untangle these complexities before they could truly understand how the oceans' uptake of CO2 would influence the future climate.

Plenty of surprises were still coming from new data. Especially striking were studies in the 1980s that turned up layers of tiny pebbles in North Atlantic deep-sea cores. The debris could have traveled across thousands of kilometers of ocean in only one way: rafted within far-traveling icebergs. Apparently the North American ice sheet had disintegrated at the edges--perhaps in a gigantic surge?--so that great numbers of icebergs had broken off and sailed the North Atlantic as far as Spain. This fitted with speculations about the breakup of Arctic Ocean ice that had been circulating for decades. In 1988, a German graduate student, Hartmut Heinrich, published evidence that the "iceberg armadas" had swarmed across the North Atlantic regularly at particular phases of the glacial cycle. (18) Further studies showed that these "Heinrich events" connected with the more frequent "Dansgaard-Oeschger" periods of cooling. The exact sequence of cause and effect was obscure, but there was evidence of a link to massive surges of the North American ice sheet and changes in the thermohaline circulation. (19) In any case, it was now certain that catastrophic climate shifts, connected with shifts in ocean circulation, could affect the entire North Atlantic region, and probably other parts of the globe as well.

Whatever had set off the abrupt shifts, they seemed to have been a feature of glacial epochs, not of warmer times like the present. However, many oceanographers suspected that the present climate was not immune. (20) Experts looking into the complexities of the North Atlantic system began to think that it might have a variety of possible modes, not just "glacial" and the present stable "interglacial." Meanwhile, the traditional preoccupation with the North Atlantic was giving way to a broader perspective. Oceanographers began to feel that the tropical oceans could be easily as important in rapid climate change. For one thing, it was becoming plain that even ordinary El Niño events in the tropical Pacific seriously affected weather right around the world. For another, new studies showed that equatorial waters had undergone major changes during past ice ages. Climatologists had believed for generations that ice ages had scarcely affected the equatorial jungles. This was now replaced by a view of the globe as a system where every region reacted to changes everywhere else. Suppose, to take just one possibility, a variation of tropical climate altered how winds carried moisture from the Atlantic to the Pacific, altering the salinity? Broecker and co-workers argued that such variations could drive a feedback cycle that might bring "massive and abrupt reorganizations of the ocean-atmosphere system." (21) And there were probably other mechanisms, so far barely visible, adding their own complexities.

Only computer models could say which of these ideas might really work, if any. Modelers successfully simulated abrupt shifts of the North Atlantic circulation, confirming that during a glacial period it could shut off and on by itself. A change of circulation also looked only too likely within the next few centuries as global warming took hold. Paradoxically, that might bring severe cooling to regions from Chicago to Moscow. The modelers' best guess was that the ocean circulation would gradually slow down during the 21st century. But they could not rule out a possibility that the expected greenhouse warming would push the circulation system across some threshold, bringing an abrupt and complete shutdown. (22) The most widely used and elaborate models could hardly be expected to show abrupt changes, for they had been built to be stable. Indeed these models failed to show the sort of climate system jumps that were abundantly clear in the actual geological record of glacial times.

Modelers had not yet fully grasped even the current global ocean circulation. Among other shortcomings, their grid boxes were still too large to realistically represent giant eddies or narrow currents like the Gulf Stream. The models' failings were underlined by new data, which hinted that much of the heat energy that was carried vertically from layer to layer in the oceans was not transported by some kind of average convection (as the models had assumed), but was moved by tides. (23) The strength of ocean tides varies in predictable long-term cycles, ranging from a few years to well over a thousand. Strong tides would mix the waters, perhaps introducing lunar rhythms into the climate system. Could that explain the hints that had been cropping up of an oceanic cycle a thousand or so years long, which in turn would explain the main temperature trends of the past millennium? (24) No matter how that particular idea turned out, it remained a necessary but daunting task to find how water mixed up and down through the layers. (25)

There was also evidence that the North Atlantic Ocean, all on its own, goes through irregular oscillations. Did that explain the cycles around 60-80 years that Dansgaard and others had found for the region? Perhaps it was this slow sloshing of water masses that had made temperatures around the North Atlantic rise so noticeably until the 1940s, then dip into the 1970s, and then rise again? Since the 1920s, meteorologists had been talking about an irregular decades-long variation of weather patterns in the region. Whatever the cause, would a new long-term fluctuation bring another temporary halt in the warming of this crucial region--once again confusing the public about the greenhouse future? (26) Until scientists understood such major effects, and constructed better models, and stopped interrupting one another with surprising new evidence and ideas, the ocean circulation would remain one of the biggest uncertainties in the equation of climate change.

Progress would depend on data, and oceanographers still had sampled only a minute fraction of the world-ocean. Beginning in the 1970s, collaborative projects mobilized thousands of people from scores of nations. The march of acronyms started under the international Global Atmospheric Research Program (GARP) with regional studies like the groundbreaking GARP Atlantic Tropical Experiment (GATE), carried out in 1974. Next came a Tropical Ocean-Global Atmosphere study (TOGA) that surveyed the equatorial Pacific, inspired by the devastating El Niño of 1982-83. The advent of realistic supercomputer models in the 1980s fostered a more global view of ocean dynamics, by calculating how waters from the North Atlantic circulated all the way to the mid-Pacific and back. To feed the models, there were now satellites (starting with the short-lived SEASAT of 1978) that could measure winds, waves, temperatures, and currents in the remotest reaches of the seas. But the satellites could not measure everything, and what their instruments did measure required "ground truth" observations for checking and calibration. The global approach was embodied in a World Ocean Circulation Experiment (WOCE), planned in the 1980s and carried out in the 1990s by some thirty nations. It was supplemented by a Joint Global Ocean Flux Study (JGOFS) that looked at CO2 uptake and other ocean chemistry. (27)

Mining old data could also tell many things. One project burrowed through historical records to transcribe literally millions of thermometer readings, assembling a database for the most basic of all climate numbers--the temperatures within the seas. Since the world-ocean absorbs dozens of times more heat than any other component of the climate system, it was here if anywhere that the reality of global warming should be visible. The team found that the heat content of the upper oceans had risen markedly in the second half of the 20th century, in just the way computer modelers predicted from the greenhouse effect. (28)

It would be harder to get reliable measures for the more complex numbers that represented currents, movement of heat into the deeps, and other dynamic features. Improvements would have to wait on data from yet more grand international data-gathering projects, and on a clearer grasp of fundamental processes, worked into computer models. The coupled ocean-atmosphere models were now good enough to give a general idea of the warming that was likely to come in the 21st century as greenhouse gases built up in the atmosphere. But nobody could confidently rule out the possibility of some future climate shock, caused by processes perhaps not yet imagined in the convoluted systems that linked air, ice, and seas. (29)


Rapid Climate Change

General Circulation Models of the Atmosphere

1. Ruddiman and McIntyre (1981a); Boyle and Keigwin (1982) (using Cd as tracer for nutrients); for further refs., see Broecker et al. (1985); later Boyle and Keigwin, using a core from a spot where deposits had built up exceptionally fast, found that "the deep ocean can undergo dramatic changes in its circulation regime" within 500 years, Boyle and Keigwin (1987), p. 36.

2. "A basinwide change of deep water occurred," Schnitker (1979), quote p. 265; Schnitker (1982) speculated about unstable ocean feedback loops; "dramatic change": Shackleton et al. (1983), p. 242; Rooth (1982) wrote that "catastrophic transitions in the structure of the thermohaline circulation are not only possible, but have probably occurred on many occasions...," p. 131.

3. "large-scale circulation changes," Oeschger et al. (1984), p. 303; he cited Broecker (1982b); for meltwater effect he cited Worthington (1968); and in more detail Ruddiman and McIntyre (1981a); in 1990 Broecker cited Oeschger's paper as the first suggestion "that the Greenland events constitute jumps between two modes of operation of the climate system," Broecker et al. (1990).

4. Oeschger to Broecker, 11/23/95 and reply 12/4/95, Broecker office files, Lamont-Doherty Geophysical Observatory, Palisades, NY. "Disbelief:" Stocker (1999); Siegenthaler and Wenk (1984).

5. "Idea hit:" Broecker (2000), p. 13; Broecker also recalls seeing Oeschger at a 1984 Florida meeting. On this and faulty data: Broecker, interview by Weart, Nov. 1997, AIP.

6. "Astounded": Broecker, interview by Weart, Nov. 1997, see also Dec. 1997, AIP. Broecker et al. (1985), "jumps... speculate" p. 25; "conveyor belt" and "staggering" heat flow were publicized in Broecker (1987b), p. 87, and laid out fully in Broecker (1991).

7. Broecker, interview by Weart, Dec. 21, 1997, AIP.

8. Broecker et al. (1985), p. 25.

9. Broecker (1987a), p. 123.

10. Broecker (1987a); Broecker (1987b); U.S. Senate, Subcommittee on Environmental Protection, Hearings, Jan. 26-28 1987, pp. 21-23.

11. A typical modern model shows a roughly 2C drop in Europe: Vellinga and Wood (2002).

12. Bryan and Spelman (1985); the question is the title of Broecker et al. (1985).

13. Bryan (1986). N.b. this is Frank Bryan, not Kirk; in 1985 Broecker suspected the meltwater pulse was the entire cause of the Younger Dryas, but later he suggested it was only the trigger that set the timing for a shift between thermohaline modes. Broecker et al. (1989); Broecker et al. (1990); another model found that a cold North Atlantic surface sufficed to bring a Younger Dryas-like climate: Rind et al. (1986).

14. Bryan and Spelman (1985), p. 11,687.

15. Manabe and Stouffer (1988), p. 841.

16. Schlesinger and Mitchell (1987), p. 796; McGuffie and Henderson-Sellers (1997), pp. 55-56; 1980s work is reviewed in Haidvogel and Bryan (1992); Meehl (1992).

17. E.g., Wood et al. (1999); see summary: Rahmstorf (1999).

18. Heinrich (1988); earlier speculations: Mercer (1969); Ruddiman and McIntyre (1981a).

19. Bond et al. (1992); Bond et al. (1993); Broecker suggested that when fresh water was brought into the North Atlantic in a million melting icebergs, it might have halted the North Atlantic thermohaline circulation. Broecker et al. (1992).

20. Bauch et al. (2000); Alley (2000), ch. 15.

21. Broecker and Denton (1989), quote p. 2489; Broecker et al. (1990); for the evaporation cycle Warren (1983); a detailed review is Broecker and Denton (1990); for a more recent review, Broecker (2000).

22. Manabe and Stouffer (1993) pioneered the demonstration of a transition under future warming; an improved model showed a shutdown was especially likely with rapid increase of greenhouse gas emissions, Stocker and Schnitter (1997); see also Broecker (1997); Ganopolski and Rahmstorf (2001) for instability during a glacial period; IPCC (2001), pp. 439-40.

23. Egbert and Ray (2000).

24. See discussion in Keeling (1998), pp. 70-73.

25. Mapping global patterns and incorporating the results in models was described as "...a daunting task... requires a large effort, but ... feasible" for one important type of mixing, the breaking of the internal waves on the surfaces between layers of different densities, Gregg et al. (2003).

26. New studies suggested that this "North Atlantic Oscillation" was driven not by ocean interactions, but by changes in upper atmosphere wind patterns around the entire hemisphere, Wallace and Thompson (2002), see also Science 289 (28 July 2000): 547-48.

27. http://ads.smr.uib.no/jgofs/Intro.htm#HISTORY, Thompson et al. (2001).

28. Levitus et al. (2001) (the data compilation was by the NOAA group Levitus headed); Barnett et al. (2001).

29. E.g., "The threshold separating stable and unstable climate regimes represents a relatively small departure from the modern ice sheet configurations," according to McManus et al. (1999), p. 1.