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 area 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 years 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 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)