DEFINITION OF SEA ICE

Ice that has broken off an ice cap, polar sea, or

calved off a glacier and is floating in open water. Sea ice presents

a serious hazard to ocean traffic and shipping lanes and

has sunk numerous vessels, including the famous sinking of

the Titanic in 1912, killing 1,503 people.

There are four main categories of sea ice. The first comes

from ice that formed on polar seas in the Arctic Ocean and

around Antarctica and is typically about 10–13 feet (3–4 m)

thick. Antarctica becomes completely surrounded by this sea

ice every winter, with the Arctic Ocean typically about 70 percent

covered in the winter. During summer, many passages

open up in this sea ice, but during the winter they re-close,

forming pressure ridges of ice that may be up to tens of meters

high. Recent observations suggest that the sea ice in the Arctic

Ocean is thinning dramatically and rapidly and may soon disappear

altogether. The icecap over the Arctic Ocean rotates

clockwise, in response to the spinning of the Earth. This spinning

is analogous to putting an ice cube in a glass and slowly

turning the glass; the ice cube will rotate more slowly than the

glass because it is decoupled from the edge of the glass. About

one-third of the Arctic sea ice is removed every year by the

East Greenland current. This ice then moves south as icebergs

and becomes a hazard to shipping in the North Atlantic.

Recent studies have revealed that the Arctic Ice Cap has

thinned dramatically in recent years (by nearly 50 percent),

and that the aerial extent of the ice cap is rapidly shrinking.

These changes are probably a result of the 10°F average

warming of the climate in the Arctic over the past 10 years. It

is not certain if this is a human-induced change, but it very

likely is a response to increased carbon dioxide, aerosols, and

other greenhouse gases put into the atmosphere by human

use. Some of the heating may alternatively be the result of

natural cycles in global temperature.

Icebergs from sea ice float on the surface, but between

81 and 89 percent of the ice will be submerged. The exact

level that sea ice floats in the water depends on the exact density

of the ice, as determined by the total amount of air bubbles

trapped in the ice, and how much salt got trapped in the

ice during freezing.

A second group of sea ice forms as pack ice in the Gulf

of St. Lawrence, along the southeast coast of Canada, in the

Bering, Beaufort, and Baltic Seas, in the Seas of Japan and

Okhotsk, and around Antarctica. Pack ice builds up especially

along the western sides of ocean basins where cold currents

are more common. Occasionally, during cold summers, pack

ice may persist throughout the year.

Several scenarios suggest that new ice ages may begin

with pack ice that persists through many summers, gradually

growing and extending to lower latitudes. Other models and

data show that pack ice varies dramatically with a four or

five-year cycle, perhaps related to sunspot activity, and the

El-Niño–Southern Oscillation (ENSO).

Pack ice presents hazards when it gets so extensive that it

effectively blocks shipping lanes, or when leads (channels)

into the ice open and close, forming pressure ridges that

become too thick to penetrate with icebreakers. Ships

attempting to navigate through pack ice have become crushed

when leads close and the ships are trapped. Pack ice has terminated

or resulted in disaster for many expeditions to polar

seas, most notably Franklin’s expedition in the Canadian arctic

and Scott’s expeditions to Antarctica. Pack ice also breaks

up, forming many small icebergs, but because these are not as

thick as icebergs of other origins they do not present as significant

a hazard to shipping.

Pack ice also presents hazards when it drifts into shore,

usually during spring breakup. With significant winds pack

ice can pile up on flat shorelines and accumulate in stacks up

to 50 feet (15 m) high. The force of the ice is tremendous and

is enough to crush shoreline wharves, docks, buildings, and

boats. Pack ice that has blown ashore also commonly pushes

up high piles of gravel and boulders that may be 35 feet (10

m) high in places. These ridges are common around many of

the Canadian Arctic islands and the mainland. Ice that forms

initially attached to the shore presents another type of hazard.

If it breaks free and moves away from shore, it may

carry with it significant quantities of shore sediment, causing

rapid erosion of beaches and shore environments.

Pack ice also forms on many high-latitude lakes, and the

freeze-thaw cycle causes cracking of the lake ice. When lake

water rises to fill the cracks, the ice cover on the lake expands

and pushes over the shoreline, resulting in damage to any

structures built along the shore. This is a common problem

on many lakes in northern climates and leads to widespread

damage to docks and other lakeside structures.

Icebergs present the greatest danger to shipping. In the

Northern Hemisphere most icebergs calve off glaciers in

Greenland or Baffin Island, then move south through the

Davis Strait into shipping lanes in the North Atlantic off

Newfoundland. Some icebergs calve off glaciers adjacent to

the Barents Sea, and others come from glaciers in Alaska and

British Columbia. In the Southern Hemisphere, most icebergs

come from Antarctica, though some come from Patagonia.

Once in the ocean, icebergs drift with ocean currents,

but because of the Coriolis force, they are deflected to the

right in the Northern Hemisphere and to the left in the

Southern Hemisphere. Most icebergs are approximately 100

feet–300 feet (30.5–90 m) high and up to about 2,000 feet

(310 m) in length. However, in March of 2000 a huge iceberg

broke off the Ross Ice Shelf in Antarctica that was roughly

the size of the state of Delaware. It had an area of 4,500

square miles (11,660 km2) and stuck 205 feet (63 m) out of

the water. Icebergs in the Northern Hemisphere pose a

greater threat to shipping, as those from Antarctica are too

remote and rarely enter shipping lanes. Ship collisions with

icebergs have resulted in numerous maritime disasters, especially

in the North Atlantic on the rich fishing grounds of the

Grand Banks off the coast of Newfoundland.

Icebergs are now tracked by satellite; ships are updated

with their positions so they can avoid any collisions that could

prove fatal for the ships’ occupants. Radio transmitters are

placed on larger icebergs to more closely monitor their locations,

and many ships now carry more sophisticated radar and

navigational equipment that helps track the positions of large

icebergs and themselves, so that they avoid collision.

Icebergs also pose a serious threat to oil drilling platforms

and seafloor pipelines in high-latitude seas. Some precautions

have been taken such as building seawalls around

near-shore platforms, but not enough planning has gone into

preventing an iceberg colliding with and damaging an oil

platform or damage from one being dragged across the

seafloor and rupturing a pipeline.

See also GLACIER.

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