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Making sense of rapid change

Evidence of a climate shift could only be accepted if it made sense--that is, if there existed some plausible theory of the climate system that could explain the shift. Broecker suspected that the cause might be a rapid turnover of North Atlantic ocean waters, but that was just hand-waving speculation. More influential was a 1956 paper by Ewing and William Donn, who built an elaborate model for the coming and going of ice ages.⁶ Like Brooks and others before them, Ewing and Donn began with the notion that a retreat of reflective snow and ice would bring more warming by sunlight. Their new idea was that the feedback mechanism had a hair trigger set off by ocean currents. As ice sheets melted and the sea level rose, warm water would spill into the Arctic Ocean and melt its ice cover, thus speeding up the warming. But once the Arctic Ocean was free of ice, they argued, so much moisture would evaporate that snow would fall heavily all around the Arctic, switching the feedback to cooling. Ewing and Donn thought it conceivable that the polar ocean might become ice-free and launch us into a new ice age within the next few hundred years.

Journalists alerted the public to the risk of a glacial advance within the foreseeable future. People were prepared to believe it, for they were already abandoning their old ideas about an imperturbable balance of nature. The headlong advances of population and industry were making themselves felt in ever more widespread pollution. More ominous still was the global radioactive fallout from nuclear weapons tests, alongside scientists' warnings that a nuclear war could wreck the entire planet. It was no longer inconceivable that some perturbation--even one produced from human industry--might alter the entire planet.

In fact, Ewing and Donn's theory was erroneous, as other scientists quickly pointed out. Nevertheless, it had served a useful function. For the first time, there was respectable scientific backing for a picture of rapid, even disastrous, climate change. Other scientists, even as they rejected the theory, were stimulated to broaden their thinking and to inspect data for new kinds of information.

Further stimulation came from entirely different studies. In the late 1950s, a group led by Dave Fultz at the University of Chicago carried out tabletop "dishpan" experiments in which they used a rotating fluid to simulate the circulation of the atmosphere. They created a simulacrum complete with a miniature jet stream and cyclonic storms. But when they perturbed the rotating liquid with a pencil, they found that the circulation pattern could flip between distinct modes. If the actual atmospheric circulation did that, weather patterns in many regions would shift almost instantly. In the early 1960s, climatologist Mikhail Budyko in Leningrad got disturbing results on a still larger scale from some simple equations for Earth's energy budget. His calculations indicated that feedbacks involving snow cover could indeed bring extraordinary climate changes within a short time. Other geophysical models turned up more possibilities for rapid change.

Figure 2

The most influential idea for what might bring rapid change was developed from old speculations about the circulation of the North Atlantic Ocean. In 1966, Broecker (pictured in figure 2), taking a close look at deep-sea cores, reported evidence for an "abrupt transition between two stable modes of operation of the ocean-atmosphere system."⁷ Nowadays, warm tropical water flows northward near the surface of the Atlantic; a large quantity, heavy with cold and salt, sinks near Iceland and returns southward in the deep. A change of temperature or salinity might shut down the circulation, cut off the northward transport of a huge amount of heat, and bring severe climate change. Simple numerical models involving the transport of fresh water by a changed pattern of winds showed that such a change could be self-sustaining.

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.