Environmental Engineering Reference
In-Depth Information
been marked decreases in sea-ice extent, while the Ross Sea has exhibited
increasing sea ice extent (Figure 4.12). Due to the opposing regional trends,
Antarctic sea-ice extent (Figure 4.11) shows a slight, but statistically signifi-
cant, positive trend of 0.96±0.61% to 1.0±0.4% per decade, from 1979 to
present (Cavalieri and Parkinson, 2008; Turner et al., 2009). This positive
trend is led by autumn increases in the Ross Sea followed by the western
Pacific Ocean and the Weddell Sea. Recently, the negative trends in the
Bellingshausen/Amundsen Seas have decreased, while that in the Indian
Ocean has become positive (Cavalieri and Parkinson, 2008). These regional
trends in the Antarctic sea ice have been attributed to the influence of at-
mospheric circulation mechanisms, such asthe SAM (e.g., Thompson and
Wallace, 2000), ENSO (e.g., Carleton et al., 1998), and zonal wave three
(Raphael, 2007), and stronger cyclonic flow over the Amundsen and Ross
Seas associated with stratospheric ozone depletion (Turner et al., 2009). As
in the Arctic, there have also been changes in seasonality of the sea ice.
The sea-ice maximum appears to occur slightly later in the year, while the
sea-ice minimum tends to occur earlier. However, neither trend is statisti-
cally significant.
Predictions for Arctic Sea Ice over the 21st Century
The consistent conclusion from all of the model studies is that Arctic
sea-ice extent will continue to decrease during the 21st century if emissions
of GHGs remain unchecked. This is clearly illustrated in Figure 4.13, which
shows the September results from IPCC models using 21st century forcings
based on the SRES A1B scenario, a medium-high emissions scenario. In
this scenario CO 2 concentration is projected to reach 720 ppm by 2100.
Although much smaller, the late winter trends are of similar sign. Even with
reductions in GHG emissions, Arctic sea-ice extent is predicted to decrease,
and the rate of decrease changes with the GHG emission scenario as well as
individual model physics. Washington et al. (2009) conducted an analysis
of what would occur if CO 2 were to be stabilized at 450 ppm at the end of
the 21st century. Their results for late summer/early fall (Figure 4.14) show
that sea-ice extent for an unmitigated CO 2 scenario is reduced by 76%
compared to present-day values, whereas for 450 ppm it is reduced only
by 24%, preserving 4 million Km 2 of sea ice.
Although the predicted year varies, by the end of the 21st century most
IPCC models predict an essentially ice-free Arctic (less than 1 million Km 2 )
in late summer. Estimates range from 2037 to 2100 (e.g., Arzel et al., 2006;
Boe et al., 2009; Wang and Overland, 2009). The timing depends in part
Search WWH ::




Custom Search