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WARNINGS FROM THE ICE THE CONVENTIONAL WISDOM IS THAT CLIMATE CHANGE WILL BE GRADUAL AND MODERATE. BUT WHAT IF IT IS SUDDEN AND EXTREME? A FROZEN WILDERNESS MAY HOLD THE ANSWER
BY EUGENE LINDEN/MCMURDO SOUND
Monday, Apr. 14, 1997
Through blind physics, the Antarctic can confer on a dead seal the splendor of an Arthurian burial rite. A corpse will become frozen beneath some floating ice, then rise slowly to the top as ice forms below and evaporates above. Once on the surface, the body insulates the underlying ice from the sun, causing it to form a pedestal as the surrounding ice recedes. Eventually, the ice breaks up, and the seal, mummified by the dry, cold climate, drifts out to sea. As layers of warm and cold air bend light and play tricks on the eye, it can appear that the seal is standing as it sails off toward its Avalon.
Antarctica also plays tricks with time and space. The vast, treeless continent conveys the awesome inertia of a place where motion is noticeable only on a geological time scale, as though the extreme cold flowing out from the polar plateau has slowed the pulse of life itself. On the ice sheets, ice streams 50 miles wide--glaciers within a glacier, in effect--look like frozen rapids when seen from space. Only if time-lapse photography could collapse many hundreds of years would the continent look alive as the ice flowed and cracked, redistributing its mass according to the laws governing gravity and friction.
Entombed in the Antarctic are memories of ice ages, of volcanic eruptions, of epochal changes in winds and rains. These memories are encrypted in dust particles, rare molecules and the properties of the ice itself. The tales they contain of thousands of years of climate changes provide intimations, and warnings, of our fate. That is why people like glaciologist Kendrick Taylor of Nevada's Desert Research Institute are drawn here. By drilling to the base of the ice sheet and extracting a 3,300-ft.-long series of ice cores, he hopes to answer new and urgent questions about the nature of global climate change.
The poet Robert Frost asked whether the world would end in fire or ice. Four years ago, Taylor and other geophysicists found evidence that the answer may be both. The message, extracted from an ice core taken in Greenland, at the opposite end of the earth, was that climate can change dramatically over short periods of time. Roughly 11,500 years ago, Greenland suddenly chilled, and then 1,500 years later, it suddenly warmed. The speed of the last change--an 18[degree] warming in some places in as little as three years--was fast enough, a meteorologist wryly commented, to capture the attention of politicians. To put a change of this magnitude in perspective, a mere 2[degree] drop in global temperatures during the 13th century started the "Little Ice Age" that wiped out the Vikings' Greenland colony, spurred glaciers to crush villages in Europe and contributed to periodic episodes of starvation and mass migration.
A "flickering climate" (as it was dubbed by Taylor and his colleagues) would be a biblical disaster in today's crowded world. Droughts, heat waves, floods and plagues of pests would play havoc with crops, and rapid sea-level rise would inundate cities and destroy rich agricultural lands. "The Greenland finding was like a loud noise in the dark," says Taylor. Now he and dozens of other scientists have moved their search to Antarctica in an effort to follow up on this finding.
Scientists have assumed that any change caused by humans would occur over many decades. They are no longer so sure. As climatologist Peter deMenocal put it, "When I began my Ph.D. in 1986, the conventional wisdom was that it took 1,000 years to end an ice age; in '91 that figure was lowered to 100 years, and then just two years later, Richard Alley at Penn State published a paper about climate changing in two to five years."
If climate change brings about a large rise in sea level, the principal immediate cause will be the collapse of the West Antarctic ice sheet. WAIS is the world's last remaining marine ice sheet (meaning that it sits on the ocean floor rather than floats). It is so big that volcanic eruptions at its bottom only rarely cause a dimple on its surface. Marine ice sheets persist only as long as they have enough mass to squeeze out underlying seawater, which makes them inherently unstable. Should this ice sheet collapse or float free, as other marine ice sheets have done, global sea level would rise nearly 20 ft., which would inundate most of Florida and hundreds of low-lying cities from Jakarta to London.
This process may already have started. Ted Scambos, an analyst at the National Snow and Ice Center in Boulder, Colorado, looks at a satellite image and says, "I see an ice sheet in the process of collapse." But before anyone rushes to sell off property in Florida, he hastens to add that there are still too many unknowns and that any change could take thousands of years.
Also vulnerable is the floating apron of sea ice that surrounds Antarctica. During the winter, this apron effectively doubles the continent's size, then in summer it shrinks 80%. The interaction of deep ocean currents and sea ice is crucial to the vast "conveyor belt" that redistributes the sun's heat around the globe. For all its importance, however, it is on average less than 2 ft. thick, and its stability depends on a precarious balance of factors ranging from air temperature to the salinity and temperature of the water.
Some change in Antarctic climate is already noticeable. It seems to be snowing more often at the South Pole, an area remote from any obvious sources of additional moisture. In the continent's Dry Valleys region, the lake ice seems to be thinning. It actually rained briefly at the American base in McMurdo Sound this year. The Wordie Ice Shelf on the Antarctic peninsula has all but collapsed, signaling a retreat of the northern limit of permanent ice on the continent. And in the 1970s a portion of the sea-ice apron as big as California disappeared for three years.
Do these changes fall within the normal ebb and flow of climate variation on the continent? Or, in concert with other changes afoot around the world--the retreat of glaciers in Europe and North America, the increasing range of cold-intolerant plants and insects, the increase in extreme weather events--do they signal that human tampering with the atmosphere is affecting the global climate?
Taylor and his colleagues will help answer the specific questions about the history of the West Antarctic sheet when the deep drills pull up cores from its bottom next fall. His efforts are a form of retrodicting--using the present to understand the past, and in turn to predict the future. At a site called Siple Dome, Taylor's team has dug a pit so that his colleague Christopher Shuman can study the layering pattern caused by accumulation, melting and recrystallization.
The pit is covered with plywood to block out the blinding summer sunlight, and it is artfully backlit by diffused light from another pit. The 7-ft. layer of firn (the name given to compressed snow before it becomes ice) looks like a Japanese screen, a blue-white background marked by horizontal darker bands. Shuman points out a lens of ice in the wall that memorializes a big melting in the summer of 1990. By matching isotopes taken from various points in this wall with satellite records of microwave radiation, Shuman can determine the timing of snow accumulation. In a nearby canvas building, Jeffrey Severinghaus, another veteran of the Greenland expedition, is studying the ways in which temperature changes affect the separation of isotopes, in an effort to determine how changes in CO2 and methane are related to climate changes in the past.
All this takes time. The drilling crew can bore into the ice sheet for only 40 days a year. The deeper ice-core samples are under such great pressure that they tend to shatter if not handled carefully when brought to the surface. Taylor's team has dug a giant trench where the cores will be stored for a year at -10[degrees] F before they are transported to Colorado for analysis.
Before Taylor determines whether WAIS collapsed in the past, scientists hope to have answers to other key issues: 1) what triggers collapse, 2) how long it takes, and 3) whether the ice sheet is now getting bigger or smaller. Though ice seems rigid, in great masses it behaves like a very slow-moving liquid. Once a sheet gets more than 1,300 ft. thick, notes Charles Raymond of the University of Washington, the stresses of its weight tend to force it to spread rapidly. Since WAIS can be more than 14,000 ft. at its thickest, ice is continually moving from its interior toward its edges.
As long as accumulation at the center offsets the amount of ice lost through sublimation (as ice turns directly into vapor) or the calving of icebergs, the great ice sheet remains stable. If it begins shedding ice rapidly, however, the sheet gets lighter, allowing warm seawater to intrude underneath, further speeding the flow of ice to the edges. At some point--no one knows when--the whole sheet begins to come apart. That is when sea level around the world would climb rapidly.
Robert Bindschadler, a NASA geophysicist, has been investigating the behavior of the ice streams. His hypothesis is that the ice streams are in the process of surging, which involves the rapid transport of ice from the interior toward the Ross Ice Shelf. If so, then WAIS is in the process of collapse. While this sounds dramatic, Bindschadler suspects WAIS has been collapsing for thousands of years, and final collapse may not occur for a couple of thousand more. On the other hand, Bindschadler cheerfully acknowledges, there is no guarantee that the collapse of WAIS will continue to be orderly and predictable.
Even today there is indirect evidence that WAIS may be shrinking. Global sea level has recently been rising about .08 in. a year. Scientists can account for about 44% of that rise through thermal expansion of the oceans as they slowly warm and through the melting of mid-latitude glaciers, like the one in Switzerland that exposed the 5,000-year-old "Ice Man." Stan Jacobs of Columbia University's Lamont-Doherty Earth Observatory suspects that the missing component of sea-level rise comes from Antarctica.
Some scientists see ways in which collapse could suddenly speed up. A pulse of warming that began 10,000 years ago has been moving down through the ice sheets at the rate of about 1 ft. a year. Warmer ice deforms more easily and moves more quickly, and some speculate that this heat, as it moves to the bottom of the sheet, could set off a rapid collapse.
In the near term, a more likely cause of rapid climate change might be a disruption of the Antarctic's apron of sea ice. The earth's weather is partly controlled by a Mobius strip-like system of a deep, cold ocean current linked to a warmer, shallower current. The amount of water moving through this system is equivalent to 100 Amazon rivers, and it is responsible for redistributing more than 30% of the heat that the earth receives from the sun. The currents essentially take heat from the sun at the equator and over a long, slow cycle of 1,500 years redistribute it to the poles.
According to Lamont's Arnold Gordon, who first described this system, the heat arrives in Antarctica in the form of water that comes in under the sea ice at about 34[degrees] F, gives up its meager heat as it rises, and then sinks and begins its journey to the opposite end of the world.
The stability of the system depends on the temperature and salinity of the layers of water under the ice. As long as there is a buoyant layer of relatively fresh water, ice will continue to form. As ice thickens, however, salts become concentrated in the underlying water to the point at which the upper layer of water is as dense as the warmer layer below. When this happens, according to Lamont's Douglas Martinson, warm and cold waters begin to mix, which in turn release to the surface the heat formerly trapped in the current. This heat melts the ice, forming an opening called a polynya.
This melting is what caused the California-size gap in the apron during the early 1970s. Probes showed warm and cold waters had mixed all the way down to depths of 9,000 ft. With nothing to block the release of warmth from this enormous column of water, vast amounts of heat vented through the ocean surface. While it was open, the gap is suspected to have brought about an increase in precipitation and a rise in temperatures up to 1,000 miles away.
Is the apron of sea ice currently shrinking? Martinson says an interpretation of satellite data reveals no significant trend, but he adds that detailed analysis shows tremendous variability around Antarctica. Anecdotal evidence suggests that these changes can have profound effects.
Since 1968, Gerald Kooyman of the University of California at San Diego has studied emperor penguins, beguiling flightless birds that are dependent on Antarctic sea ice. The penguins need 255 days of sea ice in order to complete the cycle from egg laying to the stage when fledglings are hardy enough to begin their wanderings through the southern ocean. Typically, the young birds jump into the water only two weeks or so before the ice breaks up. This year, says Kooyman, the ice near Franklin Island broke up in mid-December, two weeks before the fledglings were ready to embark, probably dooming the juveniles to an early death. The story is a reminder of the thin margins that sustain life, even for creatures as durable as the emperor penguin, which has thrived for millions of years in the harshest climate on earth.
The climate record shows that the whole 8,000-year span of human civilization, from the dawn of cities to space flight, has taken place during a period of extraordinary warmth and stability. The past 150 years, which have seen the industrial and information ages, have been even more remarkably clement. The experience has left humanity with the notion that climate is warm and stable. But those who look at the past know different. "Climate is an angry beast," says Lamont's Wallace Broeker, "and we are poking it with sticks."
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