Abrupt Climate Change

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 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.

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 abruptly than almost any scientist had imagined. ⁽¹⁾ For more on ice drilling, see Joel Genuth's Greenland Ice Sheet Project (GISP) http://www.ngdc.noaa.gov/paleo/icecore/greenland/gisp/gisp.html.

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, showed rapid oscillations of temperature repeatedly at irregular intervals throughout the last glacial period. Greenland had sometimes warmed a shocking 7C within a span of less than 50 years. During the Younger Dryas transition, drastic shifts in the entire North Atlantic climate were visible within five snow layers, that is, as little as five years! "The general circulation [of the atmosphere] in the Northern Hemisphere must have shifted dramatically," Dansgaard's group concluded. ⁽²⁾

There was more. In the late 1980s and early 1990s, improved carbon-14 techniques gave dates for pollen and the like at locations ranging from Ohio to Japan to Tierra del Fuego. The results suggested that the Younger Dryas events had affected climates around the world. The extent of this perturbation, and just how weather had changed in different regions, was controversial. ⁽³⁾ But scientists were increasingly persuaded that abrupt climate shifts could have global scope.

Could such variations occur not only in glacial times, but also in warm periods like the present? The layers from the previous warm period were down near bedrock, distorted by ice flow, and here the two groups' cores gave divergent results. (Antarctic cores could not help. Little snow falls there, and the layers of ice were too thin and squashed together to reveal any 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." ⁽⁴⁾

Scientists will doubt even the best set of data if they cannot explain it, but at least one plausible explanation was at hand. A flip-flop of the entire Atlantic Ocean's circulation pattern might have caused 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'." ⁽⁵⁾ But the evidence of iceberg surges was strong, and computer models suggested that such events--or even just by the natural instability of the circulation during a glacial period--could indeed have caused a drastic circulation shift with global impact.

Could the same instability hold today? There was suggestive evidence that abrupt flips of circulation had in fact happened in previous times of warmth. In particular, it appeared that during the warm period some 8000 years ago, fresh water bursting out of huge lakes left over from the last ice age had sufficed to halt the North Atlantic circulation, bringing a deep cold spell. ⁽⁶⁾ "There is surely a possibility," Broecker wrote, "that the ongoing buildup of greenhouse gases might trigger yet another of these ocean reorganizations." He was troubled by contemporary measurements of the North Atlantic that suggested that its circulation had slowed down, compared with measurements in earlier decades. (Further observations in 2001-2002 confirmed that the Atlantic's freshwater balance was changing.) Still, the record was so skimpy that nobody could say this was not just a normal, temporary fluctuation. When an international panel of experts made a best guess on the issue in 2001, they reported that a shutdown of the Atlantic circulation in the coming century was "unlikely," but "cannot be ruled out." If so, it would probably change climates all around the North Atlantic, a serious regional cooling brought on by global warming. Broecker warned that the consequences, perhaps only a few decades hence, in a world that would already be pressed to feed its soaring population, could be "widespread starvation." [In 2004, however, he cautioned against "exaggerated scenarios." As others too pointed out, there were no prospects in our era for a deluge of melted glacier water. Any reorganization of the ocean circulation would be gradual-- troublesome but not catastrophic.] ⁽⁷⁾

Other mechanisms for catastrophically abrupt shifts remained on the table. 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. ⁽⁸⁾

In the 1990s, geologists found that such titanic gas outbursts had in fact caused a spurt of warming 55 million years ago. At any rate something back then had radically changed climate, bringing mass extinctions and a new geological era, and 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. But it seemed to have come in abrupt steps, which in some centuries might have pumped greenhouse gases into the atmosphere at a rate fast enough to bring serious change within a human lifetime ⁽⁹⁾

Ominously, data showed that sudden climate shifts did not happen only during a glacial period. In 1993, Dansgaard and his colleagues reported that rapid 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. ⁽¹⁰⁾

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. ⁽¹¹⁾ 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. ⁽¹²⁾

An altogether different type of evidence for 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." ⁽¹³⁾ 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. ⁽¹⁴⁾

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." ⁽¹⁵⁾

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 climate system changes (as have occurred in the past)." ⁽¹⁶⁾ 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 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. ⁽¹⁷⁾ Policy-makers and the public lagged even farther behind in grasping what the new scientific view could mean.

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 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 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. ⁽¹⁸⁾ 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.

Related:

Ocean Currents and Climate

Aerosol Hazes

Simple Models of Climate

Supplements:

Radiocarbon Dating

Ice Sheets & Rising Seas

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

2. 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 Alley (2000); five-year Younger Dryas steps: Taylor et al. (1997); 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).

3. For references 1987-94 (including also Alaska, New Zealand, Gulf of California, etc.) see Broecker (1995), pp. 306-08; for later developments, National Academy of Sciences (2002) and Lynch-Stieglitz (2004).

4. Hammer et al. (1997), Preface, "not yet faced," p. 26,315.

5. Alley (1998).

6. Barber et al. (1999).

7. 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); "exaggerated:" Broecker (2004); see Weaver and Hillaire-Marcel (2004).

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

9. 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); a good recent overview is Kunzig (2004).

10. Dansgaard et al. (1993); Kerr (1993).

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

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

13. Rignot and Thomas (2002), p. 1505.

14. "longest stable:" Petit et al. (1999), p. 434. Ruddiman and Thomson (2001) #1679; see Kerr (2004).

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

16. IPCC (1996), p. 7.

17. National Academy of Sciences (2002), p. 121.

18. 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 abrupt change, I am grateful to Ken Brown, Daniel A. Livingstone and other respondents from the QUATERNARY and PALEOLIM listservs.