DEFINITION OF GLACIER

Any permanent body of ice (recrystallized snow)

that shows evidence of gravitational movement. Glaciers are

an integral part of the cryosphere, which is that portion of

the planet where temperatures are so low that water exists

primarily in the frozen state. Most glaciers are presently

found in the polar regions and at high altitudes. However, at

several times in Earth history glaciers have advanced deeply

into midlatitudes and the climate of the entire planet was different.

Some models suggest that at one time the entire surface

of the Earth may have been covered in ice, a state

referred to as the “Snowball Earth.”

Glaciers are dynamic systems, always moving under the

influence of gravity and changing drastically in response to

changing global climate systems. Thus, changes in glaciers

may reflect coming changes in the environment. There are several

types of glaciers. Mountain glaciers form in high elevations

and are confined by surrounding topography, such as

valleys. These include cirque glaciers, valley glaciers, and fiord

glaciers. Piedmont glaciers are fed by mountain glaciers but

terminate on open slopes beyond the mountains. Some piedmont

and valley glaciers flow into open water, bays, or fiords,

and are known as tidewater glaciers. Ice caps form domeshaped

bodies of ice and snow over mountains and flow radially

outward. Ice sheets are huge, continent-sized masses of ice

that presently cover Greenland and Antarctica and are the

largest glaciers on Earth. Ice sheets contain about 95 percent

of all the glacier ice on the planet. If global warming were to

continue to melt the ice sheets, sea level would rise by 230 feet

(66 m). A polar ice sheet covers Antarctica, consisting of two

parts that meet along the Transantarctic Mountains. It shows

ice shelves (thick glacial ice that floats on the sea), which form

many icebergs by calving, which move northward into shipping

lanes of the Southern Hemisphere.

Polar glaciers form where the mean average temperature

lies below freezing, and these glaciers have little or no seasonal

melting because they are always below freezing. Other glaciers,

called temperate glaciers, have seasonal melting periods, where

the temperature throughout the glacier may be at the pressure

melting point (when the ice can melt at that pressure and both

ice and water coexist). All glaciers form above the snow line,

which is the lower limit at which snow remains year-round,

located at sea level in polar regions, and at 5,000–6,000 feet

(1,525–1,830 m) at the equator (Mount Kilimanjaro in Tanzania

has glaciers, although these are melting rapidly).

Glaciers represent sensitive indicators of climate change

and global warming, shrinking in times of warming, and

expanding in times of cooling. Glaciers may be thought of as

the “canaries in the coal mine” for climate change.

The Earth has experienced at least three major periods of

long-term frigid climate and ice ages, interspersed with periods

of warm climate. The earliest well-documented ice age is the

period of the “Snowball Earth” in the Late Proterozoic,

although there is evidence of several even earlier glaciations.

Beginning about 350 million years ago, the Late Paleozoic saw

another ice age lasting about 100 million years. The planet

entered the present ice age about 55 million years ago. The

underlying causes of these different glaciations are varied and

include anomalies in the distribution of continents and oceans

and associated currents, variations in the amount of incoming

solar radiation, and changes in the atmospheric balance

between the amount of incoming and outgoing solar radiation.

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