Living in a climate of uncertainty

The Swiss geophysicist Hans Oeschger, one of the first researchers to lay eyes on the polar ice profile, recognized what the newly deduced pattern of natural climate behavior was likely to mean to the future. That disturbing vision inspired a sense of social responsibility in him that lasted the rest of his life. Oeschger realized that Earth's climate system does not always respond slowly to gradual "forcings" or modifying factors. Occasionally it crosses a threshold of some kind—and then it flips. The pace of change may have nothing to do with the pace of the forcing, Oeschger realized, and the climate's new equilibrium state, lasting a thousand years or more, is very different from the old one. At least two dozen of these gyrations—these Dansgaard-Oeschger events— punctuated the last 110,000 years of climate, and as he pondered their mysteries in the early 1980s, one conclusion he reached was that the nature of these changes is not friendly to humans. Like the gradual orbital influences that eventually pushed the climate system over a threshold into and out of ice ages, Oeschger warned, the gradual loading of greenhouse gases in the atmosphere could force the climate over another threshold into another state.

In his Nature obituary of Oeschger, Thomas F. Stocker, his successor at the University of Bern, wrote that as long ago as 1984, "Oeschger recognized the importance of ocean circulation in explaining abrupt climate change and used the physical analogy of a flipflop system—triggered by small perturbations, the ocean circulation might switch from one circulation mode to another. Based on these insights, he was among the first to point out that the anthropogenic increase of CO2 could represent such a perturbation. Although his early warnings were often greeted with disbelief "

Another key concept occurred to Oeschger as he pored over a job of editing some of his work with a colleague in the small office of an international climate research program in Bern. "He wrote with a Swiss accent, but his work also had a distinctive style and flare," Herman Zimmerman recalled of their work together in Bern in the 1990s. "We spent long hours editing out the accent, but keeping the flare. The term 'climate surprise' came from Hans' not being able to find quite the correct phrase in English, but we left it, because it actually described just the right thought—the abrupt, short-lived, climate changes of the glacial stages that had been unexpectedly found in the ice core records from Greenland."

To be sure, the confirmation of abrupt climate change in polar ice and other environmental archives was a big surprise to scientists in the 1990s. What really resonated with Oeschger and later researchers, however, was the sense that surprise inhabits the future in an unexpectedly important way—that the natural behavior of Earth's climate is both more dangerous in character and more uncertain than just about anyone had supposed. Most every theorist since Oeschger has employed variations on his "flip-flop" analogy and his concept of "climate surprise."

Depending on his audience, Richard B. Alley talks about a climate with dimmers and switches or compares abrupt change to a bungee jumper hanging over the side of a roller coaster car. Boing! Boing! About how "leaning slightly over the side of a canoe will cause only a small tilt, but leaning slightly more may roll you and the craft into the lake." About the drunkard who sleeps when left alone and staggers when awakened.

Wally Broecker, having conferred with Oeschger in Bern, was seeing the same writing on the same wall. In an influential 1987 commentary in Nature, Broecker wondered out loud if there would be "Unpleasant surprises in the greenhouse." With our "gigantic environmental experiment" of loading the atmosphere with carbon dioxide and other greenhouse gases, he wrote, "we play Russian roulette with climate, hoping that the future will hold no unpleasant surprises. No one knows what lies in the active chamber of the gun, but I am less optimistic about its contents than many."

Coming even before the so-called global warming debate, a dispiriting experience for many U.S. climate scientists in the 1990s, Broecker's commentary sounded a dark and pessimistic note. Records from polar ice and elsewhere indicate "that Earth's climate does not respond to forcing in a smooth and gradual way," he wrote, and the consequences of forcing from the buildup of greenhouse gases "are potentially quite serious." Our food supply and many species of wildlife could be at risk. "To date, we have dealt with this problem as if its effects would come in the distant future and so gradually that we could easily cope with them," he wrote. "This is certainly a possibility, but I believe that there is an equal possibility that they will arrive suddenly and dramatically." Researchers didn't know enough to make reliable predictions, and indeed, "reliable modeling may never be possible." Even with a great intensification of effort, he wrote, "I fear that the effects of the rise in concentration of the greenhouse gases will come largely as surprises."

In a variety of forums and formats, Broecker tried to jolt scientists and citizens out of what he saw as a dangerous complacency on the subject, warning in Natural History magazine in 1987, for example, that rather than "treating it as a cocktail hour curiosity, we must view it as a threat to human beings and wildlife that can be resolved only by serious study over many decades." Ten years later, in GSA Today, a journal of the Geological Society of America, Broecker asked: "Will Our Ride into the Greenhouse Future Be a Smooth One?" He warned that a doubling of the carbon dioxide concentration of the atmosphere would come as the planet is asked to feed double the human population during the next century. He reviewed the evidence from the Greenland ice and elsewhere, placing new emphasis on the possibility of water vapor and conditions in the Tropics triggering abrupt change.

"Whatever pushed Earth's climate didn't lead to smooth changes, but rather to jumps from one state of operation to another," he wrote. "So the question naturally arises, What is the probability that through adding CO2 we will cause the climate system to jump to one of its alternate modes of operation?" We don't have any idea, he declared, although we do know that warming temperatures can reduce the density of polar surface water and cut off the circulation that carries tropical warmth through the North Atlantic. "So we're entering dangerous territory and provoking an ornery beast," he wrote. "Our climate system has proven that it can do very strange things.

"Since we've only recently become aware of this capability, there's nothing concrete that we can say about the implications," he conceded, although he saw in this uncertainty a strength to the argument. "This discovery certainly gives us even more reason to be prudent about what we do, though. We must prepare for the future by learning more about our changeable climate system, and we must create the wherewithal to respond if the CO2-induced climate changes are large, or, worse yet, if they come abruptly, changing agricultural conditions across the entire planet. We must think all this through. Even if there is only a 1 percent probability that such a change might occur during the next 100 years, its impact would be sufficiently catastrophic that the mere possibility warrants a lot of preparation."

In the 1990s, the danger of greater risk from essentially unpredictable natural forces was about the last piece of information that policy makers wanted to know about this troublesome subject. And so it seems that most of them did not hear it, or at least did not hear it in a way that could become part of the cantankerous political dialogue of the times. In any case, there were caveats by the carload. No one could be certain, of course, but the abrupt changes detected in Greenland ice cores looked to most researchers like ice age events, whereas the Holocene (last 10,000 years) climate seemed to have been blessedly stable. Whatever the case, uncertainty is inherent in surprise, and uncertainty simply is not welcome in the political arena.

The subject of natural variation and the uncertain potential for abrupt change seldom came up in the hotly partisan 1990s U.S. political debate about "global warming" and the Kyoto Protocol. The closer Hans Oeschger had looked at the problem, the more seriously he had taken science's social responsibility. Colleagues remember him saying, "The worst for me, would be if there were serious changes in the next five to ten years and we scientists . did not have the courage to point out these dangerous developments early on." But climate scientists who took up Oeschger's challenge found themselves in an arena that was not congenial to science or receptive to their qualified trains of thought.

In the greater marketplace of ideas, transactions are brokered by people who often are immaculate of science and who have their own, more certain ideas about the future. Like other marketplaces, it is a realm in which the point of view's viability can be influenced by the amount of money its adherents have to spend. It is nothing if not democratic, such a marketplace; one idea is just as good as another and every story has two sides.

Climate scientists were treated in the mass media not as courageous researchers performing acts of social responsibility, but as just another interest group—and a poorly financed one at that. More often than not, they found themselves arguing with industry lobbyists, who were given equal billing, in circumstances designed by television producers and editors to elicit the greatest possible conflict. So disagreements were frequently passionate and often devoid of science. Before long, the subject was just another "hot button" topic, an article of political faith that had nothing to do with climate. Global warming was something one either "believed in" or most definitely did not. Even among believers, most often it was seen as a gradually worsening problem of uncertain consequences that would manifest themselves in a manageable way over time.

The United Nations-sponsored Intergovernmental Panel on Climate Change, which began raising the possibility of "climate surprises" from feedbacks in the system in 1991, was still, more than 10 years later, composing graphical representations of the future that depicted congenially smooth curves of gradual change as far out as computer models could see. And more than a decade after the Greenland ice cores confirmed abrupt change, virtually all political discourse in the United States remained devoted to the idea that climate conforms to the rules of a well-behaved, linear system.

Climate crashes such as those that punctuate the paleoclimate archives are said to be high-impact, low-probability scenarios, like asteroid impacts or other catastrophes, although even this assertion raises questions that climate scientists cannot easily manage. Who really knows? Now they can say with certainty that thresholds exist in the system and that crossing a threshold could suddenly pitch the climate into a new mode of operation that societies could find difficult to adapt to—or worse. But who can say how close or how distant the next abrupt change is?

By the turn of the new millennium, finally the idea of abrupt change had reached some kind of critical mass among climate scientists. In a coming of age for the science, the National Research Council appointed a special committee to conduct a comprehensive review of the subject. Some of the brightest climate scientists in the business were asked to identify "critical knowledge gaps" and to recommend a research strategy. In 2002, the NRC committee issued its report Abrupt Climate Change: Inevitable Surprises.

"We do not yet understand abrupt climate changes well enough to predict them," Richard Alley, who chaired the NRC committee, wrote in a preface to the report. "The models used to project future climate changes and their impacts are not especially good at simulating the size, speed, and extent of the past changes, casting uncertainties on assessments of potential future changes. Thus, it is likely that climate surprises await us."

A problem that Alley and other paleoclimate scientists refer to as the "insensitivity of models" or the "model-data gap" sounds like a technical issue but really is more fundamental. It means that the models are unable to reproduce accurately the numerous episodes of abrupt change that show up clearly in many environmental archives around the world. The reasons for this failure are not yet known, but the implications are plain enough. Until these highly sophisticated numerical representations of Earth's climate system—running on the world's most powerful computers—are able to get the past right, what reason is there to believe they can get the future right?

This is a divide that separates two general trains of thought about climate's behavior in the future. How good are the models? How complete is the theory of climate? It is such questions that most divide climatologists' opinions—not the fact that changes are coming or that greenhouse gases will cause them, or the basic chemistry, so much as the underlying physics: the steepness and slipperiness of the slope.

On one side is the well-established idea that the models, although imperfect, still best represent the behavior of warm-era climate—they get the recent past right—so they are the best guides to the future. This line of thought assumes that climate will continue to behave as it has much of the time—a small push usually leads to a small response—and the record of the recent past proves the point. On the other side is the upstart idea, born out of the Greenland ice cores, that climate is a precariously balanced nonlinear system that lurches between very different states of coldness, dryness, wetness, and warmth. This is the meaning of the "climate surprise" warning flag that abrupt-change researchers are waving—small changes can provoke big responses, and the paleoclimate record proves the point.

"Although abrupt climate changes have shocked ecosystems and societies over the last few millennia," the NRC committee observed, "these changes have not been as dramatic as those that occurred during the last ice age. It is probably no coincidence that stability of the climate increased when ice-sheet size and atmospheric carbon dioxide concentration largely leveled off at the end of the ice age."

Could the continuing buildup of greenhouse gases in the atmosphere during the modern industrial era provoke climate to change abruptly? Yes, the NRC committee concluded, it could. Abrupt change could happen any time in a chaotic system such as Earth's climate, said the report, but the "existence of a forcing greatly increases the number of possible mechanisms. Furthermore, the more rapid the forcing, the more likely it is that the resulting change will be abrupt on the timescale of human economies or global ecosystems."

The paleoclimate record shows that "the abrupt climate changes of the past were especially prominent when orbital processes were forcing the climate to change most rapidly during the cooling into and warming out of the ice age, consistent with the results from modeling that forcing of climate increases the possibility of crossing thresholds that trigger abrupt change," the committee wrote. "Given our understanding of the climate system and the mechanisms involved in abrupt climate change, this committee concludes that human activities could trigger abrupt climate change. Impacts cannot be predicted because current knowledge is limited," but there are many possibilities. Such well-known modes of annual variation such as El Niño or the North Atlantic Oscillation could change their behavior. Droughts could become more frequent or widespread or show up in unexpected places. And the circulation of the North Atlantic could slow down or speed up.

Across different timescales, almost every important climate change, from the advance and retreat of glaciers to the comings and goings of El Niño, is driven by closely coupled feedback loops between conditions in the oceans and the atmosphere. Winds control ocean currents that control sea surface temperatures that control atmospheric pressure differences that control winds—and so on. The North Atlantic Oscillation is another ocean-atmosphere feedback loop that steers the tracks of storms over much of the Northern Hemisphere as it waxes and wanes. In the new context of abrupt change, of a chaotic system, is it really possible for climate science to untangle all of the linkages, to categorically ascribe some changes to human-induced greenhouse gas loading and others to natural variation and to predict the future?

The modern climate system slips in and out of preferred modes of behavior such as a stronger or weaker North Atlantic Oscillation or Pacific Ocean patterns that seem to alter the intensity and frequency of the El Nino-Southern Oscillation. These various changes in "decadal variability" not only have powerful and far-ranging impacts on global precipitation, drought, and critical monsoon patterns, but also may affect one another. If the system is nearing a threshold of abrupt change, could one of these natural shifts push it over the edge? It is a measure of the nonlinear nature of the system, this Gordian knot of climate, that potential upstream causes of change and potential downstream effects cannot readily be distinguished from one another.

Perhaps it is no wonder that economists and social scientists have only begun to take up the idea of abrupt change and assess its potential to alter the economic well-being and stability of modern societies. In 2003, Alley and other members of the NRC committee noted in Science: "Although there is a substantial body of research on the ecological and societal impacts of climate change, virtually all research has relied on scenarios with slow and gradual changes. In part, this focus reflects how recently the existence of abrupt climate changes gained widespread recognition, and how difficult it has been to generate appropriate scenarios of abrupt climate change for impacts assessments." In addition, the scientists noted, the United Nations Framework Convention on Climate Change "has focused attention on anthropogenic forcing, whereas abrupt climate change is a broader subject covering natural as well as human causes."

An increasing number of studies that link economic and climate models "indicate that many sectors of the economy can adapt to gradual climate changes over the coming decades. But this research sheds little light on the impacts of abrupt climate changes, particularly where these involve major changes in precipitation and water availability over periods as short as a decade," and there is "virtually no linked research on abrupt climate change." While economic estimates based on the gradual warming scenario indicate that climate change could be slowed by "modest but increasing emissions reductions and carbon taxes . efficiently avoiding abrupt change may involve much larger abatement costs."

Even in the United Kingdom and northwestern Europe, a region that is in the cross-hairs of the dominant scenario for abrupt change, economists and social scientists have only begun to study the potential impact of a slowdown or collapse of thermohaline circulation in the North Atlantic.

In 2002, a preliminary report by the UK's Natural Environmental Research Council noted that a "sudden strong cooling could be catastrophic for agriculture, fisheries, industry and housing," although another UK study noted that "neither the probability and timing of a major ocean circulation change nor its impacts can be predicted with confidence yet."

Ominous signs of change are being detected in the North Atlantic. In 2002, in Nature, Robert Dickson of the UK's Centre for Environment, Fisheries, and Aquaculture Science described "a widespread, sustained, rapid and surprisingly uniform freshening of the deep and abyssal North Atlantic, south of the Greenland-Scotland Ridge, over the past four decades." It was the greatest change in oceanography ever recorded in the modern era, although still smaller than ice age changes recorded in seafloor sediment cores. Alley and the NRC committee noted in Science this "invasion of low-salinity deep waters that spread over the entire subpolar North Atlantic Ocean and the seas between Greenland and Europe in just the regions critical for abrupt shifts in thermohaline circulation, which has been implicated in many abrupt climate-change events in the past." In 2003, also in Nature, physical oceanographer Ruth Curry reported marked salinity changes in surface waters through the western basins of the North Atlantic between the 1950s and 1990s. A "systematic freshening" is occurring near both poles, Curry wrote, and a "large increase of salinity" is being detected at low latitudes. These shifts are worldwide, she wrote, and "suggest links to global warming and possible changes in the hydrologic cycle of the Earth." In spring 2004, in Science, oceanographers Sirpa Häkkinen and Peter Rhines reported satellite measurements detecting a slowing of the North Atlantic current known as the subpolar gyre.

Other researchers are monitoring critical conditions in the polar ice sheets, where abrupt changes could lead to global sea level rise. In 2004, two reports in the journal Geophysical Research Letters reported rapid changes in West Antarctica following the 2002 collapse of the Larsen B ice shelf. Using satellite images, the two teams reported glacier acceleration and thinning and accelerated ice discharge from the Antarctic Peninsula. Other satellite reports cited accelerated melting of the Greenland ice sheet and a rapid decline in Arctic sea ice. In the fall of 2004, a multinational comprehensive study of the Arctic reported that the region was experiencing "some of the most rapid and severe climate change on Earth."

In "The Perfect Ocean for Drought," as researchers Martin Hoerling and Arun Kumar reported in Science in 2003, cold sea surface temperatures cover much of the eastern Pacific and warm surface temperatures prevail in the western tropical Pacific and Indian Oceans. Simulations by these climate modelers and other researchers have linked these ocean temperature patterns to drought conditions across the globe. The persistence of this pattern from 1998 to 2002 accounts for four years of drought across North America, southern Europe, and southwestern Asia. And still, through 2004, unusually warm temperatures prevailed in the Indo-western Pacific Tropics, and the U.S. Southwest and central-southwest Asia continued to endure drought. Some researchers saw the signature of global warming in the pattern. Paleoclimatologist Jonathan Overpeck called it the biggest drought in the United States since records have been kept and wondered if "we've pushed the climate system over a threshold."

Overpeck's and other researchers' focus on the Pacific Ocean is part of a noticeable shift in the attention of climate scientists toward events in the Tropics as a region from which abrupt changes are driven. While the North Atlantic may have been the critical center of action in a world dominated by ice sheets (the argument is not settled), events in the Tropics seem especially potent in a world dominated by warming. The tropical specialist Raymond T. Pierrehumbert heralded the new perspective in the Proceedings of the National Academy of Sciences in 2000: "Climate Change and the Tropical Pacific: The Sleeping Dragon Wakes."

The failure of computer models that simulate shutdowns in Atlantic Ocean circulation to accurately reproduce the effects that these abrupt changes left behind in the climate record may mean that the trigger for such events is to be found elsewhere in the system. Moreover, "given that we don't know what accounts for the unusual stability of the recent climate, we don't know what it would take to break it," Pierrehumbert wrote. "This thought is unsettling in a world seemingly committed to substantial warming from anthropogenic CO2 increases in the next 2 centuries."

Pierrehumbert identified "an intimate link between events on the margin of Antarctica, and the tropical Pacific processes that govern both El Niño and Pacific oceanic heat transport." Thus, events in and around Antarctica, along with changes in the Tropics, bear watching.

"Whatever the world ocean does, the tropics may respond with some surprising reorganizations of convection," he wrote. There is "nothing inevitable about the present configuration" of the tropical Pacific—of a "warm pool" of water in the far western Pacific, the "cold tongue" of sea surface temperatures extending out toward the central ocean from the equatorial shore of South America, and a band of towering convective storms known as the Intertropical Convergence Zone that snakes north and south of the Equator with the seasons. It's just possible that the whole thing could break down—that the easterly trade winds could be replaced by a "westerly superrotation" leading to far-reaching changes in climate. It's an "exotic possibility," to be sure, and there is no evidence that such a state ever existed in Earth's climate. But the same might be said of the near future. A state that includes especially high concentrations of carbon dioxide in the atmosphere at the same time that ice covers the poles is "different from any that has ever before held sway on the planet," he wrote. "If one is tugging on the dragon's tail with little notion of how much agitation is required to wake him, one must be prepared for the unexpected."

In 1997, Wally Broecker wrote in GSA Today that he had been "humbled" by his lifetime study of Earth's climate, a circumstance that may have surprised a few graduate students at Columbia University. "I'm convinced that we have greatly underestimated the complexity of this system," he wrote. "The importance of obscure phenomena, ranging from those that control the size of raindrops to those that control the amount of water pouring into the deep sea from the shelves of the Antarctic continent, makes reliable modeling very difficult, if not impossible. If we're going to predict the future, we have to achieve a much greater understanding of these small-scale processes that together generate large-scale effects."

The more likely circumstance, as Broecker was among the first to suggest, is that we are not going to predict the future with any degree of confidence—that climate surprises are inevitable. As Alley and the NRC Committee on Abrupt Climate Change, writing in Science in 2003, put the case: "The difficulty of identifying and quantifying all possible causes of abrupt climate change, and the lack of predictability near thresholds, imply that abrupt climate change will always be accompanied by more uncertainty than will gradual climate change."

In 2002, in a lecture to the American Geophysical Union meeting in San Francisco, Alley reviewed the performance of the major climate models on which the Intergovernmental Panel on Climate Change has based its forecasts for a globally warming Earth. When it comes to simulating the past, across the board the models underestimate the changes that are known to have taken place. "On average, the models got two-thirds of what happened," he said. "It's not a cold bias, it's not a warm bias. It's an insensitivity to changed boundary conditions ... how sensitive the model is when you change things."

"The least astonishing hypothesis that I get from this is that either the future warming projections are accurate or they have underestimated what we face in the future," he said. "There's a lot of paleoclimate work to be done here to test this hypothesis," but if climate models are systematically underestimating reality, then the more radical-looking "high side" of temperature projections may be more accurate than the conservative-looking "low side."

Not only are policy makers presented with what are likely to be overly optimistic expectations of the future, they are presented a profile with contours that look nothing like the record of climates past. The changes of the past are single lines or narrow bands that represent real data, whereas the projections of the future prepared for policy makers are large smoothed curves that represent the average results of many computer model simulations and a wide range of possibilities. "This tendency of the policy maker to see a smooth curve has to be really disturbing," said Alley. "Because whatever it's going to do, if it's smooth, we're going to be really surprised. It's going to stagger, it's going to jump. What happens regionally is not going to happen globally. And we really, I think, need to look at how variable it will be. What's possible in the system, and where does it go?"

The news from Greenland, unfortunately, is that much more is possible in the climate system than anyone would have supposed. Changes can be big and fast and potentially dangerous to societies that are heavily invested in stability and resistant to adaptation. In the event of an abrupt change—a climate surprise—political arguments probably will no longer be about industrial emission controls, their fairness or economic viability. More urgent political and economic problems will command the attention of nations around the globe. In the event, it is in nature's power to so change our world that even such basic questions as cause—whether the crash came naturally or not—could seem sadly beside the point.

0 0

Post a comment