In 1925 the respected climate expert C. E. P. Brooks, 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.” 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.
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. 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.
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.
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. Most of their colleagues found such a rise barely plausible. 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.
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.
By 1981, cylinders of ice from more than two km deep showed what the researchers called “violent” changes; “a dramatic cooling of rather short duration, perhaps only a few hundred years.”
Nevertheless, experts were scarcely prepared for the shock that came from the Greenland ice plateau in 1993. The comparison between cores showed convincingly that climate could change more rapidly than almost any scientist had imagined. Swings of temperature that were believed in the 1950s to take tens of thousands of years, in the 1970s to take thousands of years, and in the 1980s to take hundreds of years, were now found to take only decades. Greenland had sometimes warmed a shocking 7°C within a span of less than 50 years. More recent studies have reported that, during the Younger Dryas transition, drastic shifts in the entire North Atlantic climate could be seen within five snow layers, that is, as little as five years!
People can see only what they find believable. Over the decades, many scientists who looked at tree rings, varves, ice layers, and such had held evidence of decade-scale climate shifts before their eyes. They easily dismissed it. There were plausible reasons to dismiss global calamity as nothing but a crackpot fantasy.
First, scientists had to convince themselves, by shuttling back and forth between historical data and studies of possible mechanisms, that rapid shifts made sense, with the meaning of “rapid” gradually changing from millennia to centuries to decades. Without that gradual shift of understanding, the Greenland cores would never have been drilled. The funds required for those heroic projects became available only after scientists reported that climate could change in damaging ways on a time scale meaningful to governments. In an area as difficult as climate science, in which all is complex and befogged, it takes a while to see what one is not prepared to look for.”