Climate 101: Glaciers | National Geographic

• [Narrator] Glaciers have been shaping our world for millions of years. But as climate change warms the planet, glaciers are disappearing, not only altering the landscapes they leave behind but changing our oceans, weather, and life on earth as we know it.

A glacier is a huge mass of ice that moves slowly over land. Glaciers can be classified into two general groups: alpine glaciers and ice sheets. Alpine glaciers form on mountainsides and move downward through valleys. Sometimes, they create valleys by pushing dirt, soil, and other materials out of their way. These glaciers are found on every continent except Australia.

Ice sheets, unlike alpine glaciers, are not limited to mountainous areas. They form broad domes and spread out from their centers in all directions. As ice sheets spread, they cover everything around them with a thick blanket of ice, including valleys, plains, and even entire mountains. The largest ice sheets are called continental glaciers. They cover vast areas, including most of Antarctica and the island of Greenland.

Glaciers can form over years when more snow piles up and melts. Soon after falling, the snow begins to become denser and more tightly packed. When new snow falls and buries the previous year's snow, the bottom layer becomes even more compressed. The dense, grainy ice that has survived one year's melt cycle is called firn. When the ice grows thick enough, the firn grains fuse into a huge mass called glacial ice. The glacier may begin to move under its own weight, through a process called compression melting.

As they move, glaciers erode or wear away the land beneath and around them. When glaciers began their latest retreat less than 20,000 years ago, they left behind many landscape features, such as lakes, valleys, and mountains. Glaciers provide people with many useful resources. Glacial till provides fertile soil for growing crops. And, deposits of sand and gravel are used to make concrete and asphalt. Many rivers are fed by the melting ice of glaciers. The most important resource provided by glaciers is freshwater.

Earth's average temperature has been increasing dramatically for more than a century. Glaciers can act as indicators of global warming and climate change in several ways. Melting ice sheets contribute to rising sea levels. As ice sheets in Antarctica and Greenland melt, they raise sea levels, adding fresh water to the ocean every day. The loss of glacial ice also reduces the amount of fresh water available for plants and animals on land.

Large additions of fresh water change the ocean ecosystem, as well as ocean currents. Additionally, less salt in the ocean could disturb the Gulf Stream, drastically changing the weather on land as well. Since glaciers are so sensitive to climate change, the increased speed of glacier melt is an early warning system for the rest of the planet. And if global warming goes unchecked, many, if not all, alpine glaciers could disappear completely. (soft entertaining music)