Science Digest
December 1982, p. 73-75

COMING SOON: ANOTHER

ICE AGE


SCIENTISTS TELL WHY

By John Gribbin

IS IT TOUCHED OFF BY A LAND SHIFT, BY EARTHLY WOBBLES OR BY COSMIC DUST CLOUDS?


Contributors.- John Gribbin, Ph.D., astrophysicist and science writer; Steve Fishman, an editor on UPIs international desk; Richard Hoagland, former editor of Star and Sky and author of a new book on space.

Twenty thousand years ago, Iowa was buried under a layer of ice three times higher than the world's tallest buildings. Portions of the now-lush Pennsylvania countryside were barren tundra, and high peaks of the Rocky Mountains were linked from California to Mexico by rivers of ice. Things have warmed up recently-but we're still living in an ice epoch.

Evidence today suggests that over the past billion years, the Earth has been subjected to three major periods of glaciation, or ice epochs, of various lengths. The most recent-still in progress-began with the opening of the Pleistocene Epoch 2 million years ago. Each major ice epoch is composed of numerous ice ages separated by warm spells called interglacials. Within the past 700,000 years, as many as seven ice sheets have pushed toward the Equator; each global chill persists for about 90,000 years and is followed by a 10,000-year interglacial.

Scientists say that we currently live in one of these warm interglacial periods; the last massive ice sheet retreated about 10,000 years ago. The Earth's climatic pendulum, it seems, is now poised to swing us back into an ice age within the next 5,000 years.

Why, during an ice epoch, does our planet plunge into a deep freeze every 100,000 years or so? Explanations abound; some complement each other, some conflict. What follows is a sampler of theories.

Astronomical Theory

Also known as the Milankovitch theory, after one of its strongest advocates, geophysicist Milutin Milankovitch, this idea is based on a simple premise: the distribution of sunlight over the Earth holds the key to why the planet periodically cools. This distribution is always changing, say Milankovitch and his scientific descendants, as the Earth's attitude shifts with respect to the sun. Although this shift is a complicated combination of motions, it is thoroughly predictable.

Ice sheets may wax and wane due to the changing distribution of solar energy over the globe. A factor contributing to this change is the shift in attitude of the Earth's axis with respect to the sun.
One of the movements is a global wobble. Because of the gravitational tug of the sun and moon on the Earth's equatorial bulge, the Earth's axis does not always point in the same direction. Rather, if viewed from the top, its axis describes a circle in space, with a full turn taking 25,800 years. If the same motion could be viewed from the side, the Earth, like a giant gyroscope, would appear to wobble back and forth. Thus portions of the Earth have been closer or farther away from the sun at different times in history.

In a second, separate motion, the Earth's axis tilts. The angle made by the axis and the plane of the Earth's orbit (the ecliptic) closes slightly and then opens over the course of 41,000 years. From the side, the sphere would seem to nod. The difference between the two extremes is only about 3 degrees, but this is enough to alter the amount of solar energy reaching portions of the Earth's surface.

The third motion concerns the shape of the Earth's orbit. Every 100,000 years, the orbit changes from elliptical to nearly circular and then back to elliptical as the result of the gravitational pull of other planets. The difference between our planet's nearest and farthest distance to the sun is now about 3,000,000 miles, but in the past it has been greater.

By computing the combined effects of these three factors on the distribution of solar energy over the globe, Milankovitch was able to calculate when certain sectors would get the least sunlight and thus be most likely to experience glacial advances.

Recent sophisticated investigations of ice age chronology, using sediments from the floor of the ocean, suggest that Milankovitch's timetable may indeed correspond to periods of deep glaciation. A correlation seems to exist between times of intense cold and a nearly circular Earth orbit, which the orbit is shifting toward now.

Over the past few years, the astronomical theory has become a favorite among experts. According to science writer Nigel Calder, "The astronomical theory has emerged from among a host of rival theories as the most plausible explanation for the repeated advance and withdrawal of the ice sheets." But the theory gets a chilly reception from astronomer Fred Hoyle, who says, "If I were to assert that a glacial condition could be induced in a room liberally supplied during winter with charged high-storage heaters simply by taking an ice cube into the room, the proposition would be no more likely than the [astronomical] theory."

Crustal-Wandering Theory

When continents have drifted over or near the poles during Earth's annals, they become cold enough for glacial growth. Evidence indicates that the enormous landmass above, positioned at the South Pole over 100 million years ago, was capped in ice.
Continents have been on the move for millions of years, migrating around the globe at the pace of I to 15 centimeters per year. When this phenomenon, known as continental drift, positions a landmass in the cool high latitudes, north or south, where winter snows don't completely melt, snow can build up and gradually turn into layer upon layer of ice. But this can't happen without enough precipitation. (And, because polar regions tend to be arid, they are not always the best locations for glacial growth.) The glaciers then advance to the coasts, where icebergs break free and cool the oceans. Conditions are thus set for the spread of ice sheets on a large scale.

In support of this theory, scientists have found that, over 100 million years ago, ice capped much of the supercontinent Gondwanaland (what is now South America, Africa, India, Australia and Antarctica), which was then located over the South Pole. Evidence also shows that Antarctic land was again buried by ice during the latest series of glacial episodes. This occurred after Gondwanaland broke up and left Antarctica in wintery solitude over the Pole.

Rising-Land Theory

The rise of mountains due to the collision of the Earth's crustal plates, propose some scientists, prepared the planet for the formation of glaciers. At the start of the Pleistocene ice epoch 2 million years ago, the world underwent a massive uplifting of land. The Alps climbed 6,000 feet; the Himalayas, 9,000 feet. More and more land was pushed. above the snow line, the boundary above which snow never melts. Highland glaciers formed and eventually pushed into the lowlands, creating pockets of cold air and leading to the development of ice sheets.

Greenhouse Effect

In the atmosphere, carbon dioxide acts much like the glass in a greenhouse: it serves as a radiation trap, allowing the sun's rays to penetrate to the ground but blocking the escape of the reflected heat energy into space. Plant life, such as blue-green algae, thrive under such conditions, consuming the CO2 and reproducing at astounding rates. (Under ideal conditions, a single algae cell could generate a colony with a mass equal to that of the Earth in two weeks.) Such a population explosion, according to this theory, would almost totally deplete the CO2 in the atmosphere. Heat would rapidly escape, chilling the Earth and initiating a glacial epoch. Reduced temperatures and CO2 supply would cause plants to die and decay. This would allow CO2 to build up again in the atmosphere and boost temperatures. Thus, an oscillation in atmospheric CO2 could be an ice age regulator. By one estimate, merely cutting in half today's quantity of atmospheric CO2 (0.03 percent) would trigger an ice age.

Veil-of-Dust Theories

Debris recently spewed from El Chichon volcano
in Mexico has caused sulfuric acid droplets to form
high in the sky. These droplets--not ash--have
reduced our sunlight by 5 percent.
At various times in history, dust has slipped between the Earth and the sun-in different ways according to several theories-and shielded the planet from warming solar energy.

In one theory, the dust originates with volcanoes that erupt with such force and such frequency that clouds of ash darken the sky around the globe. After Krakatoa exploded in 1883 in Indonesia, a French solar observatory found that for three years radiation received from the sun was 10 percent below normal.

The recent eruption of El Chich6n in Mexico has provided a natural laboratory for studying this theory. Scientists with NASA's Aerosol Climatic Effects Program are tracking and measuring the height of the erupted materials with satellites and ground-based lasers and are sampling the debris with a high-flying NASA U-2 aircraft. They have found that most of the material spewed from the mountain is sulfur dioxide gas, which combines with the water vapor at high altitudes to produce enormous quantities of sulfuric acid droplets. It is these droplets-not ash particles-that are currently reducing the amount of sunlight by as much as 5 percent. Because of their minute size (0.0001 inch in diameter) and low density, these droplets are expected to remain aloft for years, unlike the heavier ash, which typically falls out within months. Owen Toon, a NASA scientist, predicts that, as a result, over the next few years temperatures "will fall by about one degree Fahrenheit, mainly in, the Northern Hemisphere."

As the Milky Way makes a full spin every 250 million years, it may sweep our solar system through a huge dust cloud that would shield us from some warm solar rays. Could this have been the cause of the three major ice epochs, which have a similar periodicity?
Another veil-of-dust theory is galactic in scope. The two arms of our galaxy swirl through space, making a full turn every 200 to 250 million years. Some think it is not mere coincidence that Earth's great ice epochs have occurred with the same periodicity. They suggest that with each turn of the galaxy, our solar system sweeps through a gigantic cloud of interstellar dust. When the debris comes between the Earth and sun, temperatures on Earth plunge. However, some experts argue the opposite. They claim that the dust would be drawn into the sun by gravity and increase its luminosity, causing warmer temperatures.

The Warm Before The Cold

As illogical as it sounds, this theory suggests that a slight heating precedes a dramatic cooling. This is based on the fact that glacier formation depends not only on low temperatures but also on the presence of precipitation.

A brief period of warmer-than-average temperatures, proponents suggest, could melt Arctic Ocean ice, allowing entry of warm currents from the south. The subsequent increase in evaporation would lead to heavier snowfall, the food of glaciers. The reflectivity of snow and ice would cool temperatures and escalate glaciation. But once the Arctic Ocean froze over again, snowfall would decrease and the ice would gradually recede. The melt off would raise the level of the oceans, once again allowing warm currents to flow far north.

Ice-Sheet-Surge Theory

At the base of a glacier, freezing conditions may no longer exist due to high pressures and warmth from the Earth. The temperature 6,000 feet below the surface of an ice sheet may be 25 degrees Fahrenheit warmer than that at the top. And high pressures can reduce the melting point of ice to as low as 29.3 degrees Fahrenheit. Indeed, using radio echo-sounding techniques, scientists have discovered that underneath parts of the west Antarctic ice sheet lie huge lakes.

Because of its great weight, a glacier normally acts like a viscous solid and creeps over the landscape. But this watery undercoat would work as a lubricant and send the ice sheet surging toward the ocean at several times normal speed. It could eventually form a vast oceanic ice shelf, covering 10,000 million square miles. Such a massive ice cube would chill the oceans and slip the world into an ice age.

Magnetic-Pole Reversal

Magnetic north hasn't always been north. Many times in the Earth's history (171 times in the past 76 million years), the magnetic poles have reversed; north becomes south, south becomes north (a compass's north needle would point south). No one is certain why these reversals occur, but the evidence for them exists in the rocks spreading from around the Mid-Atlantic Ridge.

It is widely thought that Earth's magnetic field is somehow related to climate. Advocates of this theory have linked some of the magnetic reversals to major climatic changes that have triggered the birth or death of an ice age. Although the frequency of reversals has varied drastically in the distant past, it has remained about 5 every 1 million years over the past 45 million years. Scientists predict that we will undoubtedly undergo another reversal sometime in the future; whether this will actually generate an ice age remains to be seen.

Professor J. K. Charlesworth once wrote that the theories of the causes of ice ages range "from the remotely possible to the mutually contradictory and the palpably inadequate"; hence it is not surprising that many experts subscribe to a blend of two or more of these theories. They point out that several forces will work in concert to bring on the imminent ice age. But don't be concerned now about ice sheets plowing through Canadian towns. The onset of glaciation will creep up on our descendants, giving them thousands of years to adapt (as our ancestors did)--or allowing them time to develop the technology that will prevent another chilling ice advance.

Diagrams by Ian Worpole

For Further Reading:
Dance of the Tiger by Bj6rn Kurt6n. New York: Pantheon, 1980.
Future Weather and the Greenhouse Effect by John Gribbin. New York: Delacorte, 1982.
Ice: How the Next Ice Age Will Come and How We Can Prevent It by Fred Hoyle. London: Hutchinson, 1981.
Ice Ages: Solving the Mystery by John and Kath- erine Imbrie. New Jersey: Enslow, 1979.
The Roots of Civilization by Alexander Mar- shack. New York: McGraw-Hill, 1972.
Secrets of the Ice Age: A Reappraisal of Prehistor- ic Man by Evan Hadingham. New York: Walk- er, 1982.