The Discovery of Rapid Climate Change (part 2)

Spencer Weart

August 2003, page 30

At the University of Wisconsin-Madison, Reid Bryson scrutinized entirely different types of data. In the late 1950s, he had been struck by the wide variability of climates as recorded in the varying width of tree rings. He was also familiar with the dishpan experiments that showed how a circulation pattern might change almost instantaneously. To take a new, interdisciplinary look at climate, Bryson brought together a group that even included an anthropologist who studied the ancient Native American cultures of the Midwest. From radiocarbon-dated bones and pollen, they deduced that a prodigious drought had struck the region in the 1200s--the very period when flourishing towns of the Mound Builders had gone into decline. Compared to that drought, the Dust Bowl of the 1930s had been mild and temporary. By the mid-1960s, Bryson was announcing that "climatic changes do not come about by slow, gradual change, but rather by apparently discrete 'jumps' from one atmospheric circulation regime to another."⁸ His group further reported pollen studies showing a rapid shift around 10 500 years ago; by "rapid" they meant a change in the mix of tree species within less than a century. Perhaps the Younger Dryas was not just a local Scandinavian anomaly.

Still, no major climate change was required to transform any particular forest. Many experts continued to believe it was sheer speculation to imagine that the climate of a region, let alone of the entire world, could change in less than a thousand years or so. But confirmation of changes at that rate, at least, was coming from a variety of studies. As the respected climatologist J. Murray Mitchell Jr explained in 1972, in place of the old view of "a grand, rhythmic cycle," the new evidence showed a "much more rapid and irregular succession" in which Earth "can swing between glacial and interglacial conditions in a surprisingly short span of millennia (some would say centuries)."⁹

Figure 3

The most convincing evidence came from a long core of ice drilled at Camp Century, Greenland, by Willi Dansgaard's Danish group, in cooperation with Americans led by Chester Langway Jr. The proportions of different oxygen isotopes in the layers of ice gave a fairly straightforward record of temperature. Mixed in with the expected gradual cycles were what the group called "spectacular" shorter-term shifts, including the Younger Dryas oscillation. Some of the shifts seemed to have taken as little as a century or two (see figure 3).

During the early 1970s, most climate experts came to agree that interglacial periods tended to end more abruptly than had been supposed. Many concluded that the current warm period could end in a rapid cooling, possibly even within the next few hundred years. Bryson (pictured in figure 4), Stephen Schneider, and a few others took this new concern to the public. They insisted that the climate we had experienced in the past century or so, mild and equable, was not the only sort of climate the planet knew. For all anyone could say, the next decade might start a plunge into a cataclysmic freeze, drought, or other change unprecedented in recent memory, although not without precedent in the archaeological and geological record.

Figure 4

Cooling was not the only change that experts were starting to worry about. Since the late 1950s, attentive scientists had acknowledged the potential value of the old idea that human emissions of carbon dioxide gas (CO₂) might lead to global warming. (See Physics Today, January 1997, page 34.) Most experts assumed that if such a greenhouse-effect warming did occur, it would come as they expected for any climate change--gradually over the course of a few centuries. But some suggested swifter possibilities. In 1972, pursuing his calculations of ice-cover feedbacks, Budyko declared that, at the rate we were pumping CO₂ into the atmosphere, the ice covering the Arctic Ocean might melt entirely by 2050. And glacier experts were developing models that suggested how warming might cause the ice sheets of Antarctica to break up swiftly and shock the climate system. Bryson and others worked harder than ever to bring their concerns to the attention of the broader scientific community and the public.

Most scientists spoke more cautiously. When leading experts had to state a consensus opinion, as in a 1975 NAS report on climate research,¹⁰ they reported that they saw nothing that would bring anything beyond relatively small changes that would take centuries or longer to develop. They did warn that there could be significant noise, the usual irregularities of weather patterns. And they admitted that they might have failed to recognize some mechanisms of change. If there was a threat, experts in the 1970s could not agree whether it was from global warming or cooling. The one thing that all scientists agreed on was that they were seriously ignorant about how the climate system worked. So the only step they recommended to policymakers was to pursue research more aggressively.

Jumps within centuries—or less

In the late 1970s and early 1980s, a variety of new data revealed surprising climate shifts. To take one example, a study of beetles that had been preserved in peat bogs since the end of the last glacial epoch turned up changes in the mix of species; those changes represented climate shifts of 3°C in well under 1000 years. Meanwhile, computer modelers produced plausible calculations for rapid climate shifts involving snow-cover feedbacks, a shutdown of North Atlantic circulation, or ice-sheet collapse.¹¹ During the 1980s, the list of plausible mechanisms grew. Perhaps a rise in global temperature would cause methane to bubble out of the vast expanses of boggy tundra. Because methane is a greenhouse gas that blocks heat radiation even more effectively than CO₂, such a release would cause even more warming in a vicious feedback cycle. Or what about the clathrates--peculiar ices that lock up huge volumes of methane in the muck of seabeds? Perhaps those would disintegrate and release greenhouse gases.

Many scientists continued to look on such speculations as little more than science fiction. The evidence for rapid shifts, as it sometimes turned up in odd data sources like bog beetles, was never entirely convincing. Any single record could be subject to all kinds of accidental errors. The best example of a problem was in the best data on climate shifts, the odd wiggles in measurements from the Camp Century core. Those data came from near the bottom of the hole. Skeptics argued that the ice layers there, squeezed tissue-thin, were folded and distorted as they flowed over the bedrock.

To get more reliable data, the ice drillers went to a second location, some 1400 kilometers distant from Camp Century. By 1981, after a decade of tenacious labor, they hit bedrock and extracted gleaming cylinders of ice 10 cm in diameter and more than two km deep; the deepest ice came from the last ice age, 14 000 years ago. The ratios of oxygen isotopes within the ice layers gave a temperature record showing what the researchers called "violent" changes. The most prominent of those, corresponding to the Younger Dryas oscillation, showed "a dramatic cooling of rather short duration, perhaps only a few hundred years."¹²

Since the 1950s, jumps had persistently turned up in weather and climate models, whether built from rotating dishpans or from sets of equations run through computers. Scientists could have dismissed those models as too crude to say anything reliable--but the historical data showed that the notion of radical climate instability was not absurd after all. And scientists could have dismissed the jumps in the scattered data as artifacts, due to merely regional changes or simple errors--but the models showed that global jumps were physically plausible.

Figure 5

Nevertheless, experts were scarcely prepared for the shock that came from the Greenland ice plateau in 1993. Plans had been laid to drill at the summit of the ice cap, where irregularities due to the deep flow of ice would have been minimal. Early hopes for a new cooperative program joining Americans and Europeans broke down and each team drilled its own hole, some 3 km deep (see figure 5). Competition was transmuted into cooperation by a decision to put the two boreholes just far enough apart (30 km) so that anything that showed up in both cores must represent a real climate effect, not an artifact due to bedrock conditions. The match turned out to be remarkably exact for most of the way down. The comparison between cores showed convincingly that climate could change more rapidly than almost any scientist had imagined. Swings of temperature that were believed in the 1950s 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. Greenland had sometimes warmed a shocking 7°C within a span of less than 50 years. More recent studies have reported that, during the Younger Dryas transition, drastic shifts in the entire North Atlantic climate could be seen within five snow layers, that is, as little as five years!¹³

Studies of pollen and other indicators--at locations ranging from Ohio to Japan to Tierra del Fuego, and dated with greatly improved radiocarbon techniques--suggested that the Younger Dryas event affected climates around the world. The extent of the climate variations was controversial (and to some extent remains so). Likewise uncertain was whether such variations could occur not only in glacial times, but also in warm periods like the present. Computer modelers, now fully alerted to the delicate balance of salinity and temperature that drove the North Atlantic circulation, found that global warming might bring future changes in precipitation that could shut down the current heat transport. The 2001 report of the Intergovernmental Panel on Climate Change, pronouncing the official consensus of the world's governments and their climate experts, reported that a shutdown in the coming century was "unlikely" but "cannot be ruled out." If such a shutdown did occur, it would change climates all around the North Atlantic--a dangerous cooling brought on by global warming.¹⁴

Now that the ice had been broken, so to speak, most experts were prepared to consider that rapid climate change--huge and global change--could come at any time. "The abrupt changes of the past are not fully explained yet," wrote the NAS committee in its 2002 report, "and climate models typically underestimate the size, speed, and extent of those changes. Hence, . . . climate surprises are to be expected."¹ Despite the profound implications of this new viewpoint, hardly anyone rose to dispute it.¹⁵

Although people did not deny the facts head-on, many denied them more subtly by failing to revise their accustomed ways of thinking. "Geoscientists are just beginning to accept and adapt to the new paradigm of highly variable climate systems," wrote the NAS committee. And beyond geoscientists, "this new paradigm has not yet penetrated the impacts community"--the economists and other specialists who try to calculate the consequences of climate change.¹⁶ Policymakers and the public lagged even farther behind in grasping what the new scientific view could mean. As a geologist once remarked, "To imagine that turmoil is in the past and somehow we are now in a more stable time seems to be a psychological need."¹⁷

A gradual discovery process

How abrupt was the discovery of abrupt climate change? Many climate experts would put their finger on one moment: the day they read the 1993 report of the analysis of Greenland ice cores. Before that, almost nobody confidently believed that the climate could change massively within a decade or two; after the report, almost 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 they have waited until they 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 such had held evidence of decade-scale climate shifts before their eyes. They easily dismissed it. There were plausible reasons to dismiss global calamity as nothing but a crackpot fantasy. Sometimes the scientists' assumptions were actually built into their procedures: When pollen specialists routinely analyzed their clay cores in 10-cm 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—the short-term fluctuations in ice cores would have been passed over as meaningless noise.

First, scientists had to convince themselves, by shuttling back and forth between historical data and studies of possible mechanisms, that rapid shifts made sense, with the meaning of "rapid" gradually changing from millennia to centuries to decades. Without that gradual shift of understanding, the Greenland cores would never have been drilled. The funds required for those heroic projects became available only after scientists reported that climate could change in damaging ways on a time scale meaningful to governments. In an area as difficult as climate science, in which all is complex and befogged, it takes a while to see what one is not prepared to look for.

This article is based on The Discovery of Global Warming by Spencer Weart (Harvard U. Press, 2003) and was supported in part by the NSF program in history and philosophy of science. A more complete account, with full bibliographic references to the scientific work, may be found in hypertext online at http://www.aip.org/history/climate/rapid.htm.

Spencer Weart (sweart@aip.org) directs the Center for History of Physics at the American Institute of Physics.

References

1. 1. National Academy of Sciences, Committee on Abrupt Climate Change, Abrupt Climate Change: Inevitable Surprises, National Academy Press, Washington, DC (2002), pp. 1, 16. Available online at http://books.nap.edu/books/0309074347/html.
2. 2. C. E. P. Brooks, Q. J. R. Meteorol. Soc. 51, 83 (1925).
3. 3. See, for example, R. F. Flint, Am. J. Sci. 253, 249 (1955).
4. 4. H. E. Suess, Science 123, 355 (1956).
5. 5. W. S. Broecker, M. Ewing, B. C. Heezen, Am. J. Sci. 258, 429 (1960).
6. 6. M. Ewing, W. L. Donn, Science 123, 1061 (1956).
7. 7. W. S. Broecker, Science 151, 299 (1966).
8. 8. D. A. Barreis, R. A. Bryson, Wis. Archeologist 46, 204 (Dec. 1965).
9. 9. J. M. Mitchell Jr, Quaternary Res. 2, 436 (1972).
10. 10. National Research Council, US Committee for the Global Atmospheric Research Program, Understanding Climatic Change: A Program for Action, National Academy of Sciences, Washington, DC, (1975), appendix A.
11. 11. For beetle data, see G. R. Coope, Philos. Trans. R. Soc. London, Ser. B 280, 313 (1977); for albedo feedback theory, see M. I. Budyko, EOS Trans. Am. Geophys. Union 53, 868 (1972); for a discussion on ocean circulation, see K. Bryan, M. J. Spelman, J. Geophys. Res. 90, 11679 (1985) [INSPEC]; for ice-sheet theory, see H. Flohn, Quaternary Res. 4, 385 (1974).
12. 12. W. Dansgaard et al., Science 218, 1273 (1982) [INSPEC].
13. 13. For a summary and popular account, see P. A. Mayewski, F. White, The Ice Chronicles: The Quest to Understand Global Climate Change, University Press of New England, Hanover, N.H. (2002).
14. 14. Intergovernmental Panel on Climate Change, Climate Change 2001--The Scientific Basis: Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, J. T. Houghton et al., eds., Cambridge U. Press, New York (2001), p. 420. Available online at http://www.ipcc.ch/pub/reports.htm.
15. 15. Current thinking is reviewed in R. B. Alley et al., Science 299, 2005 (2003) [INSPEC].
16. 16. See ref. 1, p. 121.
17. 17. E. Moores, quoted in J. McPhee, Annals of the Former World, Farrar, Straus and Giroux, New York (1998), p. 605.
18. 18. W. Dansgaard et al., in The Late Cenozoic Glacial Ages, K. K. Turekian, ed., Yale U. Press, New Haven, Conn. (1971), chap. 3.

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