El Nino and the Earths Climate: from Decades to Ice Ages
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During the s, additional speculations lengthened the list. That was methane, a greenhouse gas that traps heat radiation even more effectively than CO 2 , so it could cause more warming still in a vicious feedback circle. It was getting easier for scientists to consider such colossal transformations, for uniformitarian thinking was under attack.
By the early s, some geologists were stressing the importance of rare events like the enormous floods that had drained temporary lakes during the melting of the continental ice sheets. In biology, Stephen Jay Gould and a few others were arguing that some species had evolved in "punctuated" bursts. Had a stunning climate change, following the fall of a giant asteroid, exterminated the dinosaurs in a single frozen year? Could something like that befall us?
Many scientists continued to look on such speculations as little better than science fiction. The evidence of abrupt shifts that turned up in occasional studies may seem strong in retrospect, but at the time it was not particularly convincing. Any single record could be subject to all kinds of accidental errors. The best example was in the best data on climate shifts, the wiggles in measurements from the Camp Century core. These data came from near the bottom of the hole, where the ice layers were squeezed tissue-thin and probably folded and distorted as they flowed over the bedrock.
Broecker later remarked that the relatively smooth temperature record of oxygen isotopes in deep-sea sediments "tended to lull scientists into concluding that the Earth's climate responds gradually when pushed. These scientists should have realized that the top few meters of ocean exchange heat only slowly with the rest.
And they should have recalled that at most places in the deep sea, sediments accumulate at only a few centimeters per thousand years. Thus further progress would depend on getting more and better ice cores.
Ice drilling was becoming a little world of its own, inhabited by people of many nations Dansgaard's "Danish" team spoke eight different languages. Their divergent interests made for long and occasionally painful negotiations. But the trouble of cooperation was worth it for bringing in a variety of expertise, plus what was also essential a variety of agencies that might grant funds. The outcome was a series of engineering triumphs, which could turn into maddening fiascos when a costly drill head got irretrievably stuck a mile down.
Engineers went back to their drawing-boards, team leaders contrived to get more funds, and the work slowly pushed on. The first breakthrough came after the ice drillers went to a second Greenland location, a military radar station named "Dye 3" some 1, kilometers distant from Camp Century.
By , after a decade of tenacious labor and the invention of an ingenious new drill, they had extracted gleaming cylinders of ice ten centimeters in diameter and in total more than two kilometers long. Dansgaard's group cut out 67, samples, and in each sample analyzed the ratios of oxygen isotopes. Moreover, the most prominent of the changes in their record corresponded to the Younger Dryas oscillation seen in pollen shifts all over Europe. It showed up in the ice as a swift warming interrupted by "a dramatic cooling of rather short duration, perhaps only a few hundred years.
A particularly good correlation came from a group under Hans Oeschger. An ice drilling pioneer, Oeschger was now measuring oxygen isotopes in glacial-era lake deposits near his home in Bern, Switzerland. That was far from Greenland, but his group found "drastic climatic changes" that neatly matched the ice record. The severe cold spells became known as "Dansgaard-Oeschger events. As ice drillers improved their techniques, making ever better measurements along their layered cores, they found a variety of large steps not only in temperature but also in the CO 2 concentration. Since the gas circulates through the atmosphere in a matter of months, the steps seemed to reflect world-wide changes.
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Yet clearly something had made spectacular jumps. A variety of other evidence for very abrupt climate changes was accumulating, and some began to entertain the notion of such change on a global scale.
Most of these scientists, after presenting their data, could not resist adding a few suggestive words about possible causes. Dansgaard's group was typical in speculating about "shifts between two different quasi-stationary modes of atmospheric circulation. It implied transient variations of wind patterns within broad limits, and mostly concerned how weather might change in a particular region.
The new thinking about grand global shifts urged a broader view. It was hard to see how the atmosphere could settle into an entirely new state unless something drastic happened in the oceans. For it is sea water, not air, that holds most of the heat energy and most of the moisture and CO 2 of the climate system. The question of century-scale shifts, now a main topic in climatology, came to rest on the desks of ocean scientists.
Their response was prompt. Experts mooted various hypotheses about how changes in the surface waters might affect CO 2 levels. There were complex links among temperature, sea water chemistry, biological activity, and the chemical nutrients that currents brought to the surface. Oceanographers also increasingly found it plausible that the pattern of North Atlantic Ocean circulation could change on a short timescale, as Broecker had proposed to explain the Younger Dryas.
Since the circulating waters carry tremendous quantities of heat northward from the tropics, if the circulation ground to a halt, temperatures in many regions of the Northern Hemisphere would immediately plunge. In , he wrote that scientists had been "lulled into complacency. They failed to realize that these models, in the very way they were constructed, allowed only smooth and gradual changes.
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The authors of an "unstable" model would rework it until it yielded more consistent results. Broecker strongly suspected that "changes in climate come in leaps rather than gradually" — posing a drastic threat to human society and the natural world. As computer modelers labored to incorporate interactions between air and sea, their new simulations hinted that he was right. Early in the s, further revelations startled climate scientists. The first shock came from the very 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 in the s scientists had believed would take tens of thousands of years, in the s thousands of years, and in the s 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 oscillations of temperature repeatedly at irregular intervals throughout the last glacial period. 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 later studies found a big temperature jump within a single year. 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 change.
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In some places the sediments accumulated very rapidly, while in others, the sea water lacked enough oxygen to sustain life. The first results, from the Norwegian Sea in , 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.
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Changes in dustiness were meanwhile noted in the ice itself, indicating at least continental scope for the change; later, a hemisphere-wide Younger Dryas temperature step in less than a decade was confirmed by an abrupt change in the methane gas in the ice. It was only decades later, around , that scientists firmly established that the Younger Dryas cooling had been restricted to the Northern Hemisphere, with the Southern Hemisphere continuing to warm up.
Meanwhile, in the late s and early s, improved carbon techniques gave the first accurate dates for sediments containing pollen and other carbon-bearing materials at locations ranging from Japan to Tierra del Fuego.