Melting Glaciers: Clues to Climate Change

The Coming and Going of an Ice Age

Certain landscapes in Canada, the northern U.S., and northern Europe are bona fide strange: Lone boulders squat on grassy, gentle hills. Rocks are raked with deep scratches. Rectilinear piles of gravel are aligned almost too perfectly in a single direction. Swiss geologist Louis Agassiz was among the first to realize that ancient, monstrous sheets of ice spanning entire continents produced these odd geological leftovers. In 1837, such an idea was highly controversial.

However, 150 years of follow-up research is finding that Earth’s climate has actually undergone many such glaciations in the last 2.5 billion years. In the past two million years alone, Earth has experienced around 20 ice ages—cycles of advance and retreat of large continental ice sheets. Currently, Earth is between glaciations. If nature has its druthers, we’re probably not due for the next big chill for tens of thousands of years. How exactly our anthropogenically influenced global warming is forcing large-scale natural cycles, however, remains to be seen.

Plotting the Pleistocene

The glaciation that scientists in the 1800s noticed was our most recent one. At the maximum extent of its ice sheets 21,000 years ago, Earth’s air temperature was, on average, about 4 degrees C cooler than today. Around 30 percent of the land surface was covered with ice up to 3 km thick. The sheets carved the basins of the Great Lakes and bulldozed the gravelly ridge we now call Long Island.

The event took place in the Pleistocene Epoch, which began about two million years ago and ended about 10,000 years ago. For roughly the first half of it, every 40,000 years or so contained a single cycle of prolonged, extensive glaciation, then a shorter warm period. For roughly the last half, each cycle took 100,000 years.

This timing became clear only in the early 1970s. That’s when researchers working in the Indian Ocean drilled the first cores of deep-sea sediment deposited throughout the entire Pleistocene up until current times. By measuring oxygen-isotope ratios in the carbonate–rich shells of tiny marine organisms buried in the strata of these cores, scientists estimated the temperature of the ocean’s surface when these organisms lived. Using computer models, scientists were able to infer average global temperatures during the entire Pleistocene from this data. (To learn how oxygen isotopes act as paleo-thermometers, click here.)

Milankovitch Cycles

The effort to explain how glaciers retreat and advance began decades before scientists studied these cores, however. The Indian Ocean work simply confirmed long-debated speculation that astronomical cycles may have timed the Pleistocene’s glaciation.

Cyclical changes in the way Earth orbits the Sun and spins in space were worked out by Serbian mathematician Milutin Milankovitch in the early 20th century, based on calculations made by two earlier scientists. Three changes were scrutinized:

• Orbit: Earth orbits the Sun in a slightly elliptical path. Sometimes, the orbit is more elliptical than at other times. The shape of the orbit changes the maximum distance of Earth from the Sun, and with it the amount of solar radiation Earth receives. The transition from “most circular” to “most elliptical” and back again takes about 96,000 years.
  
  
• Tilt: The tilt of Earth’s axis of rotation also varies. It shifts between 21.5 and 24.5 degrees in a cycle of 41,000 years. The tilt affects where the globe is receiving the most solar radiation. During times of more tilt, higher latitudes receive more sunlight.
  
  
• Precession: Earth doesn’t rotate perfectly around its axis. Instead, it wobbles like a top, a motion called precession. Precession influences the amount of solar radiation striking a given location for a given season. This causes the difference of temperature between seasons to be either large or small. For example, sometimes winters will be extra frigid and summers extra warm (large difference). Other times, mild winters are followed by cool summers (small difference). Precession operates on a 21,000-year cycle.

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