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.

Glaciers can deposit large boulders, called erratics, hundreds of kilometers from their source.

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.

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.

Milankovitch reasoned that these cycles could work together to vary the amount of sunlight a given place on Earth receives by 20 percent, especially at high latitudes. That could nudge the advance of the polar ice caps: Less radiation at the poles would mean more snow would survive until the next season. Snow would therefore increasingly accumulate into glacier ice.

Milankovitch cycles explain much about how and when climate changes have occurred in the last 2.5 billion years. But they’re not the whole story. The magnitude of large-scale climate change is influenced by many Earth-bound factors. Among them are changes in topography and plate motions, the hydrosphere, the biosphere, and the atmosphere. The concentration of atmospheric greenhouse gases is important: The higher the concentration, the more these gases trap escaping radiation close to Earth’s surface. Ice-core analysis indicates that levels of greenhouse gases were lower during glacial periods than interglacials.

Welcome to the Holocene
map showing the greatest amount of ice to cover North America in the last ice age
The greatest extent of ice sheets on North America and Greenland during the last glaciation, 21,000 years ago.

After continental glaciation reached its largest extent 21,000 years ago, the Pleistocene began to warm. Prodded by Milankovitch cycles, the ice sheets shrank. After about 11,000 years ago, humans began to cultivate food, domesticate animals, and build cities in the continuously stable climate. Enter a new interglacial period, and with it, a new epoch: The Holocene. This, as they say, is our time.

“There’s no danger of an ice age popping in now,” says Penn State glaciologist Richard Alley. “I believe that most people studying this field think that, without any human intervention,…a new ice age should arrive 20,000 years into the future.” Likewise, it’s generally accepted that our civilization's increase of atmospheric greenhouse gases could make it tougher for that ice age to get going. However, William Ruddiman, a climatologist at the University of Virginia, is one scientist that suggests that the transition to the next glaciation should have already begun. Ruddiman's research says that early, pre-industrial societies generated enough greenhouse gases to actually stop a current-day ice age from happening altogether. But whether nature or humans avoided an ice age recently, we still have 20,000 years or more to wait before the next one.

“It’s a fascinating idea,” says Alley, “but it doesn’t mean anything for the future. If you’re concerned about staving off a new ice age, the wise thing would be to save your fossil fuels for 20,000 years from now and burn them then. But nobody has 20,000-year planning in their blood.”