Explaining Variations in Ice Age Onsets - Brief Article

USA Today (Society for the Advancement of Education), June, 2000

METEOROLOGY

Slippery winter sidewalks may inconvenience and endanger pedestrians, but they are nothing compared to what our animal skin-clad ancestors faced. During the last ice age, which peaked approximately 18,000 years ago, ice in what is now New York City lay as much as a mile thick.

On average, ice ages occur about every 100,000 years, and the next one should begin with plenty of notice in about 60,000 years, indicates Jose Rial, professor of geophysics at the University of North Carolina at Chapel Hill. The timing is not exact, he says, and Rial believes he has figured out why. "The Earth circles the sun every year in an orbit that becomes either more elliptical or less elliptical over tens of thousands of years. Variations in the orbit over time chiefly account for the ice ages as differing amounts of sunlight warm the planet."

Scientists are able to date those Northern Hemisphere glacial periods fairly precisely by drilling into the sea floor and measuring the amount of heavy oxygen-oxygen containing two additional neutrons in its nucleus compared with standard oxygen--present in sediment layers. More heavy oxygen corresponds to colder temperatures.

The fact that the geologic record shows ice ages occur every 80,000 to 120,000 years has confounded researchers who felt such astronomically controlled events should occur on a more precise schedule. Some suggested it was changes in the plane of the Earth's orbit, rather than its shape, that spawned the glaciers. Periodic dipping into a dust ring that also circles the sun might have shaded and chilled the planet instead.

Rial mathematically reanalyzed records showing when ice ages cccurred during the past 1,000,000 years and compared them with the planet's known orbits during those years. He discovered that the variable, roughly 100,000-year cycle was the result of a combination of the effects of other cycles and found the process to be equivalent to the way radio engineers "frequency modulate" signals to generate FM radio and television broadcasts.

In simpler terms, the clock-like precision of the timing of ice ages still occurs, but over a much longer span. The seemingly irregular durations from one ice age to the next eventually can be predicted if one takes into account astronomical events such as changes in the orbit's shape, the Earth's tilting on its axis, and the gravitational effects of other planets.

"Understanding the climate of the recent geological past is important because finding out how Earth's environment has transformed heat from the sun into climate variations can help predict future global climate change," Rial maintains. "Eventually, this kind of work will teach us how the whole climate system works not only over very long time scales, but also perhaps over very short spans of 50 to 100 years, which are what concern us." The long-term effect of changing the global climate by burning fossil fuels, for example, remains to be determined, but is likely to be dramatic and possibly catastrophic for humans, he says.