Part Two
FOUR
Climate scientists entered the 1980s wondering when they might see a signal of climate change. They didn’t have to wait long. The first year of the decade, 1981, was the hottest ever recorded based on reliable records, followed by another record breaker just two years later.
The 1980s were the decade in which the rise in global temperatures first began to separate itself from the noise of annual variations; it was also the decade during which scientists began focusing on what those record temperatures signified. Crucial to that understanding were investigations into an event at the end of the last ice age. Called the Younger Dryas, it was named for an Arctic flower that proliferated during its 1,300-year span and represented the last frigid paroxysm of the most recent ice age. First identified in the 1930s, the Younger Dryas followed an 1,800-year warming period that ended around 12,800 years ago. The question that bedeviled geochemists, glaciologists, and others concerned with climate dynamics was how suddenly it occurred.
After all, in the 1950s, a sudden climate change might mean a thousand years, and if that was the fastest that climate could change, society could rest easy about the threat of global warming. As early as 1960, however, some scientists, most notably Wallace Broecker, had found evidence suggesting that the onset of the Younger Dryas occurred in far less than a thousand years. The problem was that Broecker’s evidence came from an analysis of deep sea and lake deposits, and no one could be sure that over the course of thousands of years those deposits hadn’t been rearranged and thus provided a false record. The rearranger could be anything from sea worms to underwater landslides.
Because the question of how suddenly the shift occurred would influence how seriously the public should take the threat of climate change, what happened during the Younger Dryas became much more than a question of academic interest. Understanding what happened during the period required proxies used to reconstruct past climates, but how reliable were they? Unfortunately, at the beginning of the decade, many of the proxies were either subject to disturbance, or had insufficient resolution to capture annual changes, or were too short-term to use in reconstructing a long-term climate record. For instance, the familiar carbon-14 dating technique, which is based on the known decay rate of its radioactive material, has an error range of roughly one hundred years and so could not capture a really rapid change. Moreover, the technique is useless for dating material more than fifty thousand years old, and reconstructing climate patterns requires records that go back hundreds of thousands to millions of years.
Another complicating factor is that regional climates can change quite suddenly, an aspect of climate that has long been recognized by scientists. Tree rings and many other proxies reliably document the sudden massive droughts that may have brought down the ancient Pueblo and Mayan civilizations. The Little Ice Age too was a well-studied phenomenon, but it represented but a minor blip compared with the Younger Dryas. Moreover, in the 1980s there was still active debate about whether the Little Ice Age was a regional rather than global event (or even whether the Little Ice Age was one event or a string of several events). Those concerned with greenhouse gas emissions needed to know whether climate could undergo major shifts rapidly and whether they could occur on a global scale.
What they needed, in a word, was a paleothermometer, something that provided a precise record of annual changes going back hundreds of thousands of years, and they needed one they could trust. During the 1980s, there was a veritable renaissance in developing proxies that had much finer resolution and were subject to less distortion than the ones inherited from previous decades. By the end of the eighties, after many such breakthroughs, climate scientists had their “thermometer.” It was not one, but rather a composite picture drawn from many different sources. In some cases, this meant developing entirely new proxies for past climate, while in others it entailed looking at previously collected material in an entirely new way. While scientists showed boundless ingenuity in teasing a picture of the past from cave formations, pollens, dust, ocean and lake sediments, and the remnants of long-dead shelled creatures, perhaps the most convincing evidence came from something long appreciated as a record of past climate but very difficult to study—ice.
Just as Antarctica and the summit of Mauna Loa offered ideal locations to collect data on carbon dioxide in the atmosphere, the great ice sheets of Greenland and Antarctica drew scientists looking for a long-term record of climate. The Greenland Ice Sheet is at least one million years old, and Antarctica’s ice sheets are much older. What Charles Keeling was to monitoring carbon dioxide in the atmosphere, a Danish geochemist named Willi Dansgaard was to teasing out information about the past from ice cores.
Collecting and interpreting a core turned out to be a fiendishly difficult problem. You have to know where to drill the borehole (the bottom of an ice sheet can be subject to folding and other changes that distort the record of the deeper parts). Then you have to be able to drill without contaminating the cores, and you have to be able to extract a 10,000-foot-long core in such a way that the pieces fit together precisely and seamlessly. Cores toward the base of the ice sheet exist under extreme pressure, more than 150 times the pressures at the surface, enough such that air bubbles are squeezed to invisibility. Unless handled perfectly, the cores can shatter or even explode if exposed immediately to the higher temperatures and lower pressures at the surface. And then there’s the problem of measuring what’s contained in the cores.
Scientists had to figure out how gases moved around in the top layer of the ice sheet before they became entrapped in bubbles. They had to know how to adjust for summer snowfalls, how to read the signals that marked when one year ended and another began, and they had to ground-truth the thermometer once they had readings.
They needed to find something in an ice core that was ten thousand or a hundred thousand years old that memorialized temperatures, precipitation, and winds. They found past winds in the amount of dust blown in annually from the plateaus of central Asia. A precipitation record was entombed in the thickness of the core. As for temperature, that was where Willi Dansgaard made his most valuable contribution.
Dansgaard had been interested in the composition of water from his teenage years. At age thirty, he studied rain samples from a storm that passed over Denmark. He had studied the isotopes in water and had an idea that they could be used to reconstruct the temperatures when the raindrops had formed. An isotope is an atom with extra neutrons. For instance, oxygen normally has eight protons and eight neutrons, giving it an atomic weight of 16. In a sample of rainwater, however, one will also find oxygen atoms with nine or ten neutrons, known as oxygen-17 and oxygen-18. As temperatures fall, the air can hold less moisture, and Dansgaard reasoned that as precipitation formed and fell, the heavier oxygen isotopes would fall out—condense—first. Consequently, as temperatures fell, the remaining isotopes in the air would tend to be the lighter ones. From this it followed that if Dansgaard could calibrate the ratios of the heavy and normal oxygen atoms in air at different temperatures, air trapped in bubbles in ice would provide a record of temperatures at the point at which the bubble formed. It was forensic geochemistry that any CSI detective might admire.
After twelve years of further study and collecting samples from all over the globe, Dansgaard published his findings in 1964, providing scientists with one of the keystones for reconstructing past climates. Immediately after publication, Dansgaard dived right into applying his paleothermometer to the study of ice cores extracted from an American site called Camp Century on the Greenland Ice Sheet. Dansgaard was principally concerned with long-term shifts, but the ice core record he recovered also revealed what looked like major short-term changes right around the time of the Younger Dryas. Focused on long-term changes, the team barely paid attention to this anomaly.
Dansgaard’s next major venture in ice coring took place from 1979 to 1981, again at an American installation, this one about 800 miles south of Camp Century. The enclave was part of the DEW (Distant Early Warning) Line that was developed during the Cold War to provide early warning of an attack by the Soviet Union. Neither Camp Century nor the DEW Line site was ideally situated for extracting a climate record, but Dansgaard had to make do with what was at hand. Dissatisfied with the available drill heads, Dansgaard commissioned one designed for the specific task and brought it with the team to the site, named Dye 3. The extracted ice core, more than a mile and a half long, showed the same rapid changes Dansgaard had found at Camp Century more than a decade earlier. With other investigations using other proxies finding the same indications of sudden jerks in temperatures, people began paying attention.
One of those people was Wally Broecker. In 1984, he listened to a talk by Hans Oeschger, who had partnered with Dansgaard in the Dye 3 drilling. As noted, Broecker had long argued that climate was subject to sudden jerks, not gradual change, and that these jerks might pose problems for humanity. One of the points Oeschger made was that the borehole readings showed that CO2 changed in lockstep with temperatures. Struck by the evidence that both CO2 and temperatures might change suddenly, Broecker started wondering what possible mechanism could bring about such shifts. Indeed, this derivative question raised by what appeared to be rapid climate changes in the paleo record turned out to be a far more difficult question than documenting the shifts themselves.
“Rapid” in the early 1980s meant a hundred to a few hundred years. Scientists confirmed these changes during the Younger Dryas through studies of lake bed sediments in Switzerland and pollen records. As various scientists turned their attention to refining the resolution of paleo records, things were coming into focus that had been easily overlooked when the assumption was that climate changed gradually. It was like a spy reading a clandestine message but not realizing that the real information was contained in a microdot in the period at the end of the sentence.
Over the course of the decade, climate scientists got better at seeing the dot and also peering into it. They were discovering climate cycles that had been concealed by the poor resolution of earlier proxies and the self-imposed blinders entailed in the assumption that climate cycles were long and stately (at one point, Broecker complained that the computer models of the time actually smoothed over results that might show sudden change).
As Broecker puzzled over what might cause the sudden changes found by Dansgaard and Oeschger, he returned to an idea he had speculated about early in his career. In 1985, he and colleagues published their theory in Nature. Broecker argued that only the oceans could bring about such rapid changes. He knew that enormous amounts of heat were distributed around the world by what he called the Great Ocean Conveyor. The Conveyor was an enormous river of saline water that moved through the oceans of the world and, in the North Atlantic, warmed Europe far more than other territories at equivalent latitudes. He calculated that this ocean circulation contributed as much heat to the North Atlantic above 45 degrees latitude as did the sun. As the pieces fell into place, Broecker offered his theory that periodic shutdowns of this conveyor were the mechanism that plunged the world into thousand-year deep freezes such as the Younger Dryas.
If Broecker was to argue that the Conveyor occasionally shut down, he also had to explain what might cause it. Along with other scientists, he posited that the warming at the end of the last ice age produced an enormous lake atop the vast Laurentide Ice Sheet that covered much of North America, and that when the ice sheet cracked, the lake drained in one of the great floods in human history. The flood emptied into the North Atlantic and, because fresh water is lighter than salt water, the surge left a pool of fresh water on the surface of a portion of the North Atlantic in the area where this great river within the ocean would dive, forming what’s called “deep water.” The lighter lid of fresh water would interrupt this flow, and without the downflowing saline water pulling (“entraining” is the word used to describe this motion) it along, the Conveyor would slow down and stop. Without warmer water being pulled northward by this massive heat pump, Europe would suddenly cool.
As temperatures fell during the Younger Dryas, sea ice spread as far south as Great Britain. The extended ice compounded the cooling as it both trapped ocean heat beneath it and reflected solar heat back into space (a phenomenon called “albedo”). Others have proposed different mechanisms for the start of the Younger Dryas (including a theorized large meteor impact), but the positing of a massive influx of fresh water remains the leading candidate, more than thirty years after the theory was published.
Also in 1987, Broecker was instrumental in launching two new studies of the Greenland Ice Sheet that would help propel the rapid climate change hypothesis toward its current role as the conventional wisdom about the nature of climate shifts since the beginning of the ice ages. What prompted Broecker’s intervention was that the National Science Foundation (NSF) balked at funding Dansgaard’s newly proposed ice-drilling project in Greenland since it was a European, not American, effort. Broecker convinced the head of the Lamont-Doherty Earth Observatory to pay to fly the European scientists to Boston for a sit-down with American scientists.
The idea was to mount a joint project at the thickest part of the ice sheet, in part to minimize the distortions caused by ice folding as they pulled up cores more than one hundred thousand years old. They couldn’t figure out how to share ice from a single hole, but then, as Broecker told the story, Dansgaard made a dramatic suggestion. He suggested drilling two holes, one paid for by the Americans and another paid for by the Europeans. It wouldn’t be a competition but a collaboration, and with two holes, the teams could confirm each other’s findings. After some negotiating with the NSF, the project was funded, and the Europeans and Americans launched the two projects, separated by about 18.5 miles.
The two efforts, the American GISP2 and European GRIP, illustrate the structural lag inherent in studying climate. The Boston meeting was in January 1987. The projects launched the following year, and the drilling commenced during the following summer. Neither project reached the depths commensurate with ice formed during the Younger Dryas until the next decade, during the summer of 1993.
Once both teams saw the sharp lines that demarked the sudden changes more than eleven thousand years ago, they knew that what they were seeing would overturn the understanding of how fast climate could turn from warm to cold. Looking at the ice laid down before the dawn of human civilization, the scientists could see dramatic changes in temperature that had occurred in as little as three years.
Publication by both teams in Nature came shortly after the discovery—lightning speed for a peer-reviewed journal—but even then, there was much more work to be done. Were these changes unique to Greenland, were they regional, or were they global? Did other proxies show such rapid changes? What geophysical mechanism could explain them? These and other questions drove a good deal of research in the next decade.
As we’ve seen, at the dawn of the 1980s, the conventional scientific wisdom held that climate change was a smooth and gradual process. Throughout the decade, what were at first blurry glimpses that dramatic, rapid changes had happened in the past became ever more distinct, and by the end of the decade, most climate scientists became converts to this new paradigm of climate change. They knew immediately that if climate could undergo drastic changes in just a few years, then humanity was indeed playing a dangerous game by pumping ever-increasing amounts of CO2 into the atmosphere.
In 1987, Broecker published a paper in Nature entitled “Unpleasant Surprises in the Greenhouse?” In it he explicitly warned that increasing CO2 might produce sudden shifts in climate. Around this time, he began using his “angry beast” metaphor. Over the years, the phrase caught on, albeit within what was at first a very small world.
The scientific clock accelerated in the 1980s as institutions around the world mobilized to understand climate change. But it still lagged reality, and by the end of the decade, as hottest years ever began to accumulate, a number of researchers began to wonder whether the phenomenon of rapid climate change was already happening today even as they were studying it in the past. They also knew if that was true, science could never catch up, much less get ahead.
There were other major breakthroughs in atmospheric chemistry in the 1980s, one of which had great relevance to efforts to deal with climate change, even though it was looking at an entirely different problem. This was the discovery of the ozone hole in the stratosphere over Antarctica. It serves as a cautionary tale about how difficult it is to prompt international action on a global atmospheric issue. I will come back to this complicated dance in subsequent chapters, but here are the bare bones of the scientific timeline of the discovery.
In the mid-1970s, atmospheric chemists noticed that certain chlorine compounds, called chlorofluorocarbons, or CFCs, would break down ozone in certain circumstances. Sherwood Rowland, Mario Molina, and Ralph Cicerone showed how the release of these chemicals (which were used as refrigerants and propellants in spray cans) threatened the ozone layer, which protects life on earth from dangerous ultraviolet radiation coming from the sun.
In 1982, the story took a dramatic turn. Joe Farman, who had been monitoring ozone levels in the upper atmosphere above Antarctica since 1957, noticed something funny in his readings. Farman, like Keeling, loved the drudge work of monitoring. Unlike Keeling, he didn’t have a PhD, which relegated him to even lower status in the scientific great chain of being. He was, however, meticulous. At first, he wrote off the anomalous readings to faulty measurement—Farman operated on a shoestring budget and his equipment was practically obsolete.
When he took his readings the following year, however, the drop-off in ozone levels was even more dramatic, with concentrations plummeting 50 percent. To confirm that this was not just a local phenomenon, he collected data from the skies a thousand miles from his base. NASA got interested and began investigating. When they did, they discovered to their chagrin that their data actually showed the same ozone drop-off; somehow their analysts with their billions of dollars in equipment had missed a hole in the ozone layer as big as the United States.
Between that discovery and 1987, when NASA sent a flight in to collect data on the prevalence of chlorine compounds and the destruction of ozone, a whole host of questions were raised and answered. That 1987 flight produced the data—the “smoking gun”—that settled the issue of what was causing the ozone hole.
In the 1980s, climate scientists would have loved to have had such a smoking gun. They would get the equivalent in a few years, but it would not have the metaphoric power of the ozone hole or its relatively simple solution. Even so, it took years for the world to muster the consensus to take action on CFCs, and it might not have happened but for a fortuitous set of circumstances that did not apply in the case of global warming. But as it turned out, the international effort to deal with ozone depletion would have reverberations for climate change long after the ozone issue was considered solved.