The Discovery of Rapid Climate Change (part 1)
Only within the past decade have researchers warmed to the possibility of abrupt shifts in Earth's climate. Sometimes, it takes a while to see what one is not prepared to look for.
August 2003, page 30
How fast can our planet's climate change? Too slowly for humans to notice, according to the firm belief of most scientists through much of the 20th century. Any shift of weather patterns, even the Dust Bowl droughts that devastated the Great Plains in the 1930s, was seen as a temporary local excursion. To be sure, the entire world climate could change radically: The ice ages proved that. But common sense held that such transformations could only creep in over tens of thousands of years.
In the 1950s, a few scientists found evidence that some of the great climate shifts in the past had taken only a few thousand years. During the 1960s and 1970s, other lines of research made it plausible that the global climate could shift radically within a few hundred years. In the 1980s and 1990s, further studies reduced the scale to the span of a single century. Today, there is evidence that severe change can take less than a decade. A committee of the National Academy of Sciences (NAS) has called this reorientation in the thinking of scientists a veritable "paradigm shift." The new paradigm of abrupt global climate change, the committee reported in 2002, "has been well established by research over the last decade, but this new thinking is little known and scarcely appreciated in the wider community of natural and social scientists and policymakers."1
Much earlier in the 20th century, some specialists had evidence of abrupt climate change in front of their eyes. The evidence was meaningless to them. To appreciate change occurring within 10 years as significant, scientists first had to accept the possibility of change within 100 years. That, in turn, had to wait until they accepted the 1000-year time scale. The history of this evolution gives a good example of the stepwise fashion in which science commonly proceeds, contrary to the familiar heroic myths of discoveries springing forth in an instant. The history also suggests why, as the NAS committee worried, most people still fail to realize just how badly the world's climate might misbehave.
Was a 1000-year climate change possible?
During the early decades of the 20th century, a very few meteorologists did speculate about possibilities for rapid change. The most striking scenario was offered in 1925 by the respected climate expert C. E. P. Brooks, who suggested that a slight change of conditions might set off a self-sustaining shift between climate states. Suppose, he said, some random decrease of snow cover in northern latitudes exposed dark ground. Then the ground would absorb more sunlight, which would warm the air, which would melt still more snow--a vicious feedback cycle. An abrupt and catastrophic rise of tens of degrees was conceivable, Brooks wrote, "perhaps in the course of a single season."2 Run the cycle backward, and an ice age might suddenly descend.
Most other professional climatologists dismissed the idea as preposterous. The continental glaciers of an ice age, a kilometer thick, would surely require vast lengths of time to build up or melt away. Beyond that elementary reasoning lay a deeper rejection of all such speculations. It was the climatologists' trade to compile statistics on past weather in order to advise a farmer what crops to grow or tell an engineer what sort of floods were likely over the lifetime of a bridge. The climatologist's career thus rested on a conviction that the experience of the recent past reliably described future conditions. That belief was supported by a paucity of data, for hardly any accurate records of daily temperatures and the like went back more than half a century or so. The limitation scarcely worried climatologists, who assumed that significant changes took place only over thousands of years. In their textbooks, climate was introduced as the long-term average of weather over time, by definition, static over centuries.
The experts held a traditional belief that the natural world is self-regulating: If anything started to perturb a grand planetary system like the atmosphere, natural forces would automatically compensate. Scientists came up with various plausible self-regulating mechanisms. For example, if temperatures rose, then more water would evaporate from the seas; in response, clouds would thicken and reflect more sunlight, which would restore normal temperatures. The perception of self-regulation reflected a view of the world held deeply in almost every human culture: Stability was guaranteed, if not by Divine Providence, then by the suprahuman power of a benevolent "balance of nature."
Those beliefs were not disturbed by the few long-term climate records available at the time. The best of those data were compiled in the 1920s by an Arizona astronomer, Andrew Ellicott Douglass, who noted that the rings in trees were thinner in dry years. Analyzing old logs, Douglass reported a major century-long climate perturbation around the 17th century. But most other scientists doubted that tree rings (if they reflected climate at all) gave information about anything beyond random regional variations.
Signs of climate shifts were also visible in varves, a Swedish word for the layers laid down each year in the mud on the bottom of northern lakes. From bogs and outcrops where the beds of fossil lakes were exposed, or from cores of slick clay drilled out of living lakes, the layers were painstakingly counted and measured. Ancient pollen told what plants had lived in the region when the layers were laid down. Major changes in vegetation suggested that the last ice age had not ended with a uniformly steady warming, but with peculiar oscillations of temperature. Scandinavian data revealed a particularly striking shift around 12 000 years ago, when a warm period gave way to a spell of bitterly cold weather, dubbed the Younger Dryas, after Dryas octopetala, a hardy Arctic flower whose pollen signals frigid tundra. In 1955, the timing was pinned down by a radiocarbon-dating study, which revealed that the temperature change had been rapid; for climate scientists at midcentury, "rapid" meant a change that took place over as little as 1000 years.3
Ice-age changes over a thousand years or so in a restricted region, although surprising, seemed acceptable. The rate of advance and retreat of the great glaciers would be no faster than present-day mountain glaciers were seen to move. That perception was compatible with the so-called uniformitarian principle, a geological tenet that the forces that molded ice, rock, sea, and air did not vary over time. Through most of the 20th century, the uniformitarian principle was cherished by geologists as the very foundation of their science: How could one study anything scientifically unless the rules stayed the same? The idea had become central to their training and theories during a century of disputes, when scientists painfully gave up traditions that explained certain geological features by invoking Noah's Flood or other supernatural interventions. In human experience, temperatures apparently did not rise or fall radically in less than millennia, so the uniformitarian principle declared that such changes had never happened in the past. Scientists found themselves insisting on this principle as they were confronted, time and again, by cranks and religious fundamentalists who publicly proclaimed ideas about apocalyptic global cataclysms.
Something resembling catastrophic climate jumps could in fact show up in varves. But the silt layers could have been distorted in countless ways that had nothing to do with climate: a forest fire perhaps, or a shift of stream drainage. Scientists saw the jumps not as climate data to be analyzed, but as mere local noise. They did not worry about the fact that old radiocarbon dates were accurate only within a thousand years or so, so that the chronologies of different sites could not be matched well enough to point to any rapid and widespread change.
In 1956, studying variations in the shells of plankton that were embedded in cores of clay pulled from the deep seabed (see figure 1), radiocarbon expert Hans Suess discovered what was at the time the fastest change that anyone expected. Suess reported that the last glacial period had ended with a relatively rapid rise of temperature, about 1°C per thousand years.4 It scarcely bothered him and his colleagues that no faster change could have been seen in most cores. In many places, the mud was constantly stirred by burrowing worms or by seafloor currents and slumping, which blurred any differences between layers. Yet the data curves did sharpen as cores were pulled from regions of rapid deposition and as radiocarbon dating improved. By 1960, a trio of scientists at what is now the Lamont-Doherty Observatory—Wallace Broecker, Maurice Ewing, and Bruce Heezen—were reporting a variety of evidence, from deep-sea and lake deposits, that a global climate shift of as much as 5-10°C had taken place in less than a thousand years.5 Most of their colleagues found such a rise barely plausible.
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.
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.
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