Sea ice is frozen water floating on top
of the ocean. The extent of sea ice expands and shrinks seasonally,
responding relatively swiftly to rising or falling air temperatures
(unlike glaciers, which may take years to begin retreating
or advancing in response to climate trends). Time series data
on annual sea ice concentration can provide some indication
of general changes in climate.
Using microwave sensors on satellites, scientists can monitor
sea ice changes on a daily basis. Accurate monitoring of global
snow and ice cover is a key component in the study of climate
as well as daily weather forecasting. Sea ice plays an important
role in regulating the amount of heat, moisture, and salinity
in the polar oceans. With an insulating cover of ice, less
heat is lost from the ocean to the cold polar air. In places
where sea ice is cracked, there is a greater loss of heat
and water vapor to the atmosphere. This in turn can affect
local cloud cover and precipitation.
When sea water freezes, its salt content drops. So freezing
sea water increases the salt content of the water locally,
making it denser. This increase in density helps to drive
some of the ocean currents.
In late winter, sea ice typically covers 17 to 20 million
square kilometers in the Antarctic's Southern Ocean. At the
end of the summer, typically only 3 to 4 million square kilometers
of ice remain.