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Polar MeltdownFebruary 2000 The Arctic ice pack is melting. The natural air conditioning system
for much of the Earth has shrunk dramatically in both thickness and
extent over the past 40 years. Thickness has declined by more than 40
percent over the past 40 years, and the extent of pack ice older than
one year has declined by 14 percent over the past 20 years. According
to scientists from NASA and the UK Meteorological Office, computer models
strongly suggest that the only plausible explanation is a build-up of
greenhouse gases from the burning of coal, oil and natural gas. As other fact sheets in this series show, unique
Arctic wildlife including polar bears and walrus, as well as Arctic
indigenous peoples such as the Inuit are already being affected by
this sea ice decline. The implications for global
climate are enormous:
Introduction Late 1999 was a watershed for Arctic science as a series
of important new papers published in the first week of December confirmed
that dramatic changes are occurring in the Arctic ice pack. Andrew Rothrock
and his colleagues at the University of Washington in Seattle, examined
sea ice data collected by Arctic submarine patrols and concluded that
the Arctic ice pack has thinned by about 40 percent over the past few
decades, from an average thickness of 3.1 metres over the deep ocean in
1958-1976, to 1.8 metres for the 1990s.[1]
Ola M. Johannessen and his colleagues at the Nansen Environmental
and Remote Sensing Centre in Bergen, Norway, using microwave data from
satellites concluded that the area in the Arctic ocean covered by ice
more than one year old has shrunk by some 14 percent over the past two
decades.[2] If these trends in sea ice
cover and thickness continue, then the permanent Arctic ice cover will
be transformed to thin, seasonal ice that melts every summer.
A large group of leading sea ice scientists from NASA,
NOAA and the UK Meteorological Office ran computer models with and without
greenhouse gas emissions, and concluded that the odds that the changes
observed in the Arctic ice pack over the past forty years are due to natural
variation are less than 0.1 percent. The odds that the changes were human-induced
are therefore greater than 99.9 percent. The model run including greenhouse
gas emissions were much closer to the observed sea ice decline than the
model run without greenhouse gas emissions. The scientists warned that
their models "project continued decreases in sea ice thickness and extent
throughout the next century."[3]
Mechanisms What mechanisms are causing the rapid sea ice decline?
The most obvious is the direct increase in global temperature, which made
the 1990s the hottest decade in the past 1000 years in the northern hemisphere,
and possibly for the Earth as a whole.[4]
This has increased the length of the season during which ice melts in
the Arctic Ocean.[5]
However, another influence is the periodic change in Arctic
air pressure called the Arctic Oscillation - a phenomenon second only
to the El Nino/Southern Oscillation in influencing the Earth's climate.[6]
In the early 1990s, the Arctic Oscillation index reached the highest level
recorded in the past 150 years.[7] This
high level was associated with a significant drop in air pressure over
the Arctic Ocean.[8] This intensified the
polar vortex (the whirling air masses that surround the North Pole), creating
increased storminess and allowed warm air masses from the south to invade
the Arctic Ocean.[9] At the same time,
there has been an increased flow of warm water from the Atlantic into
the Arctic Ocean.[10]
The combination of rising global temperature, increased
exposure to warm air and water masses and more storminess has led to less
and thinner sea ice. Although the Arctic Oscillation Index has dropped
down from its 1993 peak, global temperatures continued to rise, with 1998
being the hottest year ever recorded, and 1999 being the fifth hottest
year since records began about 140 years ago.[11]
Moreover, greenhouse gases may also be influencing the
Arctic Oscillation. Climate modeling suggests that increased greenhouse
gas levels may be forcing the Arctic Oscillation Index to a more positive
mode and therefore amplifying sea ice decline beyond what temperature
increase itself would cause.[12]
Implications Greenhouse gas levels in the atmosphere are expected
to reach double pre-industrial levels by the year 2050-80 if emissions
increases continue.[13] What effect would
a greenhouse gas doubling have on the Arctic ice pack? This is a difficult
question because Arctic warming will have complex effects on air and ocean
circulation, clouds and precipitation. Nevertheless, most computer models
project a dramatic decline in Arctic sea ice.[14]
Perhaps the most sophisticated analysis published to date
is from Warren M. Washington and Gerald A. Meehl at the National Center
for Atmospheric Research (NCAR) in Boulder, Colorado. The NCAR model includes
coupled simulations of both the atmosphere and the oceans, with special
attention paid to modeling sea ice and clouds. The model projects that
warming the Arctic would create low hanging clouds that would tend to
cool down the region and mitigate the warming to some degree. Nevertheless,
sea ice thickness is reduced to less than half a meter in most Arctic
seas during the winter, and no ice remains by the end of the summer melt
season with the exception of an ice accumulation, less than a meter thick
and perhaps four times the size of Iceland, floating off the Laptev Sea
in the Russian Arctic[15].
Such an enormous change in the Arctic ice pack would have
huge implications for the Arctic and far beyond.
The white surface of the Arctic ice pack reflects 80 percent
of the solar energy it receives back into space. In sharp contrast, the
dark surface of ocean and tundra absorb 80 per cent of the solar energy
that strikes them, re-radiating this energy in the form of heat.[16]
A disappearing Arctic ice pack therefore, means a darker Arctic and a
much warmer Northern Hemisphere.
Other factsheets in this series look at the implications
of these changes for Arctic wildlife,
extreme weather events, and
long-term climate change in
northwestern Europe.
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