rapid - abrupt / Abrupt Climate Change

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By the 20th century scientists, rejecting old tales of world catastrophe, were convinced that global climate could change only gradually over many tens of thousands of years. But in the 1950s a few scientists found evidence that some changes in the past had taken only a few thousand years. During the 1960s and 1970s other data, supported by new theories and new attitudes about human influences, reduced the time a change might require to hundreds of years. Many doubted that such a rapid - abrupt shift could have befallen the planet as a whole. The 1980s and 1990s brought proof (chiefly from studies of ancient ice) that the global climate could indeed shift, radically and catastrophically, within a century--perhaps even within a decade.... This essay covers large one-way jumps of climate. For short-term cyclical changes, see the essay on The Variable Sun.


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After 1988

Early in the 1990s, further revelations startled climate scientists. The quantity, variety, and accuracy of measurements of ancient climates were increasing at a breakneck pace—compared with the data available in the 1970s, orders of magnitude more were now in hand. The first shock came from the summit of the Greenland ice plateau, a white wasteland so high that altitude sickness was a problem. From this location all ice flowed outward, so glacier experts hoped that even at the bottom, three kilometers (two miles) down, the layers would be relatively undisturbed by movement. Early hopes for a new cooperative program joining Americans and Europeans had broken down, and each team drilled its own hole. An ingenious decision transmuted competition into cooperation. The two holes were drilled just far enough apart (30 kilometers) so that anything that showed up in both cores must represent a real climate effect, not an accident due to bedrock conditions. The match turned out to be remarkably exact for most of the way down. A comparison of variations in the cores showed convincingly that climate could change more rapidly than almost any scientist had imagined.

Swings of temperature that scientists in the 1950s believed to take tens of thousands of years, in the 1970s to take thousands of years, and in the 1980s to take hundreds of years, were now found to take only decades. Ice core analysis by Dansgaard’s group, confirmed by the Americans’ parallel hole, showed rapid - abrupt oscillations of temperature repeatedly at irregular intervals throughout the last glacial period. Greenland had sometimes warmed a shocking 7°C within a span of less than 50 years. For one group of American scientists on the ice in Greenland, the “moment of truth” struck on a single day in midsummer 1992 as they analyzed a cylinder of ice, recently emerged from the drill hole, that came from the last years of the Younger Dryas. They saw an obvious change in the ice, visible within three snow layers, that is, scarcely three years! The team analyzing the ice was first excited, then sobered—their view of how climate could change had shifted irrevocably. The European team reported seeing a similar step within at most five years. “The general circulation [of the atmosphere] in the Northern Hemisphere must have shifted dramatically,” Dansgaard’s group eventually concluded.

The record of dust found in the ice certainly suggested that a wide region had been involved, but might the change have been restricted to parts of the world near Greenland? The first hints of the answer came from oceanographers, who had been hunting out seabed zones where bioturbation by burrowing worms did not smear any record of rapid - abrupt change. In some places the sediments accumulated very rapidly, while in others, the seawater lacked enough oxygen to sustain life. The first results, from the Norwegian Sea in 1992, confirmed that the abrupt changes seen in Greenland ice cores were not confined to Greenland alone. Later work on seabed cores from the California coast to the Arabian Sea, and on chemical changes recorded in cave stalagmites from Switzerland to China, confirmed that the oscillations found in the Greenland ice had been felt throughout the Northern Hemisphere. Meanwhile, in the late 1980s and early 1990s, improved carbon-14 techniques gave the first accurate dates for sediments containing pollen and other carbon-bearing materials at locations ranging from Japan to Tierra del Fuego. Good dates finally allowed correlation of many geological records with the Greenland ice. The results suggested that the Younger Dryas events had affected climates around the world. The extent and nature of the perturbation was controversial. But scientists were increasingly persuaded that abrupt climate shifts could have global scope, even if they affected different places differently—colder here and warmer there, wetter here and drier there.

Could such drastic variations happen not only during glacial times, but also in warm periods like the present? That was the most interesting question in 1992, as the European drillers penetrated clear through the last glacial epoch to the preceding “Eemian” period, more than 100,000 years back—a time similar to our own, or even warmer. There, too, they saw dramatic shifts. However, further analysis cast that into doubt. The layers from the Eemian warm period were down near bedrock, distorted by ice flow. Comparison of the two groups’ cores gave divergent results. Again scientists had benefitted from drilling parallel cores. But this time the lesson, valuable if unwelcome, was that they must do more work. (Another grand multinational project to drill Greenland ice from the previous warm period was completed in 2004. The team’s three-kilometer core showed a reassuringly stable climate for the last part of the Eemian warm period. But they failed again to get reliable results for most of the period, including the crucial early time most similar to our own when the atmosphere had been warming up.)

Antarctic cores could not help. Little snow falls there, and the layers of ice were too thin and squashed together to reveal rapid - abrupt variations. Certainly no climate variation of Younger Dryas magnitude had been seen recently. So there was reason to hope that our present climate was relatively stable, at least for the moment. The Europeans and Americans nevertheless agreed that through most of the last 100,000 years the global climate had oscillated “on a scale that human cultural and industrial activities have not yet faced.”

Scientists will doubt even the best set of data if they have no theory to explain it, but at least one plausible explanation was at hand. A flip-flop of the entire North Atlantic Ocean’s circulation pattern might have been involved in the Dansgaard-Oeschger events. People came up with various proposals for things that might have triggered a switch, such as the surge of an ice sheet that released a flotilla of icebergs.+

That was not easy to swallow. As one scientist remarked, many of his colleagues “do not believe that the small, energy-starved polar ‘tail’ can wag the large, energy-rich tropical ‘dog’.” But the evidence of iceberg surges was strong, and computer models suggested that such a surge could indeed have caused a drastic global circulation shift. Oceanographers began to work out how the tropical oceans could take part in a sudden global change. Or perhaps set it off — if the system was so unstable, the trigger might not lie in the well-studied North Atlantic. The tropical Pacific and Atlantic ocean and wind systems seemed to have feedbacks that, once perturbed, might reorganize the entire system of clouds, rainfall and currents. “Taken to a logical extreme,” one expert insisted in 2004, “a competing ?tropical driver’ hypothesis for abrupt climate changes could easily explain the observed global synchronization.” For example, a “permanent El Niño” might move the Earth back to a state not seen since several million years ago, when so much ice had been melted that the sea level stood roughly 25 meters above the present level.

Did the same instability exist today? There was suggestive evidence that abrupt flips of circulation had in fact happened in previous times of warmth. “There is surely a possibility,” Broecker wrote, “that the ongoing buildup of greenhouse gases might trigger yet another of these ocean reorganizations.” The media picked up the dramatic image of Europe returning to the frigid conditions of the Younger Dryas — global warming could bring on a new Ice Age almost instantly! When an international panel of experts made their best guess on the matter in 2001, they concluded that a shutdown of the Atlantic circulation in the coming century was “unlikely” but “cannot be ruled out.” If the shutdown did come, Broecker warned, it could mean “widespread starvation” within decades. In the next few years, scientists reported that the Atlantic waters were indeed growing less salty, thanks to fresh water from increased rainfall and the melting of ice. Still more troubling was a 2005 announcement that the amount of heat carried southward by the North Atlantic circulation had decreased by as much as 30% since the 1950s.

However, the observational record was so skimpy, and the system so noisy, that this could be just a normal and temporary fluctuation. A replay of the catastrophic Younger Dryas glacial scenario was not likely under the very different conditions of the present. Computer modelers redoubled their attention to the question, and their simulations showed only gradual, centuries-long changes in the ocean circulation. Broecker admitted that he had overestimated the danger, and in 2004 he publicly cautioned against the “exaggerated scenarios” that had recently appeared in a Hollywood summer spectacle. However, the models were deliberately constructed to give stable solutions, and they could never include all the complex feedbacks that might conceivably cause a sudden shift. As one group that studied the Younger Dryas remarked, "the geological understanding of past abrupt climate changes is only preliminary. This does not bode well for predicting future, abrupt climate changes."

Other mechanisms for drastic shifts also came under detailed scrutiny. An example was the clathrate ices, frozen in layers spread through sea floor muds. Clathrates might hold more carbon compounds than all the world’s coal and oil. New studies made it plausible that warming of the oceans could cause some of the deposits to disintegrate in a landslide-like chain reaction, which would vent enough methane and CO2 into the atmosphere to redouble global warming. The idea sounded like science fiction (indeed some science fiction writers used it), and it seemed highly unlikely to happen anytime soon.

In the 1990s, geologists found that such titanic greenhouse gas outbursts had probably caused a spectacular warming 55 million years ago. At any rate something back then had radically changed climate, with global heating and an abrupt change of the deep ocean circulation, bringing mass extinctions and a new geological era. Clathrates were the leading suspect. The total carbon release that caused this havoc was roughly comparable to the amount of carbon that humanity would emit if we burned all available coal and oil. Back then, the rise in temperature had apparently stretched over tens of thousands of years, “rapid” only to a geologist, and the same seemed likely for any future replay. But the past emissions had come stepwise, and in some future century too, clathrates might pump gases into the atmosphere at a rate fast enough to bring serious change within a human lifetime. There seemed little risk of truly catastrophic eruptions in the foreseeable future, but methane from the seabed could make global warming more rapid - abrupt and severe.+

Ominously, data showed that sudden climate shifts did not happen only during a glacial period. In 1993, Dansgaard and his colleagues reported that rapid - abrupt oscillations had been common during the last interglacial warm period—enormous spikes of cooling, like a 14-degree cold snap that had struck in the span of a decade and lasted 70 years. The instability was unlike anything the ice record showed for our current interglacial period. The announcement, Science magazine reported, “shattered” the standard picture of benign, equable interglacials.+

Others soon showed that these measurements, made near the bottom of the core, were distorted by ice flow that stirred together layers from warm and cold periods. Interglacials were perhaps not so horrendously variable. Yet in terms of how scientists thought about the present climate system, one might say that the ice had been broken. People recalled that the present system was certainly subject to abrupt but harrowing droughts, like the one revealed by Bryson that had devastated native North American cultures. Persuasive new geological evidence blamed extreme prolonged droughts for the downfall of ancient Mayan and Mesopotamian civilizations too.

An altogether different type of evidence for rapid - abrupt change came from improved observations of Arctic and Antarctic regions. New views from satellites, plus vigorous programs of precise measurements from airplanes and on the ground, showed that enormous glaciers could quickly change their speed of travel, while entire ice sheets could break up within a matter of months. As one expert remarked, this “ran counter to much of the accepted wisdom regarding ice sheets.” That accepted wisdom, he explained, “lacking modern observational capabilities, was largely based on ‘steady-state’ assumptions.” Now the plausible possibility that a swift alteration of land or sea ice could transform climate had to be added to all the other potential feedbacks from global warming.+

The new view of climate was reinforced by one of the last great achievements of the Soviet Union, an ice core drilled with French collaboration at Vostok in Antarctica. The record reached back through nearly four complete glacial-interglacial cycles—and drastic temperature changes peppered almost every stretch of data. This Antarctic record was too fuzzy to say whether any of these changes had come and gone on the decade-size timescale of the Younger Dryas. But warm interglacial periods had certainly been subject to big swings of temperature lasting for centuries. Especially striking to the researchers, by contrast, was our own era, the ten thousand years since the last glaciation. It was, “by far, the longest stable warm period recorded in Antarctica during the past 420 [thousand years].” When Bryson, Schneider, and others had warned that the century or so of stability in recent memory did not reflect “normal” long-term variations, they had touched on an instability grander than they guessed. The entire rise of human civilization since the end of the Younger Dryas had taken place during a period of warm, stable climate that was unique in the long record. The climate known to history seemed to be a lucky anomaly. (Paleoclimatologist William Ruddiman suggested that this was no coincidence. Perhaps the rise of agriculture, with its deforestation and rice paddies, had added enough methane and CO2 to the atmosphere to dampen the normal ice-age cycle?) The well-recorded history of the most recent century or so happened to show even more unusual stability, compared with what new evidence was revealing about severe variations in earlier millennia.

The accumulation of evidence, reinforced by at least one reasonable explanation (the reorganization of ocean circulation) destroyed long-held assumptions. Most experts now accepted that abrupt climate change, huge change, global change, was possible at any time. A report written by a National Academy of Sciences committee in 2001 said that the recognition, during the 1990s, of the possibility of abrupt global climate change constituted a fundamental reorientation of thinking, a “paradigm shift for the research community.”

The first strong consensus statement had come in 1995 from the Intergovernmental Panel on Climate Change, representing the considered views of nearly all the world’s climate scientists.+ The report included a notice that climate “surprises” were possible—“Future unexpected, large, and rapid - abrupt climate system changes (as have occurred in the past).”+ The report’s authors did not emphasize the point, however, and the press seldom mentioned it.

Despite the profound implications of this new viewpoint, hardly anyone rose to dispute it. Yet while they did not deny the facts head-on, many denied them more subtly, by failing to revise their accustomed ways of thinking about climate. For example, few of the scientists studying pollen in bogs went back to their data and took on the difficult task of looking for catastrophically rapid - abrupt shifts in the past. “Geoscientists are just beginning to accept and adapt to the new paradigm of highly variable climate systems,” said the Academy committee in 2001. Beyond geoscientists, “this new paradigm has not yet penetrated the impacts community,” that is, the economists and other specialists who tried to calculate the consequences of climate change. Policy-makers and the public lagged even farther behind in grasping what the new scientific view could mean.+

Within a few years that changed. Media ranging from science magazines to movies offered scenarios of a climate that could change abruptly, within a few decades (or even, according to Hollywood, a few weeks). Around 2005 the phrase “tipping point” appeared in stories on climate, an admission that change could be not only rapid - abrupt but irreversible. Attention focused on Antarctic and Arctic ice changes. For example, new evidence and theories suggested that Greenland’s ice might melt much faster than had been suspected, raising the sea level significantly within the next few centuries. Even more sensational were changes already visible in startling “before and after” satellite pictures of dwindling sea ice. “At the present rate,” scientists reported, “a summer ice-free Arctic Ocean within a century is a real possibility.” That had last been seen millions of years ago, in an epoch with a sea level roughly 25 meters above the present. The changes foreseen by Budyko, a generation earlier, were underway (see above). Reviewing the familiar feedback where less ice and snow meant more sunlight absorbed, one group commented dryly that “Thresholds may produce unexpected system responses.”

Another possible “tipping point” that research could not dismiss came from biological feedbacks in the carbon cycle. Although data were spotty (mostly short-term studies of only a few of the countless living systems), results in the early 2000s were discouraging. More likely than not, as plants and soils got warmer they were releasing additional CO2, methane and other greenhouse gases. Possibly the worst problem was tundra, where researchers saw greenhouse gases bubbling out of the sodden ground before their eyes. A Russian researcher said it was an “ecological landslide that is probably irreversible and is undoubtedly connected to climatic warming.”*

The good news was, nobody had yet found a mechanism that, outside Ice Age conditions, could plausibly bring a massive global climate change in less than a decade. The bad news was... well, there were several items. The feedbacks long anticipated were kicking in, so that large changes were happening within the timescale of a human lifetime. And scientists kept turning up more possible mechanisms for feedbacks that could accelerate warming. And much of the carbon system was still so poorly understood that the real pace of change could not be confidently predicted. And, finally, the known feedbacks were so strong that it seemed likely that — unless human civilization rose to the challenge very soon — global warming would become self-sustaining and irreversible, so that the world of the 22nd century would look very different indeed from the world of the 20th.

A lesson about how science proceeds can be learned from this history.+ Asked about the discovery of abrupt climate change, many climate experts today would put their finger on one moment: the day they read the 1993 report of the analysis of Greenland ice cores. Before that, nobody confidently believed that the climate could change massively within a decade or two; after the report, nobody felt sure that it could not. So wasn’t the preceding half-century of research a waste of effort? If only scientists had enough foresight, couldn’t we have waited until we were able to get good ice cores, and settle the matter once and for all with a single unimpeachable study?

The actual history shows that even the best scientific data are never that definitive. People can see only what they find believable. Over the decades, many scientists who looked at tree rings, varves, ice layers, and so forth had held evidence of rapid - abrupt climate shifts before their eyes. They easily dismissed it. There were plausible reasons to believe that global cataclysm was a fantasy of crackpots and Bible fundamentalists. Records of the past were mostly too fuzzy to show rapid - abrupt changes, and where such a change did plainly appear, scientists readily attributed it (usually correctly) to something other than climate. Sometimes the scientists’ assumptions were actually built into their procedures. When pollen specialists routinely analyzed their clay cores in 10-centimeter slices, they could not possibly see changes that took place within a centimeter’s worth of layers. If the conventional beliefs had been the same in 1993 as in 1953—that significant climate change always takes many thousands of years—scientists would have passed over the decade-scale fluctuations in ice cores as meaningless noise.

First scientists had to convince themselves, by shuttling back and forth between historical data and studies of possible mechanisms, that it made sense to propose shifts as “rapid” as a thousand years. Only then could they come around to seeing that shifts as “rapid” as a hundred years could be plausible. And only after that could they credit still swifter changes. Without this gradual shift of understanding, the Greenland cores would never have been drilled. The funds required for these heroic projects came to hand only after scientists reported that climate could change in damaging ways on a timescale meaningful to governments. In an area as difficult as climate science, where all is complex and befogged, it is hard to see what one is not prepared to look for.


55. GISP interviews, records of Study of Multi-Institutional Collaborations, AIP. Firsthand accounts are Mayewski and White (2002); Alley (2000); Dansgaard (2004) BACK

56. Dansgaard et al. (1989); increasingly abrupt changes were seen on further study, Johnsen et al. (1992); Grootes et al. (1993); jumps of Greenland snow accumulation "possibly in one to three years" were reported by Alley et al. (1993), see also Mayewski (1993); five-year steps: Taylor et al. (1997); changes in dust had been noted, indicating at least continental scope for the change, and a Younger Dryas temperature step in less than a decade was found to be hemisphere-wide since methane gas changed as well: Severinghaus et al. (1998). Good histories are Alley (2000) and Cox, (2005), ch. 8. BACK

57. First ocean results: Karpuz et al. (1992), Lehman and Keigwin (1992). For references 1987-94 (including also Alaska, Ohio, New Zealand, etc.) see Broecker (1995), pp. 306-08; for later developments, National Academy of Sciences (2002) and Lynch-Stieglitz (2004), also Cox (2005), ch. 8. BACK

58. Divergence in cores: Taylor et al. (1993), Grootes et al. (1993). 2004 work: NGRIP (2004) (North Greenland Ice Core Project members, K.K. Andersen et al.); see report by Cuffey (2004) and also Cox (2005), ch. 8. Hammer et al. (1997), Preface, "not yet faced," p. 26,315. BACK

59. Wag the dog: Alley (1998); "easily explain": Chiang and Koutavas (2004). El Niños: Cane and Evans (2000); Federov et al. (2006). BACK

60. Barber et al. (1999). BACK

61. Broecker et al. (1992); quotes: Broecker (1997), p. 1588; IPCC (2001), p. 420; Atlantic freshening: Hansen et al. (2001); Dickson et al. (2002); Curry et al.(2003); Curry and Mauritzen (2005); slowed circulation: Bryden et al. (2005). "Exaggerated:" Broecker (2004); see Weaver and Hillaire-Marcel (2004); "does not bode well": Lowell et al. (2005). BACK

62. Science fiction: notably the award-winning Robinson (1994). BACK

63. Appenzeller (1991); for the late Paleocene event, Kennett and Stott (1991); Koch et al. (1992); Dickens et al. (1995); Norris and Röhl (1999); Katz et al. (1999); Nunes and Norris (2006); an overview is Kunzig (2004). Harvey and Huang (1995) estimate clathrates could bring at worst a 10-25% increase in warming. BACK

64. Dansgaard et al. (1993); Kerr (1993). BACK

65. Alley et al. (1995); Chappellaz et al. (1997), comparing with Vostok cores. BACK

66. Maya: Hodell et al. (1995); Mesopotamia: Weiss et al. (1993); for global climate shifts throughout the postglacial period, see also deMenocal et al. (2000). BACK

67. Rignot and Thomas (2002), p. 1505. BACK

68. "longest stable...": Petit et al. (1999), p. 434. Ruddiman and Thomson (2001); on Ruddiman see Kerr (2004a). BACK

69. National Academy of Sciences (2002), p. 16, see also pp. 1, 119, 121. BACK

70. IPCC (1996), p. 7. BACK

71. National Academy of Sciences (2002), p. 121. BACK

71a. Overpeck et al. (2005), see also Lindsay and Zhang (2005). BACK

71b. "Tipping point" suggested i.a. by Foley (2005). Carbon cycle: e.g., Bellamy et al. (2005), Heath et al. (2005), Govindasamy et al. (2005). Russian (Sergei Kirpotin) quoted by Pearce (2005), along with a report that "the permafrost of western Siberia is turning into a mass of shallow lakes as the ground melts," lakes were expanding on the North Slope of Alaska, Katey Walter had found methane hotspots in eastern Siberia where the bubbling gas kept the surface from freezing in winter, etc. See also Lawrence and Slater (2005). Also, melting of permafrost allowed dark shrubs to spread, which would increase local heating, Chapin et al. (2005). BACK

71c. For example, Best (2006) suggested that increased organic sediments sent down rivers due to deforestation, factory farming, etc., once buried in the seabed, will be converted to methane by bacteria, perhaps seriously augmenting the greenhouse gas level within the next century. BACK

72. There is a famous comparable case in another field of science. In the 1930s, physicists used thin screens to block extraneous large particles from their instruments as they measured the tiny particles resulting from nuclear reactions. Since they never imagined that an atom could split into two large chunks, they automatically prevented themselves from discovering uranium fission. For discussion on the difficulties of detecting rapid change, I am grateful to Ken Brown, Daniel A. Livingstone and other respondents from the QUATERNARY and PALEOLIM listservs.