135 PAN-CRYOSPHERE FEEDBACKS: ALBEDO, PERMAFROST MELT & SEA LEVEL RISE
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Some impacts of rapid climate change in the cryosphere occur across the different regions, with contributions
from each region.
Albedo
When highly reflective snow and ice surfaces melt away, they reveal darker land or ocean surfaces that absorb
more of the sun’s energy. This process becomes especially apparent at the edges of mountain glaciers (though
extremely debris-covered glaciers seem to show slower rates of loss due an insulation effect when the debris field
is sufficiently thick). The result is enhanced warming of the Earth’s surface and the air above it. The
impact of potentially greater cloud cover in these regions remains a matter of some debate, since clouds both
insulate and warm the surface — and also reflect sunlight.
A growing body of evidence nevertheless indicates that albedo-associated warming is happening over the Arctic as
the extent of sea ice decreases and snow cover retreats earlier in the spring (Lawrence et al. 2008). Recent
satellite observations of reflectivity (Riihelä et al. 2013) have concluded that albedo during the summer months
has decreased in every month expect May (when cover remains thick) over the past 28 years. The highest albedo loss
has occurred in August, (the month of the highest sea ice melt rates), with a decrease in reflectivity of about 3
percent per decade. The loss of albedo also speeds processes related to permafrost and sea-level rise.
Permafrost
Temperatures in some parts of the Arctic permafrost have risen by up to 2 degrees over the past 30 years, faster
than surface air temperatures. In AR5, the IPCC noted with “high confidence” that permafrost temperatures
have increased in most regions since the early 1980s, with observed permafrost warming of up to 3°C in parts of
Northern Alaska, and up to 2°C in parts of the European Russia, with a considerable reduction in thickness and
extent of permafrost in both regions.
The SWIPA report (2011) attributed much of this permafrost temperature rise to an 18-percent decrease in snow
cover since the 1960s. Although most permafrost exists in the Arctic region, some occurs in most alpine systems and
a large area exists on the Tibetan Plateau. Globally, permafrost together with deposits in frozen near-shore
seabeds are thought to hold about 1,700 Gt of carbon, compared to 850 Gt of carbon currently in the Earth’s
atmosphere. Release of even a portion of this carbon into the atmosphere could drastically compound the
challenge already presented from anthropogenic sources, potentially wiping out any hard-won mitigation gains.
In regions such as Siberia, Alaska, and Tibet, loss of permafrost threatens infrastructure from homes, roads, and
trains to oil and gas pipelines that may leak when placed under stress.
Permafrost carbon is released as CO2 under dry conditions, but some portion of the release will
occur as methane under wet conditions (i.e., swamplands or methane hydrates coming from coastal seabeds).
Permafrost scientists estimate that release of just one percent of stored carbon in the form of methane will double
current rates of warming due to methane’s more powerful near-term forcing effects. About 12 percent, or 190
Gt of permafrost carbon, is stored in the upper 30 cm of permafrost layers considered most vulnerable to permanent
melting (Zimov et al. 2006). The IPCC estimated in AR5 that anywhere between 50-250 Gt carbon(5-30 percent of
current atmospheric carbon) could be released by the end of this century; great uncertainty surrounds even this
wide range, and it does not include potential releases of near-shore methane hydrates.
Some of the more dramatic observations in the past few years have involved the release of large bubbles of
methane hydrates off the coast of Siberia in 2010 and 2011 (Shakhova, Alekseev, and Semiletov, 2010). Some
arctic methane researchers estimate that 50 Gt of carbon could be released in the east Siberian sea in the coming
few decades, and cannot rule out this occurring in very brief timeframes, as a “pulse” over a few years.
Siberian shelf methane is essentially flooded permafrost in shallow waters, and may be highly sensitive to warmer
waters arising from lower sea ice extent in the past decade (Shakhova et al. 2010). Recent modeling (Whiteman
et al. 2013) estimates that such a release from coastal seabeds, or emission from land permafrost along
the same scale (attributed in part to loss of sea ice albedo, per the above) could raise Arctic temperatures
by 0.6°C by 2050. This could add perhaps $60 trillion to the cost of adaptation (mostly in developing
countries) by bringing forward the date when the globe exceeds 2°C of warming over pre-industrial levels to as
early as 2035-40. Keeping as much carbon as possible within the permafrost in the near-term, by maintaining
lower temperatures in the Arctic and alpine permafrost regions, is therefore an issue of global importance.
Sea-level Rise
A rise in the level of the world’s oceans occurs from melting land ice (not sea ice), as well as thermal
expansion as overall global temperature rises: warmer water takes up more space than colder water. Various
factors such as the earth’s spin, ocean currents, and gravity also cause sea-level rise to be non-uniform across
the globe. The greatest relative increase is expected to be near the equator (especially Western Australia,
Oceania, and small atolls and islands, including Hawaii and Micronesia) (Spada et al. 2013). Recent
observations have noted accelerated sea-level rise in the northeastern United States at 3-4 times that of the
global mean (Sallenger et al. 2012).
The 2011 SWIPA report revised earlier estimates of sea-level rise based on observations of accelerated ice loss
from Greenland and land glaciers; these are factors the previous IPCC Assessment Report (AR4, 2007) could not take
into account. SWIPA noted that accelerating melt from Arctic glaciers and ice caps, at approximately 40
percent of the total sea-level rise, was contributing much more than previously thought. SWIPA
therefore revised AR4 estimates upward to between 0.9-1.6 meters in sea-level rise by 2100 (SWIPA 2011).
In its recently-released AR5 report, the IPCC increased its estimates from 2007, projecting about 0.5-1meter
sea-level rise by 2100; this is still lower than SWIPA. This is primarily because SWIPA and similar estimates
rely on more empirical approaches, whereas the IPCC ultimately found such approaches not sufficiently tested to use
in its latest assessment. [1]
Neither SWIPA nor IPCC estimates take into account the possibility of rapid disintegration (dynamical or
non-linear ice discharge) of the West Antarctic Ice Sheet (WAIS). Like all sea-based ice sheets, it is
inherently unstable and subject to rapid changes, as seen in the collapse of the far smaller Larsen B Ice Shelf on
the Antarctica Peninsula, which lost over 2,500 km2 (1,000 square miles) in the space of a few days
in March 2002. Although an increase in discharge from West Antarctica has been observed in recent years, too
little is understood about the processes around the possible disintegration of the approximately 2-million
km2 WAIS to include in current projections. Such disintegration has, however, fairly clearly
occurred in the geologic past (Pollard and DeConto 2009). The WAIS could therefore contribute significant
global sea level rise — or very little — over the next century (Bamber et al. 2009). A total disintegration
would raise global sea level from 3.3-5 meters, but in highly uncertain time spans of 100-1000 years.[2] AR5
did note that such a collapse could result in up to “several” decimeters in additional sea-level rise during this
century.
Recent studies have indicated that sea levels may have peaked at 4-9 meters higher[3] than today’s levels in the
Eemian (125,000 years ago), the geologic period closest to current temperatures and
CO2 levels[4]. Much of that sea-level rise appears to have come from West Antarctica, rather
than Greenland as believed earlier (Bamber and Aspinal 2013).
In the case of both Antarctica and Greenland, some research indicates that today’s melting could set in motion
processes that may prove difficult to stop, even with stable or falling temperatures globally by the end of this
century. Some simulations show a loss of the Greenland ice sheet at ranges within the current 400ppm of
CO2 (Stone et al. 2010). AR5 noted with high confidence that some threshold exists beyond
which a near-total loss of the Greenland ice sheet would occur, which could result in up to six meters of sea-level
rise occurring over a 1000-year period. AR5 set that threshold between one degree (which has already passed)
and four degrees, but with lower confidence at the lower temperature ranges.
Sea-level rise thus may occur slowly, but failure to slow rapid warming in these regions soon enough may commit
us to future sea-level rise that would threaten many of the world’s largest population centers — especially in
developing countries. This threat comes not just from sea-level rise but also from sea-level rise combined
with extreme events, including offshore winds and tidal surge (such as that of Hurricane Sandy in 2012). These
events could lead to enormous and costly infrastructure damage.
[1] AR5’s 2013
figure took into account more factors than AR4, but chose not to include others included by SWIPA due to continuing
difficulty in estimating their scale of contribution. AR5’s estimate should therefore not be seen as a
“lower” estimate but rather one that chooses not to include certain elements until they can be more precisely
quantified.
[2] O’Reilly,
Oreskes, and Oppenheimer (2012). The Rapid Disintegration of Projections: the West Antarctic Ice Sheet and the
Intergovernmental Panel on Climate Change. Social Studies of Science. 42(5):709-731.
[3] Dutton and
Lambeck (2012). Ice Volume and Sea Level During the Last Interglacial. Science 337 (6091):216-219
[4] Today’s CO2
levels over 400ppm are now actually higher than in the Eemian.
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