Geoscience Reference
In-Depth Information
events external to the Arctic, there is growing recognition that that changes within
the Arctic, especially linked to a sea ice extent and thickness, may alter patterns
of atmospheric circulation both within and beyond the region (e.g., Francis et al.,
2009
; Overland and Wang,
2010
, Francis and Vavrus,
2012
). Whether such impacts
have actually been observed remains highly controversial.
Albedo feedback has always been part of the Arctic sea ice system. In response
to spring warmth, dark melt ponds form and bare ice is exposed, fostering further
ice melt because of the reduction in albedo. Dark open water areas that form readily
absorb solar energy, fostering melt of surrounding ice. Without this albedo feed-
back, the amplitude of the seasonal cycle in sea ice extent would be smaller than is
observed. The downward trend in September ice extent has in part been attributed
to a growing importance of the albedo feedback (e.g., Perovich et al.,
2007
; Stroeve
et al.,
2011b
). This is because of the transition to an Arctic ice pack dominated by
thinner, first-year ice. Thus, open water areas are exposed earlier in the melt sea-
son and become more extensive throughout summer, further accentuating summer
ice melt.
Concerns have been raised over the existence of a sea ice “tipping point” involv-
ing albedo feedback. The idea is that as the climate warms and the spring sea ice
cover thins to some critical value, a strong kick from natural climate variability
(from either the ocean or the atmosphere) could induce a reduction in sea ice extent
sufficiently large to set the albedo feedback process into high gear. As a result, the
path of a general downward trend in summer ice cover would be interrupted by
sudden plunge, hastening the slide to a seasonally ice-free ocean. S. Tietsche et al.
(
2011
) examined the tipping point argument via a series of coupled global climate
model experiments. They started with reference simulations for the twenty-first
century using the A1B greenhouse-gas emissions scenario. September ice cover
in these simulations was found to typically disappear by the year 2070. They then
conducted a new set of runs, whereby every twenty years through the twenty-first
century they artificially removed the entire sea ice cover on July 1. Instead of main-
taining ice-free conditions, ice extent in September recovered to values typical of
the reference simulations within a couple of years, even in the warmer later parts of
the century. As expected, with ice-free summers, the ocean picks up a great deal of
extra heat, delaying autumn ice growth. They argue that if there was a sea ice tip-
ping point, this summer heat gain would lead to an ice cover the following spring
that is thin enough to completely melt out over the following summer. Instead, their
simulations showed that so much ocean heat is lost during polar winter that enough
ice grows to survive the next summer's melt. In other words, the strong polar winter
cooling acts as a strong negative feedback. Their interpretation of these results is
that there is no sea ice tipping point.
What actually transpires in the twenty-first century of course remains to be seen.
One issue germane to how the sea ice cover will evolve is the observation that ocean
heat gained in summer from the surface from absorbtion of solar radiation can be
stored in the Arctic Ocean in a near surface layer (25-35 m) through the winter
season. Via storm-driven winter mixing, it appears that some of this heat can used
