The supports a critical aspect of the global biosphere

The
Arctic Ocean is defined as the waters surrounding the North Pole, located within
the Arctic Circle, including the northernmost islands of Canada, Norway, and
Russia and is mostly covered by ice sheets, ice floes, icebergs and sea ice.
Sea Ice is a thin, fragile layer of frozen ocean water that forms in the
Arctic and Antarctic oceans. On average sea ice covers 20-25 km² of the Earth,
accounting for 7% of the sea surface. The maximum extent of Sea Ice in the
Arctic is recorded as 13-15x 10?km² with the minimum coverage being 7×10?km². Since Satellite monitoring in 1979,
there has been a decline in the extent of Sea Ice during winter months, with
the lowest coverage recorded in 2017 at 9.46 million square kilometers (NSIDC)
leading many to conclude it is disappearing at a ‘devastating’ rate (Perovich
et al 2002, Holland et al 2012, Liu et al 2012, Vihma 2014) (see Figure 1).
However, Stroeve (2011) states the decline to be more stable at 8-12% per
decade. In later studies Stroeve observes a difference in sea ice formation,
with it starting 3 days later and a melt season beginning 2 days earlier, per
decade (Stroeve et al 2014). Estimates suggest that the Artic could be ice free
sometime between 2030 (Liu et al 2012) and 2050 (Perovich et al 2002, Holland
et al 2012, Vihma 2014). Cumulatively it is agreed that the decline of the
Artic sea ice is globally significant as it controls the thermohaline
circulation of the world’s oceans (Dima and Lohmann 2011). It keeps
the poles cold by reflecting the suns heat back in a process known as
“albedo” (Stroeve 2011), altering the Artic Oscillation and affecting weather
regimes (Liu et al 2012), which supports a critical aspect of the global
biosphere (Langbehn 2017).

Albedo

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Albedo is the
proportion of radiation that is reflected by a surface, in this case the amount
of sunlight reflected from the ice. The high albedo of sea ice means it
reflects much of the solar radiation, maintaining cold temperatures and delaying
ice melt. However decreasing amounts of sea ice mean albedo is reduced, as melt
ponds grow and deepen. Resulting, melt ponds are associated with higher energy
absorption, which causes more sea ice to melt, creating a negative feedback
loop (Stroeve 2012). The reduction of sea ice has in part been attributed to the
growing importance of the ice-albedo feedback by both Lindsay and Zhang 2005
and Perovich et al. 2007. Perovich et al showed between 1979-2005 there was an increase
in the amount of solar energy in the upper Arctic Ocean. This supports a study
conducted by NASA observing global top-of-atmosphere radiative fluxes from 2000
to 2013, which showed annual mean absorbed solar radiation over 75°-90°N,
increased by 2.5 W m¯². This translates to an albedo change of 0.013(10?km¯²)
¯¹ year on year, variations which are closely related to the extent of sea ice.
However, this study did not consider changes in cloud cover. CERES clear-sky
estimates suggest that about half of the surface albedo decrease is because ice
melt is screened by clouds and thereby not apparent at the top of the
atmosphere (Hartmann and Ceppi 2014).

Increasing albedo
causing earlier development of open water in the melt season enhances summer
ice-albedo feedback, promoting more open water in September thus delaying ice
formation compared to 20 years ago. This increase in solar input increases
water temperatures, with some areas increasing 7° F above the long-term average
(Mcleann et al, 2006). A study from 2014 using satellite radiation and
microwave measurements suggests increasing albedo is equivalent to adding
another 25 percent to global greenhouse emissions (Pistone et al, 2014). As
solar heat input increases the rate of sea ice melt, water vapor concentrations
increase by more than 20 percent, (Stroeve et al) adding to Arctic warming,
which is also heating up the Greenland ice sheet. (Smith et al 2014).

The most significant
effect of an decrease in ice albedo, is that it has caused melting sea ice to
retreat from the ice shelves, resulting in a large area of water forming which
is darker with a low albedo (Whiteman et al 2013). This warmer water, thaws the
offshore permafrost, underneath which is a thick layer of sediment containing
large amounts of solid methane hydrates. Submarine sonar measurements carried
out by Whiteman et al has shown the release of the overlying pressure provided
by the sea ice allows the hydrates to disintegrate and turn into gaseous
methane, which bubbles up through the water and are released to the atmosphere.
Higher methane concentrations in the atmosphere will accelerate global warming
and hasten local changes in the Arctic, speeding up sea-ice retreat, reducing
the reflection of solar energy and accelerating the melting of the Greenland
ice sheet. The significance of Permafrost thaw by Artic sea ice melt is further
agreed by Parmentier et al in 2017.                                                                                   

Atmospheric
Regimes

Sea Ice is
important in the regulation of weather. The ocean is warm and the atmosphere in
the Artic is cold, the sea ice cover prevents heat in the ocean from warming
the atmosphere. With thin ice, or no ice cover there is insufficient insulation
of the ocean. The Arctic then warms, which, in turn influences the global
circulation of the atmosphere. Cvijanovic et al (2017) suggests sea ice decline
influenced the 2012–2016 drought in California. The precipitation response to
Arctic sea-ice loss is the reorganization of tropical rainfall and a northward
precipitation shift. This results in less precipitation over California—a
consequence of a geopotential ridge in the North Pacific that steers the wet
winter air masses northward into Alaska and Canada. These findings are
consistent with previous claims of a sea-ice driven component of Californian
precipitation changes by Sewall in 2005 and Cvijanovic in 2015. However, the
study does not provide compelling evidence that the California drought is totally
attributable to Arctic sea-ice changes. The intensification of dry conditions
may have been affected by other factors not discussed in this study, e.g. the
appearance of a large warm sea surface temperature anomaly off the west coast
of North America, a 2014 phase shift of the Pacific Decadal Oscillation from
negative to positive and a result of asymmetric forcing by both natural and
anthropogenic means. Liu et al (2012), suggests Artic sea ice decline has
influenced snowfall in recent years in North America, Europe and China. It
suggests Artic sea ice decline has caused a meandering of the Artic
Oscillation. This circulation change results in more episodes of blocking patterns
that lead to increased cold surges over large parts of northern continents.
Moreover, the increase in atmospheric water vapor content in the Arctic region
during late autumn and winter provides enhanced moisture sources, supporting
increased heavy snowfall in Europe and North America. A decrease of autumn
Arctic sea ice of 1 million km² corresponds to a significantly above-normal
winter snow cover in large parts of the northern United States, Europe and
China.

Thermohaline
Circulation

Deep-ocean currents
are driven by differences in water density, which is controlled by temperature
and salinity. This process is known as thermohaline circulation. Polar ocean
water gets very cold, forming sea ice. Consequently, the surrounding seawater
gets saltier, because when sea ice forms, the salt is left behind (brine
rejection). As the seawater gets saltier, its density increases, and it starts
to sink. Surface water is pulled in to replace it, which in turn becomes cold
and salty enough to sink. This initiates the deep-ocean currents driving the
global conveyer belt where the dense surface waters in the North Atlantic
generate downward mixing and southward movement of deep-water masses. This is
partially balanced by a transport of saline waters by surface ocean currents
from the tropics to mid latitudes. The melting of sea ice in the Artic is suggested
as a cause of Great Salinity Anomalies by Aagaard and Carmack (1989), Belkin et
al 1998, Hakkin 1999, Haak et al 2003, Yang 2010. The melting of sea ice was
proven to have produced a freshwater pulse entering the North Atlantic creating
the Great Salinity Anomaly of the 1960s, which altered the convection pattern
and diminished the Atlantic Meridional Overturning Circulation causing negative
anomalies (see Figure 2). This conclusion was also supported by data from
Komuro and Hasumi in their 2003 investigation into sea ice diminishing the
effect of the thermohaline circulation using sea-ice-ocean coupled models, who
along with Nummelin et al (2016) agree that increasing sea ice melt suppresses
vertical mixing in the ocean, limiting nutrient availability.

Studies by Yang
and Neelin in 2010, into the effects of declining sea ice suggest that its
effect on the thermohaline circulation is short term and not a long-term
threat. The emphasis of the study is on how sea?ice might affect the stability
and the long?term variability of the circulation through modulations of the
surface heat and freshwater fluxes. A model combining temperature, salinity and
velocity is analysed to explain qualitatively the impacts of these two
processes. The analytical solution indicates that, for the long timescales
considered here, the thermal insulation stabilizes the thermohaline circulation,
while the freshwater feedback increases the effective inertia of the coupled
ice?ocean system. Their model also suggested that any increased salinity or
rise in temperature would eventually be corrected. However, this study has not
considered the more rapid decrease of Sea Ice, we are now seeing, becoming the
norm.  

Biosphere

Sea Ice decline
is also significant for the global biosphere as stated by Post et al (2013)
which highlights sea ice as a driver of ecological system dynamics. One of
Earth’s major biomes, sea ice not only comprises unique ecosystems in, on, and
under the ice itself but also strongly influences patterns and processes in
adjacent ecosystems. Sea ice harbors an array of micro-organisms, provides
critical habitat for vertebrates, and influences terrestrial productivity and
diversity in the Arctic. Ardyna et al (2011), Post et al (2013), and Assmy et
al (2017), suggest one significant effect of sea ice reduction is the loss of
habitat for sea-ice algae and sub-ice phytoplankton, which together account for
57% of the total annual primary production in the Arctic Ocean. They are also
important to the wider marine system as both Phytoplankton and algal blooms
which form as the ice edge retreats is critical to the growth and survival of
copepod offspring. However, earlier seasonal sea-ice melt results in earlier
formation of phytoplankton and algae blooms, well before Copepods arrive. This
in turn Impacts upon Copepods, which are a major component of the marine system,
impacting the food chain in a reduction of predator populations. Arrigo et al
(2008) suggests the effect of habitat loss for algae and phytoplankton is
globally significant, it being one of the largest ways CO2 (photosynthesis) is
converted from the atmosphere. Therefore, understanding how blooming will
change with further sea ice melt is critical to seeing how global warming will
affect the Earth. Langbehn investigations in 2017 used satellite data to show
animals are beginning to be seen at high latitudes in response to decreasing
sea ice.

Conclusion

The melting of
Artic Sea Ice has been under scientific investigation since the late 1970’s
using   satellite monitoring. Since then scientists
have noted an ongoing decline with recent monitoring by the NSIDC showing a
consistently shorter extent of sea ice, especially in 2017 where there is a
recorded low (See figure 1). As public awareness of ‘global warming’ has
increased, the global significance of diminishing sea ice has been assessed by
the scientific community through different studies of observed data and model
simulations. These have found that the Artic controls key global processes such
as the thermohaline circulation and the ice albedo feedback, which alters
global temperatures and weather regimes and the wider biosphere. However, with
the rate of Artic sea ice decline currently unstable, further investigation
will be needed in light of anthropogenic changes which might heighten or
decrease the effects of global warming and consequently effect sea ice decline.

 

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