Environmental Engineering Reference
In-Depth Information
4.7 SEA ICE, SNOW, AND RELATED FACTORS
Current State of Sea Ice
Sea ice plays a critically important role in our climate system. It controls
the rate of heat exchange between the polar ocean and atmosphere, reflects
much of the solar energy incident on it, helps to maintain the equator-pole
temperature gradient, influences ocean circulation, and is of great ecological
importance. Typically, sea-ice area is about 14 to 16 million Km
2
in late win-
ter in the Arctic and 17 to 20 million Km
2
in the Antarctic Southern Ocean.
In late summer, on average, only about 3 to 4 million Km
2
remain in the
Southern Ocean while in the Arctic there are approximately 7 million Km
2
(NSIDC;
http://nsidc.org/sotc/sea_ice.html
). Sea-ice extent, defined here
as
the area of ocean with at least 15% sea-ice concentration
, is negatively
correlated to global average surface temperature so that as globally averaged
surface temperature increases, sea-ice extent decreases (e.g., Gregory et al.,
2002). This sea-ice response to the increasing global surface temperatures
occurs directly through thermally driven flux exchanges with the atmosphere
and their impact on sea-ice extent and thickness, and indirectly, through
the additional impact of increasing temperatures on dynamic mechanisms
such as ENSO (e.g., Timmermann et al., 1999) and SAM (e.g., Arblaster and
Meehl, 2006). Satellite observations show that there has been a decrease in
globally averaged sea-ice extent since 1979. This decrease has occurred in
the Arctic, while the Antarctic sea ice has increased slightly (Figure 4.11).
Since the 1950s Arctic sea-ice extent has exhibited a statistically signifi-
cant decrease (Vinnikov et al., 1999), and the rate of decrease has been faster
in summer (-7.8% per decade) than in winter (-1.8% per decade) (Stroeve
et al., 2007). Satellite observations beginning in 1978 show that the annual
average Arctic sea-ice extent has shrunk by 2.7% per decade (IPCC, 2007a)
with larger decreases at the end of summer (9.1% per decade) than at the
end of winter (2.9% per decade) (Stroeve et al., 2007). By some measures,
from 1979 to 2006, September (late summer) sea-ice extent decreased by
almost 25% or about 100,000 km
2
per year (Serreze et al., 2007). Compari-
son of observed Arctic sea-ice decline to IPCC AR4 projections show that
the observed rate of ice loss is faster than that predicted by any of the IPCC
AR4 models (Stroeve et al., 2007).
The decrease in sea-ice extent has been accompanied by thinning pe-
rennial and seasonal ice (Kwok and Rothrock, 2009), a decrease in multi-
year ice (Maslanik et al., 2007), and record minima in September sea-ice
cover (Serreze et al., 2007; Stroeve et al., 2007; Stroeve et al., 2008). Spring
