SHOW OVERALL INCREASES IN ANTARCTIC SEA ICE COVER
recent studies have shown that on the whole Arctic sea ice has decreased since
the late 1970s, satellite records of sea ice around Antarctica reveal an overall
increase in the southern hemisphere ice over the same period. Continued decreases
or increases could have substantial impacts on polar climates, because sea ice
spreads over a vast area, reflects solar radiation away from the Earths
surface, and insulates the oceans from the atmosphere.
a study just published in the Annals of Glaciology, Claire Parkinson of NASAs
Goddard Space Flight Center analyzed the length of the sea ice season throughout
the Southern Ocean to obtain trends in sea ice coverage. Parkinson examined 21
years (1979-1999) of Antarctic sea ice satellite records and discovered that,
on average, the area where southern sea ice seasons have lengthened by at least
one day per year is roughly twice as large as the area where sea ice seasons have
shortened by at least one day per year. One day per year equals three weeks over
the 21-year period.
can see with this dataset that what is happening in the Antarctic is not what
would be expected from a straightforward global warming scenario, but a much more
complicated set of events, Parkinson said.
length of the sea ice season in any particular region or area refers to the number
of days per year when at least 15 percent of that area is covered by sea ice.
Some areas close to the Antarctic continent have sea ice all year long, but a
much larger region of the Southern Ocean has sea ice for a smaller portion of
the year, and in those regions the length of the sea ice season can vary significantly
from one year to another.
calculate the lengths of the sea ice seasons, Parkinson used satellite data gridded
to 25 by 25 kilometer grid cells for the Southern Ocean region. For each grid
cell, the satellite data were used to determine the concentration, or percent
area, of the sea ice cover. Whenever the percentage was at least 15 percent, the
grid cell was considered to have ice. Using this method, Parkinson went through
the entire data set and for each grid cell had a computer count how many days
of each year had ice, then calculated trends over the 21-year record.
the area of the Antarctic with trends indicating a lengthening of the sea ice
season by at least one day per year was 5.6 million square kilometers (2.16 million
square miles), about 60 percent the size of the United States. At the same time,
the area with sea ice seasons shortening by at least one day per year was 3 million
square kilometers (1.16 million square miles).
the Ross Sea, on average, had its sea ice seasons getting longer, while most of
the Amundsen Sea and almost the entire Bellingshausen Sea had their sea ice seasons
Antarctic sea ice changes match up well with regional temperature changes,
Parkinson said. The one region in the Antarctic where the temperature records
have shown prominent warming over this period is the Antarctic Peninsula, and
indeed its immediately to the west and east of the Antarctic Peninsula,
in the Bellingshausen/Amundsen and western Weddell seas, respectively, that the
sea ice seasons have been shortening rather than lengthening.
Arctic also shows a mixed pattern of sea ice trends over the 1979-1999 period,
but in contrast to the Antarctic, the area with shortening seasons in the Arctic
is far greater than the area with lengthening seasons. The Arctic patterns suggest
some connections with major oscillations in large-scale atmospheric pressures,
called the Arctic Oscillation and the North Atlantic Oscillation, and it is possible
the ice covers of both hemispheres could be influenced by oscillations that are
still not fully identified, Parkinson said.
study used data from NASAs Nimbus 7 Scanning Multichannel Microwave Radiometer
(SMMR) and the Defense Meteorological Satellite Program (DMSP) Special Sensor
Microwave Imagers (SSMIs) and in the future will be extended with data from the
National Space Development Agency of Japan's Advanced Microwave Scanning Radiometer
for the Earth Observing System (AMSR-E) recently launched on board NASA's Aqua