CHAPEL HILL - Slippery winter sidewalks may inconvenience and endanger pedestrians, but they are small (frozen) potatoes compared to what our animal skin-clad ancestors faced. During the last ice age, which peaked some 18,000 years ago for example, ice in what's 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, says Dr. Jose Rial, professor of geophysics at the University of North Carolina at Chapel Hill. So not to worry.
But the timing is not exact, Rial says, and he believes he has figured out why.
In a paper appearing in the June 23 issue of the journal Science, the scientist explains that long-term climate change is more complicated than recent analysts believed. His new work appears to confirm a theory proposed in the 1920s by Serbian astronomer-mathematician Milutin Milankovitch and widely held until called into question during the past three decades.
"The earth circles the sun every year in an orbit that becomes either more elliptical or less elliptical over tens of thousands of years," Rial says. "Variations in the orbit over time chiefly account for the ice ages as differing amounts of sunlight warm the planet."
Scientists can date those Northern Hemisphere glacial periods fairly precisely by drilling into the sea floor and measuring the amount of "heavy" oxygen present in sediment layers, he says. More heavy oxygen - oxygen containing two additional neutrons in its nucleus compared with standard oxygen - corresponds to colder temperatures.
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, Rial says. 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, they said.
Rial, who believed Milankovitch was correct, mathematically re-analyzed records showing when ice ages occurred during the past million years and compared them with the earth'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 says. "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 are likely to be dramatic and possibly catastrophic for humans, he says.
Materials provided by University Of North Carolina At Chapel Hill. Note: Content may be edited for style and length.
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