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WEATHER: What forces affect our weather?
Ice and Snow

National Snow and Ice Data Center
Includes a glossary of snow-related terms, a snow fact sheet, and features on avalanches, glaciers, blizzards, and more. NSIDC is also home to the World Data Center-A for Glaciology.

Electron Microscopy Unit Snow Page
See images of snow crystals under an electron microscope.

Glaciers on the Move: Ice Ages in History

Twenty thousand years ago, a sheet of ice covered what is now the northern United States. Nowadays, you won't find glaciers in Massachusetts or Michigan, though the evidence of their passing is carved into the landscape. What brought about this dramatic change in climate? Why aren't these areas still covered in ice?

We're accustomed to thinking of our planet as stable, steady, and solid. In reality, Earth is dynamic. Our climate is changeable. The history of Earth's climate has been marked by many ice ages and warm spells, some measured in millenia and others in centuries. You may be surprised to find that there are some patterns in this changeability, though. Climate change seems to be cyclical.

A "wobbling top": The Milankovitch theory

A Serbian mathematician named Milutin Milankovitch was intrigued by this puzzle of climate change, and in the 1930s he presented a theory that might explain it. Milankovitch studied climate records, noting differences over time. He theorized that global climate change was brought about by regular changes in Earth's axis, tilt, and orbit that altered the planet's relationship to the Sun, triggering ice ages.

Earth doesn't rotate perfectly like a wheel about an axis; it spins like a wobbling top. Every 22,000 years, Milankovitch calculated, there is a slight change in its wobble. Every 100,000 years, there is a change in Earth's orbit about the Sun. Its almost circular orbit becomes more elliptical, taking Earth farther from the Sun. And finally, Milankovitch discovered, every 41,000 years there is a change in the tilt of the planet's axis, moving either the Northern or Southern Hemisphere farther from the Sun.

These cycles mean that at certain times there is less sunshine hitting Earth, so there is less melting of snow and ice. Instead of melting, these cold expanses of frozen water grow. The snow and ice last longer and, over many seasons, begin to accumulate. Snow reflects some sunlight back into space, which also contributes to cooling. Temperatures drop, and glaciers begin to advance.

The Little Ice Age

Not all ice ages occur on a large, 100,000-year scale. Some, such as the "Little Ice Age" that occurred in medieval times, are relatively brief. The Little Ice Age lasted only a few hundred years (from about 1400 to 1800), and its effects were fairly mild. Still, for the people who watched glaciers advance year to year, it was certainly dramatic. In England, the Thames froze and heavy snow fell. In France, bishops were ushered to the edge of glaciers to halt their advance with prayer.

During the last 10 million years, Earth has had 10 major ice ages and numerous smaller glaciations such as the recent Little Ice Age. Each major ice age has been followed by a warm period of about 10,000 years. We're in a warm period now—more specifically, we're at the end of one. Should we expect another major ice age soon? There's no way of knowing. Climate change may be cyclical, but it's not easy to forecast. Both day-to-day weather and long-term climate change remain difficult to predict.

[Back to "Ice and Snow"] [Next Topic: "Forecasting"]

 "Weather" is inspired by programs from Planet Earth.


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