The Ice Age Cometh

The Ice Age Cometh?
Last week's big chill was a reminder that the earth's climate can change at any time
By MICHAEL D. LEMONICK

Monday, Jan. 31, 1994

Just as last week's tremors were destroying highways, buildings and lives in Southern California, an even deadlier natural disaster was advancing slowly but inexorably south from Canada into the U.S. By midweek a huge mass of frigid arctic air had practically paralyzed much of the Midwest and East. Temperatures in dozens of cities dropped to all-time lows: -22 degrees F in Pittsburgh; -25 degrees in Akron, Ohio, and Clarksburg, West Virginia; -27 degrees in Indianapolis, Indiana. Chicago schools closed because of cold weather for the first time in history, Federal Government offices shut down in Washington, and East Coast cities narrowly escaped widespread power outages as overburdened electric utilities struggled to keep homes heated. Hundreds of motorists in New Jersey had to be rescued by snowmobile from an impassably icy highway, and thousands of the homeless crammed into New York City's shelters to avoid freezing. By week's end the unprecedented cold wave had killed more than 130 people.

What ever happened to global warming? Scientists have issued apocalyptic warnings for years, claiming that gases from cars, power plants and factories are creating a greenhouse effect that will boost the temperature dangerously over the next 75 years or so. But if last week is any indication of winters to come, it might be more to the point to start worrying about the next Ice Age instead. After all, human-induced warming is still largely theoretical, while ice ages are an established part of the planet's history. The last one ended about 10,000 years ago; the next one -- for there will be a next one -- could start tens of thousands of years from now. Or tens of years. Or it may have already started.

There is no way of knowing yet: an entire winter of record-shattering cold, let alone a single week, might be a meaningless blip in the overall scheme of long-term climate trends. In fact, last week's cold wave was caused by a phenomenon that is by no means rare. The jet stream, a stratospheric wind that governs the movement of air over North America, dipped temporarily south of its usual course. As it did so, the stream pulled along a vast high-pressure system from Siberia and the Arctic Ocean.

If that starts happening more and more often, though, it might mean that something bigger is going on. Climatologists once thought the world eased into ice ages, with average temperatures in parts of the Northern Hemisphere falling 15 degrees over hundreds or thousands of years. During long, frigid winters and short, cool summers, snow piled up much faster than it could melt, and mile-thick sheets of ice gradually covered much of the planet's land surface. After 100,000 years or so, scientists believed, the glaciers made a dignified retreat, stayed put for about 10,000 years and then began to grow again.

But over the past several years, researchers have dug deep into Atlantic sea-floor sediments and Greenland glaciers to study the chemistry of ancient mud and ice, and they are increasingly convinced that climate change is anything but smooth. The transition from warm to frigid can come in a decade or two -- a geological snap of the fingers. Says Gerard Bond, a geophysicist at Columbia University's Lamont-Doherty Observatory: "The data have been coming out of Greenland for maybe two or three decades. But the first results were really so surprising that people weren't ready to believe them."

There is a growing understanding as well that ice ages are not uniformly icy, nor interglacial periods unchangingly warm. About 40,000 years ago, for example, right in the middle of the last Ice Age, the world warmed briefly, forcing glaciers to retreat. And while the current interglacial period has been stably temperate, the previous one, according to at least one study, was evidently interrupted by frigid spells lasting hundreds of years. If that period was more typical than the present one, humanity's invention of agriculture, and thus civilization, may have been possible only because of a highly unusual period of stable temperature -- a fluke.

Just 150 years ago, the notion that much of the Northern Hemisphere had once been covered by thick sheets of ice was both new and highly controversial. Within a few decades, though, most scientists were convinced and began looking for explanations. Several suggested that astronomical cycles were involved, and by the 1930s the Yugoslav astronomer Milutin Milankovitch had constructed a coherent theory. The ice ages, he argued, were triggered by changes in the shape of the earth's slightly oval orbit around the sun and in the planet's axis of rotation. Studies of the chemical composition of ocean-floor sediments, which depend on climate conditions when the material was laid down, more or less supported Milankovitch's predicted schedule of global glaciation.

According to Milankovitch cycles, an ice age could start sometime within the next 1,000 or 2,000 years. But geophysicists have realized for years that while the cycles are real, and influence climate, they alone cannot explain ice ages. For one thing, Milankovitch's timing of glaciation may be broadly correct, but major glacial episodes happen when his cycles call for minor ones, and vice versa.

Besides, a simple astronomical model would predict smooth and gradual climate transitions -- the opposite of what really happens. The last Ice Age was in full retreat about 13,000 years ago when temperatures suddenly reversed and began heading lower again. They stayed low for 1,000 years, an episode known as the Younger Dryas period. The periodic "spikes" of warmer weather that have interrupted ice ages and the cold weather that often came on suddenly in the last interglacial period are also impossible to explain with astronomy. And so is the astonishingly rapid changeover from warm to cold.

A number of theories have been floated to explain these irregular, rapid variations. The leading one, advanced by Lamont-Doherty's Wallace Broecker and George Denton of the University of Maine, involves a kind of cyclic ocean current that has been likened to a conveyer belt. Broecker and Denton note that a stream of unusually salty (and thus especially dense) water flows underneath the Gulf Stream as it moves from the tropics to the North Atlantic. When this salty stream reaches the far north, it is forced to the surface as water above it is blown aside by the winds; it then discharges its tropical heat into the arctic air, cools off and sinks to the bottom, where it returns to the tropics to be heated again.

It is this current, argue Broecker and Denton, that keeps the Arctic relatively warm and glacier free. When it stops running, an ice age -- or a cold spike -- begins. What causes a turnoff? An influx of fresh water might do it, by diluting the saltiness and density of the current, preventing it from sinking and heading back to the tropics. There is evidence that at just the time the Younger Dryas began, a huge North American lake (which no longer exists) began dumping Amazonian quantities of fresh water into the North Atlantic. The discharge stopped about 1,000 years later, as did the Younger Dryas. Broecker and Denton's model, says Penn State's Richard Alley, an expert on Greenland ice cores, "is probably the trigger for these abrupt changes."

Nobody knows what other factors might help trigger climate shifts, and how sensitive they are. "It scares us," says Alley. "We know that there are times when climate is very delicately poised. We know that for the past 8,000 or 10,000 years, it hasn't flipped over. But we don't really understand it well enough to say whether it's really stable or whether we are on thin ice."

In short, while there is no reason to think the next full-fledged Ice Age is upon us, a shorter episode of frigid conditions could happen at any time. The last interglacial period was warmer than this one and also, arguably, more unstable. It is conceivable that the greenhouse effect could heat up the planet for a while but then trigger changes that could plunge the earth into a sudden chill. And for an idea of what a mini-Ice Age might be like, just imagine last week's cold wave lasting all winter, every winter -- for the next thousand years.