Sea ice is frozen seawater that
floats on the ocean surface. Blanketing millions of square kilometers, sea ice
forms and melts with the polar seasons, affecting both human activity and
biological habitat. In the Arctic, some sea ice persists year after year,
whereas almost all Southern Ocean or Antarctic sea ice is "seasonal
ice," meaning it melts away and reforms annually. While both Arctic and
Antarctic ice are of vital importance to the marine mammals and birds for which
they are habitats, sea ice in the Arctic appears to play a more crucial role in
Because they are composed of ice
originating from glaciers, icebergs are not considered sea ice
The Importance of Sea Ice
Sea ice has a profound influence
on the polar physical environment, including ocean circulation, weather, and
regional climate. As ice crystals form, they expel salt, which increases the
salinity of the underlying ocean waters. This cold, salty water is dense, and it
can sink deep to the ocean floor, where it flows back toward the equator. The
sea ice layer also restricts wind and wave action near coastlines, lessening
coastal erosion and protecting ice shelves. And sea ice creates an insulating
cap across the ocean surface, which reduces evaporation and prevents heat loss
to the atmosphere from the ocean surface. As a result, ice-covered areas are
colder and drier than they would be without ice.
Sea ice also has a fundamental
role in polar ecosystems. When sea ice melts in the summer, it releases
nutrients into the water, which stimulate the growth of phytoplankton, which are
the base of the marine food web. As the ice melts, it exposes ocean water to
sunlight, spurring photosynthesis in phytoplankton. When ice freezes, the
underlying water gets saltier and sinks, mixing the water column and bringing
nutrients to the surface. The ice itself is habitat for animals such as seals,
Arctic foxes, polar bears, and penguins.
Sea ice’s influence on the
Earth is not just regional; it’s global. The white surface of sea ice reflects
far more sunlight back to space than ocean water does. (In scientific terms, ice
has a high albedo.) Once sea ice begins to melt, a
self-reinforcing cycle often begins. As more ice melts and exposes more dark
water, the water absorbs more sunlight. The sun-warmed water then melts more
ice. Over several years, this positive feedback cycle (the “ice-albedo
feedback”) can influence global climate.
Sea ice plays many important
roles in the Earth system, but influencing sea level is not one of them. Because
it is already floating on the ocean surface, sea ice is already displacing its
own weight. Melting sea ice won’t raise ocean level any more than melting ice
cubes will cause a glass of iced tea to overflow.
The Sea Ice Life Cycle
When seawater begins
to freeze, it forms tiny crystals just millimeters wide, called frazil.
How the crystals coalesce into larger masses of ice depends on whether the seas
are calm or rough. In calm seas, the crystals form thin sheets of ice, nilas,
so smooth they have an oily or greasy appearance. These wafer-thin sheets of ice
slide over each other forming rafts of thicker ice.
In rough seas, ice crystals converge into slushy pancakes.
These pancakes can slide over each other to form smooth rafts, or they can
collide into each other, creating ridges on the surface and keels on the bottom.
Sea ice begins as
thin sheets of smooth nilas in calm water (top) or
disks of pancake ice in choppy water (2nd from top). Individual
pieces pile up on top of one another to form rafts and eventually solidify (3rd
from top). Over time, large sheets of ice collide, forming thick pressure ridges
along the margins (bottom). (Nilas, pancake, and ice raft photographs courtesy
Don Perovich, Cold Regions Research
and Engineering Laboratory. Pressure ridge photograph courtesy Ted Scambos, National
Snow and Ice Data Center.)
Some sea ice is fast
ice that holds fast to a coastline or the sea floor, and some sea ice is pack
ice that drifts with winds and currents. Because pack ice is dynamic,
pieces of ice can collide and form much thicker ice. Leads—narrow,
linear openings in the ice ranging in size from meters to
kilometers—continually form and disappear.
Larger and more persistent
openings, polynyas, are sustained by upwelling currents of warm water or steady
winds that blow the sea ice away from a spot as quickly as it forms. Polynyas
often occur along coastlines where offshore winds maintain their presence.
ice is anchored to the
shore or the sea bottom, while pack ice floats
freely. As it drifts, leads continually open and
close between ice floes. Persistent openings, polynyas,
are maintained by strong winds or ocean currents. (NASA satellite
image courtesy Jacques Descloitres, MODIS
Rapid Response Team.)
As the water and air
temperatures rise each summer, some sea ice melts. Because of differences in
geography and climate, it’s normal for Antarctic sea ice to melt more
completely in the summer than Arctic sea ice. Ice that escapes summer melting
may last for years, often growing to a thickness of 2 to 4 meters (roughly 6.5
to 13 feet) or more in the Arctic.
For ice to thicken, the ocean
must lose heat to the atmosphere. But the ice insulates the ocean like a
blanket. Eventually, the ice gets so thick that no more heat can escape. Once
the ice reaches this thickness—3 to 4 meters (10 to 13 feet)—further
thickening isn’t possible except through collisions and ridge-building.
Ice that survives the summer
melt season is called multi-year ice. Multi-year ice increasingly loses salt
and hardens each year it survives the summer melt. In contrast to multi-year
ice, first-year ice—ice that has grown just since the previous summer—is
thinner, saltier, and more prone to melt in the subsequent summer.
Salinity and Brine
Salinity is a measure of the
concentration of dissolved salts in water. Until recently, a common way to
define salinity values has been parts per thousand (ppt), or
kilograms of salt in 1,000 kilograms of water. Today, salinity is usually
described in practical salinity units (psu), a more accurate but more
complex definition. Nonetheless, values of salinity in ppt and psu are nearly
equivalent. The average salinity of the ocean typically varies from 32 to 37
psu, but in polar regions, it may be less than 30 psu. Sodium chloride (table
salt) is the most abundant of the many salts found in the ocean.
Fresh water freezes at 0
degrees Celsius (32 degrees Fahrenheit), but the freezing point of sea water
varies. For every 5 ppt increase in salinity, the freezing point decreases by
0.28 degrees Celsius (0.5 degrees Fahrenheit); thus, in polar regions with an
ocean salinity of 35 ppt, the water begins to freeze at -1.8 degrees Celsius
(28.8 degrees Fahrenheit).
When frazil ice crystals
form, salt accumulates into droplets called brine, which are
typically expelled back into the ocean. This raises the salinity of the
near-surface water. Some brine droplets become trapped in pockets between
the ice crystals. These droplets are saline, whereas the ice around them
is not. The brine remains in a liquid state because much cooler
temperatures would be required for it to freeze. At this stage, the sea
ice has a high salt content. Over time, the brine drains out, leaving air
pockets, and the salinity of the sea ice decreases. Brine can move out of
sea ice in diferent ways: Aided by gravity, the brine migrates downward
through holes and channels in the ice, eventually emptying back into the
The ice surrounding the
brine compresses and breaks the brine pockets, allowing the brine to
escape to the ocean.
When the sea ice begins to
melt during the summer, small freshwater ponds (called melt ponds) form on
the top layer of the ice. This freshwater travels through the cracks and
holes in the ice, washing out remaining brine.
When the sea ice surface
cools, brine increases in salinity to the point at which it can melt ice
at its underside. This leads to a downward migration of brine droplets,
ultimately allowing the brine to escape into the ocean below the ice
Salt plays an important role in
ocean circulation. In cold, polar regions, changes in salinity affect ocean
density more than changes in temperature. When salt is ejected into the ocean
as sea ice forms, the water's salinity increases. Because salt water is
heavier, the density of the water increases and the water sinks. The exchange
of salt between sea ice and the ocean influences ocean circulation across
hundreds of kilometers.
compiled from The British Antarctic Study, NASA, Environment Canada,
UNEP, EPA and other sources as stated and credited Researched by Charles
Welch-Updated daily This Website is a project of the The Ozone Hole Inc.
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