What were the glacial and interglacial periods in the last Ice Age?

In the last Ice Age (and in all ice ages), there were cycles of glacial (when ice covered the land) and interglacial (relatively warmer temperatures) times. Corresponding with these times, the glaciers advanced or retreated. Scientists believe the last Ice Age—also called the Pleistocene Ice Age—had eight cycles. The following lists these stages for North America (stage names for northern and central Europe differ). All dates are approximate:

Glacial/Interglacial Periods

Approximate Years Ago

North American Stage

75,000-10,000

Wisconsin*

120,000-75,000

Sangomonian (interglacial)

170,000-120,000

Illinoian

230,000-170,000

Yarmouth (interglacial)

480,000-230,000

Kansan

600,000-480,000

Aftonian (interglacial)

800,000-600,000

Nebraskan

1,600,000-800,000

Pre-Nebraskan

* Note: During the Wisconsin glacial stage, an interstadial period occurred—a time not warm or prolonged enough to be called an interglacial period.

How did the last Ice Age begin and end?

Approximately 1.7 million years ago (the beginning of the Quaternary Period, Pleistocene epoch) geologists believe the plains of North America cooled. As a result, large ice sheets began to advance south from the Hudson Bay area of Canada and eastward from the Rocky Mountains. These ice sheets advanced and retreated many times toward the end of the Pleistocene epoch in intervals lasting from 10,000 to 100,000 years. This most recent ice age ended about 10,000 years ago, when the ice retreated to its present polar positions. Currently, the Earth is nearing the end of an interglacial (warmer) period, meaning that another ice age might be due in a few thousand years.

What are major and minor ice age periods?

Of course, not all scientists agree on how to divide the periodic ice ages, with some calling for a more strict division between the times and temperatures. For example, some scientists believe a major ice age period should be defined as lasting about 100,000 years, with a 9°F (5°C) decrease in temperature between glacial and interglacial periods; a minor ice age period, lasting about 12,000 years with a 5°F (2.8°C) decrease in temperature; and a smaller ice age period, lasting about 1,000 years with a 3°F (1.7°C) decrease in temperature.

How much of the Earth was covered in ice during the various ice ages?

Because of extensive erosion, it is difficult to determine the extent of ice sheets during the various ice ages. But scientists do know some things about the last ice age—the Pleistocene Ice Age—a time when up to 10 percent of the Earth was covered (although not simultaneously) by often miles-high ice. At their greatest extent, the Northern Hemisphere glaciers and ice sheets covered most of Canada, all of New England, much of the upper Midwest, large areas of Alaska, most of Greenland, Iceland, Svalbard, and other arctic islands, Scandinavia, much of Great Britain and Ireland, and the northwestern part of the former Soviet Union. In the Southern Hemisphere, glaciers were much smaller, with the main effects being cooler and much drier weather.

During the last stage of the Ice Age (the Wisconsin stage in the United States), ice sheets covered parts of Eurasia and much of North America, extending as far south as Pennsylvania. As the climate warmed up, scientists estimate that sea level rose about 410 feet (125 meters), an average rate of an inch (2.5 centimeters) per year for roughly 5,000 years. Interestingly, although most of the huge northern ice sheets melted, the Antarctic ice sheet decreased by only 10 percent.

What was the "Little Ice Age"?

The "Little Ice Age" was an interval of relative cold, beginning about 1450 and lasting until about 1890 (the coldest periods of 1450 and 1700 are often divided into the two Little Ice Ages). It occurred during the current warm, interglacial period, but is not considered a full glacial episode, since the high latitudes of the Northern Hemisphere landmasses remained largely free of permanent ice cover.

Even so, much of the world experienced cooler temperatures during this time of at least 2°F (1°C) lower worldwide average surface temperatures. It was a time of renewed glacial advance in Europe, Asia, and North America, with sea ice causing havoc in the colonies of Greenland and Iceland. In England, the Thames River froze; in France, bishops tried to halt glacial advances with prayer. Several historians also believe the low temperatures caused social conflict and poor food production. Thus, this may have been partially responsible for war and hunger during that time.

Just like the major ice ages, no one really knows what caused the Little Ice Age, though English astronomer Edward Maunder (1851-1928) first hypothesized that it had something to do with solar activity. Other scientists attribute the cooling down to volcanic eruptions, changes in the ocean circulation, changes in the Earth's orbit, the wobbling of the Earth's axis, or even our planet's passage through clouds of interstellar dust.

Will there be an ice age in the future?

Yes, eventually the Earth will again cool and ice will cover land at higher latitudes and elevations. When this might happen, no one is really sure.

 
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