Ice age may be a climatic norm

PEOPLE who think the severe North American winter of 1976-77 felt like the ice age have a point. Climatologist Thomas Crowley says the atmospheric circulation that season was a mild version of what may be a typical ice-age pattern.

The University of Missouri scientist, who is temporarily with the National Science Foundation, notes that ice-age type circulation patterns may not be all that unusual. In fact, this may be a normal mode of atmospheric behavior.

Meanwhile, at the Australian Numerical Meteorological Research Centrecq , B.G. Hunt has run computer simulations which suggest to him that ice ages may themselves be the prevailing climatic norm. Presenting his conclusions recently in Nature, he says that intervening warm periods, such as the present, ''should be considered climatic aberrations.''

Such ideas are speculative. But they do dramatize the fact that the climatic record indicates our present equable climate may be a fragile thing.

Certainly the present geological epoch is best characterized as a long-playing ice age that is punctuated at 100,000-year intervals by brief (10, 000-year) interglacial spells. This pattern has persisted for at least the last million years. The onset of the ice age itself now seems to have been as early as 2.5 million years ago, according to analysis of deep sea sediment samples from the North Atlantic made by Nicholas Shackleton of Cambridge University in England and his colleagues.

Just what started the glaciation and what causes the interglacial breaks still is far from clear. However, meteorologists generally have accepted the theory that changes in Earth's orbital motion set the basic rhythm. These are slow, cyclical variations in the shape of the orbit and in the tilting and wobbling of Earth's axis. As suggested in 1941 by the Serbian scientist Milutin Milankovitch, these planetary changes slightly alter the timing and degree of solar heating in relation to the seasons and thus can have climatic effects.

The most obvious correlation is that between a 100,000-year cycle in orbital shape and the 100,000-year glacial-interglacial rhythm. This makes a 0.1 percent change in the total sunlight intercepted by the planet.

Thomas Crowley notes that such small changes in the amount of sunshine reaching Earth might be enough to kick the atmosphere into a preferred mode, such as an intensified version of that prevailing during the winter of 1976-77. He calls this circulation pattern ''Greenland Above,'' because Greenland and western North America have above-normal temperatures, while eastern North America and Western Europe are unusually cold.

It is a pattern that would dump a lot of snow on the land and favor ice sheet growth. Crowley notes that about 70 percent of the Januaries over the past century were characterized by only four distinct circulation patterns. The ''Greenland Above'' pattern was one of these, appearing in a form somewhat less intense than that which would prevail during an ice age. It has occurred about 20 percent of the time.

B.G. Hunt, whose findings also indicate that ice-age conditions are normal, thinks that such a small change in solar radiation could easily lead to glaciation. His computer studies suggest to him that a frozen ocean is a preferred state for the Arctic. Furthermore, he says that a frozen Arctic seems to be a ''necessary and sufficient condition . . . for the creation of ice ages.'' Thus he sees no need to hunt for special conditions to initiate glaciation. The problem is to account for the warm interglacials which may also be tied to the Milankovitch cycles.

Here Hunt thinks the mechanism may be an increase in the atmosphere's concentration of carbon dioxide (CO2). This picks up a suggestion made by some other investigators as well. CO2 traps heat radiated toward space and reradiates some of it back toward the ground. This has a warming effect. Climatologists are concerned that a buildup of CO2 released in burning fossil fuel may abnormally warm the atmosphere. But, as a natural control, CO2 increases in the past may have helped modulate the ice-age rhythm.

There is evidence of this in polar ice cores. The natural atmospheric CO2 concentration began increasing as the ice sheets started melting 16,000 years ago. It increased to some 280 parts per million, which was typical just before there was significant fossil fuel burning. At the peak of the last glaciation, the CO2 level was 40 to 100 parts per million below that figure.

Climatologists do not yet know how changes in solar radiation linked to the Milankovitch cycle might, in turn, be linked to CO2 levels. There are many pieces of the puzzle yet to be found. But it may well be that our pleasantly warm interglacial climate today is indeed an aberration on a normally icy world. If that is true, one wonders whether and to what extent the man-made CO2 buildup may help prolong the warm spell.