Thinning of the Arctic Sea-ice
By Monika Kopacz
Arctic is considered to be a region north of 70N latitude. The Arctic Ocean Basin
is a mediterranean sea surrounded by Asia, Greenland and North America. Arctic is not
the coldest place (lowest recorded temperature is –68C), but it is the most humid place on
Earth (even more humid than the Amazon forest). It covers 9 to 12 (16) million km sq.
(for comparison, the size of US is about 12 million km sq.)
Sea-ice is ice that floats in water but is not an iceberg. While icebergs originate
over land, sea ice is actually produced from ocean water and is therefore free of
sediments that can be carried in icebergs. Sea ice is formed at the boundary layer between
relatively warm ocean and cold atmosphere. It usually forms first in shallow coastal water
as the process involves substantial cooling water below. Initially, ice crystals create slush,
which then forms pancake shaped pieces up to a couple of meters in diameter. These
pieces of ice do not contain salt due to Brine rejection, but as they grow by aggregating
(especially if the process is fast), some salty water pockets can be trapped inside ice.
These can later speed up melting. In fact, sea ice can melt and grow at the same time.
When the sun shines on the ice and melts it, the fresh water slides off the sides and ice
grows underneath.
Figure 1.
Figure 2.
One of the most important issues in sea-ice research today is that its volume is
shrinking. Above, Figures 1 and 2 show how sea-ice has decreased in the last 50 years
and how the trend has been extrapolated by NOAA (National Oceanic and Atmospheric
Administration) for the next 50 years. The decrease is very substantial, especially its
projections.
The paper that addressed this decrease was published in Geophysical Research
Letters journal in 1999 and titled “Thinning of the Arctic Sea-Ice cover. Rothrock et al.
compare submarine data from 1960s to current measurements from 1990s. They find that
ice draft (amount under water) did in fact decrease by about 40% (~1.3m) and continues
to decrease at a rate of 0.1m/year. After great media coverage of the problem, these
findings were corrected in a 2003 Journal of Geophysical Research article titled “The
arctic ice thickness anomaly of the 1990s: A consistent view from observations and
models” by mostly the same authors. In this study, Rothrock et al. do not take into
consideration the old unreliable data, rather concentrating on recent measurements from
1980s and 1990s and comparing them to model simulations. Doing this, they find that sea
ice cover was decreasing until 1996 or so at which point it started recovering. This way
Rothrock et al. rejected the idea of constant decrease in the future, while still
acknowledging the decrease derived before.
Another important issue tied to arctic sea-ice is global warming. Since sea-ice
reflects about 80% of sun rays that reach it, its melting (due to global warming) could
cause increased heat absorption by ocean and atmosphere (further increase in global
warming). Melting of ice in the arctic could also contribute to decrease in salinity of the
Northern ocean and thus alter thermohaline circulation with more implications to global
climate. Aside from global warming connection, sea-ice melting has other great negative
effects on the environment. Ice protects Canadian coast lines and thus its disappearance
contributes to coastal erosion and beach exposure to winter storm waves in the Gulf of St.
Lawrence. And of course as the arctic ice melts, arctic animals lose their habitat.