By Robert Hazen, Ph.D., George Mason University
The Earth’s present ice caps began forming about 10 million years ago. The ice caps reached a maximum about two million years ago. During this time, there have been many extensive advances of ice from the polar North, covering North America and much of Europe. Then those ice sheets have retreated, in a regular, recurrent pattern.
How Close Are We to Another Ice Age?
During the most recent glacial advance, which was about 20,000 years ago, the sea level was hundreds of meters lower than today because there was less water in the ocean. The East Coast of North America was 250 kilometers farther east, for example. There was a land bridge that connected Alaska and Siberia across what is now the Bering Sea.
At the height of these glacial advances, as much as 5 percent of Earth’s water can be locked in ice; so we see big changes can occur. We’re now in a very brief period called an inter-glacial period. The present ice age may last millions of years more, so there may be more advances of ice; but we’re in an age when the ice has retreated, and we expect at some point it may come back.
However, some scientists argue that human activities—especially the extensive burning of fossil fuels, because it causes a build-up of carbon dioxide in the atmosphere—may actually be causing a period of warming, which could conceivably interrupt this large-scale cycling of glacial advance and retreat. If we warm up the Earth too much, the next cycle of advance is not going to happen quite so quickly, or as easily.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Why Do We Have Ice Caps?
Plate motions probably explain the formation of ice caps 10 million years ago. Thick ice caps can only form if you have a continent at a pole. If you have oceans at the pole, you only form a relatively thin sheet of ice over the water; but with a continent, you can build up layer after layer of ice.
About 10 million years ago, Antarctica really insinuated itself in that position across the pole and could start building up very thick layers of ice; and so it’s possible that this influenced this global climate quite a bit. Also, a lot of Eurasia and North America lies inside the Arctic Circle, so there is quite a bit of land mass—for example, Greenland and other parts of North America and Siberia—that can also accumulate ice in a period of glacial advance.
Why do we have periods of glacial advance? This was a problem that Milutin Milankovitch, a Serbian civil engineer who lived early in this century, tackled. Milankovitch explained periodic glacial advances and retreats as a result of orbital variations of the Earth. These cause a slight shift in the amount of solar radiation that reaches the Northern and the Southern hemispheres, and apparently explain the glaciation.
Here’s what he said: there are three types of orbital variations that contribute to glacial cycles. They are, first, the eccentricity of the orbit; second, the precession of the axis of the Earth; and third, the tilt of the axis.
Learn more about the Earth’s topography.
The first and most major effect is a 100,000-year orbital cycle of changing eccentricity. The Earth is now in a slightly eccentric orbit; it’s elongated only about 1 percent. At its farthest distance from the Sun in its orbit, it’s only 1 percent farther than its closest approach. It’s actually farther from the Sun in June, and closer in December, at this point.
But the elongation of the Earth’s axis is affected by the pull of other planets, and at times, in this 100,000-year cycle, it could be as much as 5 percent eccentric. That’s much greater, so the exaggeration between the closest and the most distant approaches to the Sun is quite a bit more.
As the orbit becomes more elongated during the next 50,000 years, we will experience greater seasonal contrasts in the Sun’s energy, and therefore greater contrasting temperature changes: colder winters, warmer summers in some parts of the Earth.
Learn more about the solar system.
Earth’s Rotation Axis
The second orbital variation results from the precession of the Earth’s rotation axis every 23,000 years. If we notice a toy top, after it starts spinning for a while, it starts wobbling a little bit, and that wobble is called precession: the axis actually changes orientation.
The average tilt remains 23 degrees, but the orientation with respect to the Sun changes. At present, the Northern Hemisphere is tilted toward the Sun during the summer months, when the Sun is farther away. But in 11,500 years, the tilt will have changed, so the Southern Hemisphere is tilting toward the Sun in the summer months, and that’s when the Sun is closest.
So, in this case, you’re going to have greater contrasts, once again, in climate. The net result is that the total amount of sunlight reaching the Northern versus the Southern hemisphere changes on a 23,000-year cycle.
Earth Keeps Wobbling
The third effect is somewhat more subtle; it’s a 41,000-year cycle of axial wobble. In this case, the tilt changes from an average of 21.5 degrees to 24.5 degrees. The greater the tilt, of course, the greater the seasonal contrasts because of the greater winter and summer contrast in the amount of sunlight falling on the poles.
Taken together, these three effects—that is, the 100,000-year glaciation cycle, the shorter 20,000 and 40,000-year glacial maxima— exactly match Milankovitch’s ideas about the various orbital variations of the Earth.
Common Questions about the Formation of the Earth’s Ice Caps
Plate motions probably explain the formation of ice caps 10 million years ago. Thick ice caps can only form if you have a continent at a pole. If you have oceans at the pole, you only form a relatively thin sheet of ice over the water.
Milutin Milankovitch said that there are three types of orbital variations that contribute to glacial cycles: first, the eccentricity of the orbit; second, the precession of the axis of the Earth; and third, the tilt of the axis.
As the Earth’s orbit becomes more elongated during the next 50,000 years, we will experience greater seasonal contrasts in the Sun’s energy, and therefore greater contrasting temperature changes: colder winters, warmer summers in some parts of the Earth.