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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