Cracking The Ice

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For more than a decade now, scientists have been puzzling over these ice streams, and the consensus they have reached is both reassuring and disturbing. On one hand, they note, the overall rate at which the ice streams are transporting ice — at the relatively brisk clip of hundreds of feet a year — does not seem to have increased in recent decades. On the other hand, the streams have been extremely erratic — accelerating, decelerating, sometimes even stopping. And miles inland, the streams have shown surprising sensitivity to ocean tides, raising the specter that modest rises in sea level could accelerate their flow.

Finally, there is the question of the stability of the Ross Ice Shelf itself. Ice shelves, it turns out, are important to ice sheets for a number of reasons. Among other things, they serve as buffers against currents and wave action and as buttresses that provide structural support. In fact, the absence of ice shelves may be the reason that glaciers in the remote Amundsen Sea sector of West Antarctica are speeding up their transport of ice to the sea. Earlier this year, in the Antarctic Peninsula, the Larsen B ice shelf showed what can happen when conditions warm. First, rising summer temperatures created meltwater ponds on the surface of the ice shelf, allowing water to pour into cracks. Then pressure exerted by the inflow of water deepened the cracks as relentlessly as a wedge splitting a log. Eventually the ice shelf fell to pieces, like an enormous tree reduced to a jumble of firewood.

Fossil Air: Beacon Valley
Carrying a pickax and shovel, Boston University geologist David Marchant trudges up a snow-dusted side canyon to Beacon Valley. The ground beneath his feet is as intricately patterned as a quilt, and under its rubble-strewn surface lurks a glacier of venerable age. Marchant believes this glacier has been frozen in place for millions of years — and if he's right, the ice in the glacier holds invaluable clues to an earlier epoch of global warming, one that offers a provocative parallel to the warming expected later in this century.

Three million years ago, the mean temperature of the earth was at least several degrees higher than today. Scientists still do not know precisely what caused the warming or how Antarctica responded. One group argues that during this time significant expanses of the White Continent were not merely ice free but covered with low-lying, tundra-type vegetation. But Marchant and his colleagues contend that the vast ice sheet that covers the Antarctic plateau rode out the temperature rise unperturbed.

The mystery that Marchant is grappling with is perhaps the most profound of all. Today Antarctica is synonymous with ice; 98% of its surface is covered by ice. But this was not always the case. Even though the landmass that constitutes Antarctica has occupied a polar position for well over 100 million years, for much of that time it enjoyed a rather pleasant clime. During the Cretaceous Period, for example, areas that today are obscured by ice were covered with forests of conifers and beech, and through them, scientists believe, roamed a variety of animals, including reptiles and dinosaurs.

What changed? The answer, many scientists believe, lies in the breakup of the ancient supercontinent of Gondwanaland, to which Antarctica once belonged. For tens of millions of years, Antarctica was the centerpiece of Gondwanaland, and its winds, like those of then contiguous Australia and South America, were warmed by currents flowing down from the equator. But around 25 million years ago, after the other continents had pulled away, a new current was created — one that circled endlessly round the Southern Ocean, sealing Antarctica off from tropical influence.

That is the bare-bones version of the story. Many scientists think more than that was needed to put Antarctica in its present deep freeze. Among their favorite candidates: a reduction of heat-trapping greenhouse gases, notably carbon dioxide. Supporting this idea are the provocative data scientists pulled from an ice core taken near the Russian station at Vostok. That ice, notes Marchant, contained bubbles of air that spanned the past 420,000 years, and the carbon dioxide in those bubbles tracked the temperature swings that mark the beginning and end of glacial cycles.

Now Marchant has found ice that promises to be more ancient still, as it lies beneath layers of ash that range between 1 million and 8 million years old. Just like the Vostok ice, Marchant's ice contains air bubbles, meaning that it could produce a record of carbon dioxide swings that occurred over this distant and dimly understood interval of time. First, of course, Marchant will have to convince skeptical colleagues that his ice really is that old, that it has not been reworked by geological processes — and this is likely to take some doing. But if that effort proves successful, scientists will have wrested from Antarctica's frozen fortress yet another fiercely guarded secret — in this case, an ancient secret of urgent import to the 21st century.

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