are made up of fallen snow that, over many years, compresses into large,
thickened ice masses. Glaciers form when snow remains in one location long
enough to transform into ice. What makes glaciers unique is their ability to
move. Due to sheer mass, glaciers flow like very slow rivers. Some glaciers are
as small as football fields, while others grow to be over a hundred kilometers
Presently, glaciers occupy about
10 percent of the world's total land area, with most located in polar regions
like Antarctica and Greenland. Glaciers can be thought as remnants from the last
Ice Age, when ice covered nearly 32 percent of the land, and 30 percent of the
oceans. An Ice Age occurs when cool temperature endure for extended periods of
time, allowing polar ice to advance into lower latitudes. For example, during
the last Ice Age, giant glacial ice sheets extended from the poles to cover 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
Within the past 750,000 years,
scientists know that there have been eight Ice Age cycles, separated by warmer
periods called interglacial periods.
Presently, 10% of land area is
covered with glaciers.
Glaciers store about 75% of
the world's freshwater.
Glacierized areas cover over
15,000,000 square kilometers.
Antarctic ice is over 4,200
meters thick in some areas.
In the United States, glaciers
cover over 75,000 square kilometers, with most of the glaciers located in
During the last Ice Age,
glaciers covered 32% of the total land area.
If all land ice melted, sea
level would rise approximately 70 meters worldwide.
Glacier ice crystals can grow
to be as large as baseballs.
The land underneath parts of
the West Antarctic Ice Sheet may be up to 2.5 kilometers below sea level,
due to the weight of the ice.
North America's longest
glacier is the Bering Glacier in Alaska, measuring 204 kilometers long.
Glacial ice often appears blue
when it has become very dense. Years of compression gradually make the ice
denser over time, forcing out the tiny air pockets between crystals. When
glacier ice becomes extremely dense, the ice absorbs all other colors in the
spectrum and reflects primarily blue, which is what we see. When glacier ice
is white, that usually means that there are many tiny air bubbles still in
The Kutiah Glacier in Pakistan
holds the record for the fastest glacial surge. In 1953, it raced more than
12 kilometers in three months, averaging about 112 meters per day.
In Washington state alone,
glaciers provide 470 billion gallons of water each summer.
Antarctic ice shelves may
calve icebergs that are over 80 kilometers long.
Almost 90% of an iceberg is
below water--only about 10% shows above water.
The Antarctic ice sheet has
been in existence for at least 40 million years.
From the 17th century to the
late 19th century, the world experienced a "Little Ice Age," when
temperatures were consistently cool enough for significant glacier advances.
sheets and glaciers form the largest component of perennial ice on Earth. Over
75% of the world's fresh water is presently locked up in these frozen
Glacier is any large mass of perennial ice that originates on land by the
recrystallization of snow or other forms of solid precipitation and that shows
evidence of past or present flow. A glacier occupying an extensive tract of
relatively level land and exhibiting flow from the center outward is commonly
called an ice sheet. Glaciers form when snow accumulates on a patch of land over
tens to hundreds of years. The snow eventually becomes so thick that it
collapses under its own weight and forms dense glacial ice. When enough of the
ice is compacted together it succumbs to gravity and begins to flow downhill or
spread out across flat lands. What makes glaciers unique is their ability to
move. Due to sheer mass, glaciers flow like very slow rivers.
than 90 percent of the 33 million cubic kilometers of glacier ice in the world
is locked up in the gigantic Greenland and Antarctic ice sheets.
A Short Tour of the Cryosphere Video
Most of the world's glaciers are
found near the Poles, but glaciers exist on all of the world's continents, even
Africa. Australia doesn't have any glaciers; however, it is considered part of
Oceania, which includes several Pacific island chains and the large islands of
Papua New Guinea and New Zealand. Both of these islands have glaciers.
Glaciers require very specific
climatic conditions. Most are found in regions of high snowfall in winter and
cool temperatures in summer. These conditions ensure that the snow that
accumulates in the winter isn't lost (by melt, evaporation, or calving) during
the summer. Such conditions typically prevail in polar and high alpine regions.
There are two main types of glaciers: valley glaciers and continental glaciers
(known as ice sheets).
The amount of precipitation
(whether in the form of snowfall, freezing rain, avalanches, or wind-drifted
snow) is important to glacier survival. In areas such as Antarctica, where the
low temperatures are ideal for glacier growth, very low annual precipitation
causes the glaciers to grow very slowly.
Motion and change define a
glacier's life. Glacial ice advances, then retreats. Glaciers grow and shrink in
response to changing climate.
ice cover in Greenland and Antarctica has two components - thick, grounded,
inland ice that rests on a more or less solid bed, and thinner floating ice
shelves and glacier tongues. An ice sheet is actually a giant glacier, and like
most glaciers it is nourished by the continual accumulation of snow on its
surface. As successive layers of snow build up, the layers beneath are gradually
compressed into solid ice. Snow input is balanced by glacial outflow, so the
height of the ice sheet stays approximately constant through time. The ice is
driven by gravity to slide and to flow downhill from the highest points of the
interior to the coast. There it either melts or is carried away as icebergs
which also eventually melt, thus returning the water to the ocean whence it
came. Outflow from the inland ice is organized into a series of drainage basins
separated by ice divides that concentrate the flow of ice into either narrow
mountain-bounded outlet glaciers or fast-moving ice streams surrounded by
slow-moving ice rather than rock walls. In Antarctica much of this flowing ice
has reached the coast and has spread over the surface of the ocean to form ice
shelves that are floating on the sea but are attached to ice on land. There are
ice shelves along more than half of Antarctica's coast, but very few in
Ahlenius, UNEP/GRID-Arendal based on material provided by K. Steffen, CIRES/Univ.
JPL Video: The Big Thaw October 01, 2007 A thick chunk of Arctic sea ice the
size of two states has disappeared. Is it global warming or normal causes? A new
NASA-led study found a 23-percent loss in the extent of the Arctic's thick,
year-round sea ice cover during the past two winters. Between winter 2005 and
winter 2007, the perennial ice shrunk by an area the size of Texas and
California combined. This drastic reduction of perennial winter sea ice is the
primary cause of the fastest-ever sea ice retreat on record this summer.
Scientists say the rapid decline in winter perennial ice was caused by unusual
winds. For more information go to: www.jpl.nasa.gov
An ice sheet is a mass of glacial
land ice extending more than 50,000 square kilometers (20,000 square miles). The
two ice sheets on Earth today cover most of Greenland and Antarctica. During the
last ice age, ice sheets also covered much of North America and Scandinavia.
Together, the Antarctic and
Greenland ice sheets contain more than 99 percent of the freshwater ice on
Earth. The Antarctic Ice Sheet extends almost 14 million square kilometers (5.4
million square miles), roughly the area of the contiguous United States and
Mexico combined. The Antarctic Ice Sheet contains 30 million cubic kilometers
(7.2 million cubic miles) of ice. The Greenland Ice Sheet extends about 1.7
million square kilometers (656,000 square miles), covering most of the island of
Greenland, three times the size of Texas.
sheets form in areas where snow that falls in winter does not melt entirely over
the summer. Over thousands of years, the layers of snow pile up into thick
masses of ice, growing thicker and denser as the weight of new snow and ice
layers compresses the older layers. Ice sheets are constantly in motion, slowly
flowing downhill under their own weight. Near the coast, most of the ice moves
through relatively fast-moving outlets called ice streams, glaciers, and ice
shelves. As long as an ice sheet accumulates the same mass of snow as it loses
to the sea, it remains stable.
Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Aqua
satellite captured this composite image on January 27, 2009
sheets contain enormous quantities of frozen water. If the Greenland Ice
Sheet melted, scientists estimate that sea level would rise about 6 meters
(20 feet). If the Antarctic Ice Sheet melted, sea level would rise by
about 60 meters (200 feet).
The Greenland and Antarctic
ice sheets also influence weather and climate. Large high-altitude
plateaus on the ice caps alter storm tracks and create cold downslope
winds close to the ice surface.
In addition, the layers of
ice blanketing Greenland and Antarctica contain a unique record of Earth's
The mass of ice in the
Greenland Ice Sheet has begun to decline. From 1979 to 2006, summer melt
on the ice sheet increased by 30 percent, reaching a new record in 2007.
At higher elevations, an increase in winter snow accumulation has
partially offset the melt. However, the decline continues to outpace
accumulation because warmer temperatures have led to increased melt and
faster glacier movement at the island's edges.
Most of Antarctica has yet
to see dramatic warming. However, the Antarctic Peninsula, which juts out
into warmer waters north of Antarctica, has warmed 2.5 degrees Celsius
(4.5 degrees Fahrenheit) since 1950. A large area of the West Antarctic
Ice Sheet is also losing mass, probably because of warmer water deep in
the ocean near the Antarctic coast. In East Antarctica, no clear trend has
emerged, although some stations appear to be cooling slightly. Overall,
scientists believe that Antarctica is starting to lose ice, but so far the
process has not become as quick or as widespread as in Greenland.
Ice shelves are permanent
floating sheets of ice that connect to a landmass.
Most of the world's ice
shelves hug the coast of Antarctica. However, ice shelves can also form
wherever ice flows from land into cold ocean waters, including some
glaciers in the Northern Hemisphere.
Island, Northern Canada
Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC
The northern coast of Canada's
Ellesmere Island is home to several well-known ice shelves, among them the
Markham and the Ward Hunt ice shelves.
Ice from enormous ice
sheets slowly oozes into the sea through glaciers and ice streams. If the
ocean is cold enough, that newly arrived ice doesn't melt right away.
Instead it may float on the surface and grow larger as glacial ice behind
it continues to flow into the sea. Along protected coastlines, the
resulting ice shelves can survive for thousands of years, bolstered by the
rock of peninsulas and islands. Ice shelves grow when they gain ice from
land, and occasionally shrink when icebergs calve off their edges. This
give and take helps them maintain a dynamic stability.
Because ice shelves
already float in the ocean, they do not contribute directly to sea level
rise when they break up. However, ice shelf collapse could contribute to
sea level rise indirectly. Ice streams and glaciers constantly push on ice
shelves, but the shelves eventually come up against coastal features such
as islands and peninsulas, building pressure that slows their movement
into the ocean. If an ice shelf collapses, the backpressure disappears.
The glaciers that fed into the ice shelf speed up, flowing more quickly
out to sea. Glaciers and ice sheets rest on land, so once they flow into
the ocean, they contribute to sea level rise.
Research suggests that
glaciers behind ice shelves may accelerate by as much as five times
following a rapid ice shelf retreat.
This satellite image shows
floating chunks of ice from the 2008 Wilkins Ice Shelf collapse. - Credit:
National Snow & Ice Data Center
In the last thirty years,
scientists have observed a series of unusual ice shelf collapses on the
Antarctic Peninsula. Although it is not unusual for ice shelves to calve
large icebergs, that process normally takes months to years, as cracks
slowly form in the ice. Following a calving, ice shelves generally recover
over a period of decades.
In recent years, ice
shelves on the Antarctic Peninsula and along the northern coast of Canada
have experienced rapid disintegration. In March 2008, the Wilkins Ice
Shelf in Antarctica retreated by nearly 1,100 square kilometers (425
square miles). Later that summer, several ice shelves along Ellesmere
Island in Northern Canada broke up in a matter of days.
In contrast, the collapses
in previous years happened over a period of weeks, leaving a soup of
chunky ice and small icebergs. The remaining ice shelves retreated by as
much as 90 percent, and several have experienced repeated collapses.
Scientists think that the
recent ice shelf collapses in both the Arctic and Antarctica are related
to climate change. Most of the rapidly retreating ice shelves in
Antarctica are located on the Antarctic Peninsula. The Antarctic Peninsula
juts north towards South America, into warmer waters. The peninsula has
warmed 2.5 degrees Celsius (4.5 degrees Fahrenheit) since 1950, making it
one of the fastest-warming places on Earth.
Scientists attributed rapid
ice shelf collapse to warmer air and water temperatures, as well
asincreased melt on the ice shelf surface. Retreating sea ice may also
play a role.
Warm air melts the ice
shelf surface, forming ponds of meltwater. As the water trickles down
through small cracks in the ice shelf, it deepens, erodes, and expands
those cracks. In a separate process, warmer water melts the ice shelf from
below, thinning it and making it more vulnerable to cracking. Scientists
have observed both processes in all the ice shelves that have rapidly
retreated in recent years.
However, warm temperatures
alone do not fully explain rapid ice shelf collapse. Recent research
suggests that waning sea ice surrounding the Antarctic Peninsula and the
Arctic ice shelves in Canada might also have contributed to the recent
collapses. Sea ice provides a layer of protection between an ice shelf and
the surrounding ocean, muting the power of large waves and storms. As sea
ice decreases, more waves buffet the ice shelves. The largest waves can
buckle and bend an ice shelf, increasing instability and possibly
contributing to a collapse.
Ice Shelf, Antarctica
7 Science Team and NASA GSFC Satellite: Landsat 7 Sensor: ETM+
shelves occur when ice sheets extend over the sea, floating on the water. In
thickness they range from a few hundred meters to over 1000 meters. Ice shelves
surround nearly all of the Antarctic continent. Retreating ice shelves may
provide indications of climate change.
Grobe, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven,
caps are miniature ice sheets. An ice cap covers less than 50,000 square
kilometers. They form primarily in polar and sub-polar regions that are
relatively flat and high in elevation.
Peak North Cascade Mountains
glaciers develop in high mountainous regions, often flowing out of icefields
that span several peaks or even a mountain range. The largest mountain glaciers
are found in Arctic Canada, Alaska, the Andes in South America, the Himalayas in
Asia, and on Antarctica.
oblique aerial photograph of the upper part of the Bucher Glacier, an outlet
glacier of the Juneau Icefield, Coast Mountains, Tongass National Forest,
Alaska. The length of the glacier shown is ~ 5 miles
originating from mountain glaciers or ice fields, these glaciers spill down
valleys, looking much like giant tongues. Valley glaciers may be very long,
often flowing down beyond the snow line, sometimes reaching sea level.
calving terminus of Tyndall Glacier, located at the head of Taan Fiord, Icy Bay,
Wrangell - St. Elias National Park, Alaska. The large mountain to the right is
Mt. St. Elias.
glacier with a terminus that ends in a body of water (river, lake, ocean) into
which it calves icebergs.
Space Shuttle Photograph
glaciers occur when steep valley glaciers spill into relatively flat plains,
where they spread out into bulb-like lobes. The Malaspina Glacier in Alaska,
covering over 5,000 square kilometers is one of the most famous examples of this
type of glacier.
Glaciers are named for the bowl-like hollows they occupy, which are called
cirques. Typically, they are found high on mountainsides and tend to be wide
rather than long.
oblique aerial photograph showing a small, unnamed hanging glacier located in
the Chugach Mountains, near Cordova Peak, Chugach National Forest, Alaska.
called ice aprons, these glaciers cling to steep mountainsides. Like cirque
glaciers, they are wider than they are long. Hanging glaciers are common in the
Alps, where they often cause avalanches due to the steep inclines they occupy.
oblique aerial photograph of the reconstituted Ogive Glacier, located on the
shore of Northwestern Fjord, Kenai Fjords National Park, Kenai Mountains, Alaska
glacier formed below the terminus of a hanging glacier by the accumulation, and
reconstitution by pressure melting (regelation), of ice blocks that have fallen
and/or avalanched from the terminus of the hanging glacier. Also called Glacier
oblique aerial photograph of an unnamed rock glacier heading in a cirque,
located on the southeast side of the Talkeetna Mountains, Alaska
glacier-like landform that often heads in a cirque and consists of a
valley-filling accumulation of angular rock blocks. Rock glaciers have little or
no visible ice at the surface. Ice may fill the spaces between rock blocks. Some
rock glaciers move, although very slowly.
photograph showing the stream and pond covered-surface of the Taku Glacier,
Juneau Icefield, Tongas National Forest, Coast Mountains, Alaska
glacier with a or temperature-regime in which liquid water coexists with frozen
water (glacier ice) during part or even all of the year
oblique aerial photograph of the tidewater terminus of the calving Chenega
Glacier, located at sea level in western Prince William Sound, Kenai
the name implies, these are valley glaciers that flow far enough to reach out
into the sea. Tidewater glaciers are responsible for calving numerous small
icebergs, while not as imposing as Antarctic icebergs, can still pose problems
for shipping lanes.
NASA GSFC Scientific Visualization Studio Data Source: Airborne Topographic
NASA study of Greenland's ice sheet reveals that it is rapidly thinning. In an
article published in the Magazine Science, Bill Krabill, project
scientist at the NASA Goddard Space Flight Center's Wallops Flight Facility,
Wallops Island, VA, reports that the frozen area around Greenland is thinning,
in some places, at a rate of more than three feet per year. Any change is
important since a smaller ice sheet could result in higher sea levels. "A
conservative estimate, based on our data, indicates a net loss of approximately
51 cubic kilometers of ice per year from the entire ice sheet, sufficient to
raise global sea level by 0.005 inches per year, or approximately seven percent
of the observed rise," Krabill said. "This amount of sea level rise does not
threaten coastal regions, but these results provide evidence that the margins of
the ice sheet are in a process of change," Krabill said. "The thinning
cannot be accounted for by increased melting alone. It appears that ice must be
flowing more quickly into the sea through glaciers."
ice reflects a great deal of the Sun's energy that falls on it back into space,
helping regulate the amount of energy arriving on Earth, which drives weather
and all the other atmospheric activities. Even a few percent more acres of open
water absorbing energy could tip the scales of Earth's energy balance, adding
more energy to the atmosphere, altering short- and long-term weather patterns.
Satellite Reveals Dramatic Arctic Ice Thinning-Click
Bridge Supporting Wilkins Ice Shelf Collapses-Click
sea ice younger, thinner as melt season begins-Click
of the ice bridge supporting Wilkins Ice Shelf appears imminent-Click
Ice Shelf hanging by its last thread-Click
Antarctic Ice Shelf Disintegration
Underscores a Warming World-Click Here