TheArctic Ocean is defined as the waters surrounding the North Pole, located withinthe Arctic Circle, including the northernmost islands of Canada, Norway, andRussia 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 theArctic 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 theArctic 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, withthe lowest coverage recorded in 2017 at 9.46 million square kilometers (NSIDC)leading many to conclude it is disappearing at a ‘devastating’ rate (Perovichet 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% perdecade. 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, perdecade (Stroeve et al 2014). Estimates suggest that the Artic could be ice freesometime between 2030 (Liu et al 2012) and 2050 (Perovich et al 2002, Hollandet al 2012, Vihma 2014).
Cumulatively it is agreed that the decline of theArtic sea ice is globally significant as it controls the thermohalinecirculation of the world’s oceans (Dima and Lohmann 2011). It keepsthe poles cold by reflecting the suns heat back in a process known as”albedo” (Stroeve 2011), altering the Artic Oscillation and affecting weatherregimes (Liu et al 2012), which supports a critical aspect of the globalbiosphere (Langbehn 2017). Albedo Albedo is theproportion of radiation that is reflected by a surface, in this case the amountof sunlight reflected from the ice. The high albedo of sea ice means itreflects much of the solar radiation, maintaining cold temperatures and delayingice melt. However decreasing amounts of sea ice mean albedo is reduced, as meltponds grow and deepen.
Resulting, melt ponds are associated with higher energyabsorption, which causes more sea ice to melt, creating a negative feedbackloop (Stroeve 2012). The reduction of sea ice has in part been attributed to thegrowing importance of the ice-albedo feedback by both Lindsay and Zhang 2005and Perovich et al. 2007. Perovich et al showed between 1979-2005 there was an increasein the amount of solar energy in the upper Arctic Ocean. This supports a studyconducted by NASA observing global top-of-atmosphere radiative fluxes from 2000to 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-skyestimates suggest that about half of the surface albedo decrease is because icemelt is screened by clouds and thereby not apparent at the top of theatmosphere (Hartmann and Ceppi 2014). Increasing albedocausing earlier development of open water in the melt season enhances summerice-albedo feedback, promoting more open water in September thus delaying iceformation compared to 20 years ago. This increase in solar input increaseswater temperatures, with some areas increasing 7° F above the long-term average(Mcleann et al, 2006). A study from 2014 using satellite radiation andmicrowave measurements suggests increasing albedo is equivalent to addinganother 25 percent to global greenhouse emissions (Pistone et al, 2014). Assolar heat input increases the rate of sea ice melt, water vapor concentrationsincrease 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 significanteffect of an decrease in ice albedo, is that it has caused melting sea ice toretreat from the ice shelves, resulting in a large area of water forming whichis darker with a low albedo (Whiteman et al 2013). This warmer water, thaws theoffshore permafrost, underneath which is a thick layer of sediment containinglarge amounts of solid methane hydrates. Submarine sonar measurements carriedout by Whiteman et al has shown the release of the overlying pressure providedby the sea ice allows the hydrates to disintegrate and turn into gaseousmethane, which bubbles up through the water and are released to the atmosphere.Higher methane concentrations in the atmosphere will accelerate global warmingand hasten local changes in the Arctic, speeding up sea-ice retreat, reducingthe reflection of solar energy and accelerating the melting of the Greenlandice sheet. The significance of Permafrost thaw by Artic sea ice melt is furtheragreed by Parmentier et al in 2017. AtmosphericRegimes Sea Ice isimportant in the regulation of weather.
The ocean is warm and the atmosphere inthe Artic is cold, the sea ice cover prevents heat in the ocean from warmingthe atmosphere. With thin ice, or no ice cover there is insufficient insulationof the ocean. The Arctic then warms, which, in turn influences the globalcirculation of the atmosphere. Cvijanovic et al (2017) suggests sea ice declineinfluenced the 2012–2016 drought in California. The precipitation response toArctic sea-ice loss is the reorganization of tropical rainfall and a northwardprecipitation shift.
This results in less precipitation over California—aconsequence of a geopotential ridge in the North Pacific that steers the wetwinter air masses northward into Alaska and Canada. These findings areconsistent with previous claims of a sea-ice driven component of Californianprecipitation changes by Sewall in 2005 and Cvijanovic in 2015. However, thestudy does not provide compelling evidence that the California drought is totallyattributable to Arctic sea-ice changes. The intensification of dry conditionsmay have been affected by other factors not discussed in this study, e.
g. theappearance of a large warm sea surface temperature anomaly off the west coastof North America, a 2014 phase shift of the Pacific Decadal Oscillation fromnegative to positive and a result of asymmetric forcing by both natural andanthropogenic means. Liu et al (2012), suggests Artic sea ice decline hasinfluenced snowfall in recent years in North America, Europe and China. Itsuggests Artic sea ice decline has caused a meandering of the ArticOscillation.
This circulation change results in more episodes of blocking patternsthat lead to increased cold surges over large parts of northern continents.Moreover, the increase in atmospheric water vapor content in the Arctic regionduring late autumn and winter provides enhanced moisture sources, supportingincreased heavy snowfall in Europe and North America. A decrease of autumnArctic sea ice of 1 million km² corresponds to a significantly above-normalwinter snow cover in large parts of the northern United States, Europe andChina. ThermohalineCirculation Deep-ocean currentsare driven by differences in water density, which is controlled by temperatureand salinity. This process is known as thermohaline circulation. Polar oceanwater gets very cold, forming sea ice. Consequently, the surrounding seawatergets saltier, because when sea ice forms, the salt is left behind (brinerejection). As the seawater gets saltier, its density increases, and it startsto sink.
Surface water is pulled in to replace it, which in turn becomes coldand salty enough to sink. This initiates the deep-ocean currents driving theglobal conveyer belt where the dense surface waters in the North Atlanticgenerate downward mixing and southward movement of deep-water masses. This ispartially balanced by a transport of saline waters by surface ocean currentsfrom the tropics to mid latitudes. The melting of sea ice in the Artic is suggestedas a cause of Great Salinity Anomalies by Aagaard and Carmack (1989), Belkin etal 1998, Hakkin 1999, Haak et al 2003, Yang 2010.
The melting of sea ice wasproven to have produced a freshwater pulse entering the North Atlantic creatingthe Great Salinity Anomaly of the 1960s, which altered the convection patternand diminished the Atlantic Meridional Overturning Circulation causing negativeanomalies (see Figure 2). This conclusion was also supported by data fromKomuro and Hasumi in their 2003 investigation into sea ice diminishing theeffect of the thermohaline circulation using sea-ice-ocean coupled models, whoalong with Nummelin et al (2016) agree that increasing sea ice melt suppressesvertical mixing in the ocean, limiting nutrient availability.Studies by Yangand Neelin in 2010, into the effects of declining sea ice suggest that itseffect on the thermohaline circulation is short term and not a long-termthreat. The emphasis of the study is on how sea?ice might affect the stabilityand the long?term variability of the circulation through modulations of thesurface heat and freshwater fluxes.
A model combining temperature, salinity andvelocity is analysed to explain qualitatively the impacts of these twoprocesses. The analytical solution indicates that, for the long timescalesconsidered here, the thermal insulation stabilizes the thermohaline circulation,while the freshwater feedback increases the effective inertia of the coupledice?ocean system. Their model also suggested that any increased salinity orrise in temperature would eventually be corrected.
However, this study has notconsidered the more rapid decrease of Sea Ice, we are now seeing, becoming thenorm. Biosphere Sea Ice declineis 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 ofEarth’s major biomes, sea ice not only comprises unique ecosystems in, on, andunder the ice itself but also strongly influences patterns and processes inadjacent ecosystems. Sea ice harbors an array of micro-organisms, providescritical habitat for vertebrates, and influences terrestrial productivity anddiversity in the Arctic. Ardyna et al (2011), Post et al (2013), and Assmy etal (2017), suggest one significant effect of sea ice reduction is the loss ofhabitat for sea-ice algae and sub-ice phytoplankton, which together account for57% of the total annual primary production in the Arctic Ocean. They are alsoimportant to the wider marine system as both Phytoplankton and algal bloomswhich form as the ice edge retreats is critical to the growth and survival ofcopepod offspring. However, earlier seasonal sea-ice melt results in earlierformation of phytoplankton and algae blooms, well before Copepods arrive.
Thisin 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 isglobally significant, it being one of the largest ways CO2 (photosynthesis) isconverted from the atmosphere. Therefore, understanding how blooming willchange with further sea ice melt is critical to seeing how global warming willaffect the Earth. Langbehn investigations in 2017 used satellite data to showanimals are beginning to be seen at high latitudes in response to decreasingsea ice.Conclusion The melting ofArtic Sea Ice has been under scientific investigation since the late 1970’susing satellite monitoring. Since then scientistshave noted an ongoing decline with recent monitoring by the NSIDC showing aconsistently shorter extent of sea ice, especially in 2017 where there is arecorded low (See figure 1).
As public awareness of ‘global warming’ hasincreased, the global significance of diminishing sea ice has been assessed bythe scientific community through different studies of observed data and modelsimulations. These have found that the Artic controls key global processes suchas the thermohaline circulation and the ice albedo feedback, which altersglobal temperatures and weather regimes and the wider biosphere. However, withthe rate of Artic sea ice decline currently unstable, further investigationwill be needed in light of anthropogenic changes which might heighten ordecrease the effects of global warming and consequently effect sea ice decline.