Arctic sea ice decline
In recent decades, sea ice in the Arctic Ocean has been melting faster than it re-freezes in winter. The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report states that greenhouse gas forcing is predominantly responsible for the decline in Arctic sea ice extent. A study from 2011 suggested that internal variability enhanced the greenhouse gas forced sea ice decline over the last decades. A study from 2007 found the decline to be "faster than forecasted" by model simulations. The IPCC Fifth Assessment Report concluded, with high confidence, that sea ice will continue to decrease in extent, and that there is robust evidence for the downward trend in Arctic summer sea ice extent since 1979. It has been established that the region is at its warmest for at least 40,000 years and the Arctic-wide melt season has lengthened at a rate of 5 days per decade (from 1979 to 2013), dominated by a later autumn freeze-up. Sea ice changes have been identified as a mechanism for polar amplification.
- 1 Definitions
- 2 Observation
- 3 Ice-free summer
- 4 Tipping point
- 5 Implications
- 6 See also
- 7 References
- 8 External links
The Arctic Ocean is the mass of water positioned approximately above latitude 65° N. Arctic Sea Ice refers to the area of the Arctic Ocean covered by ice. The Arctic sea ice minimum is the day in a given year when Arctic sea ice reaches its smallest extent, occurring at the end of the summer melting season, normally during September. Arctic Sea ice maximum is the day of a year when Arctic sea ice reaches its largest extent near the end of the Arctic cold season, normally during March. Typical data visualizations for Arctic sea ice include average monthly measurements or graphs for the annual minimum or maximum extent, as shown in the adjacent images.
Sea ice and climate feedbacks
Arctic Sea ice maintatins the cool temperature of the polar regions and it has an important albedo effect on the climate. Arctic Sea ice melts in the summer, and more of the ocean is being absorbed by the sun. The fast rate of the sea ice melting is resulting in the oceans absorbing and heating up the Arctic. The decline in sea ice does have a notable potential to significantly speed up global warming and the climate changes. 
Observation with satellites show that Arctic sea ice area, extent, and volume have been in decline for a few decades. Sometime during the 21st century, sea ice may effectively cease to exist during the summer. Sea ice extent is defined as the area with at least 15% ice cover. The amount of multi-year sea ice in the Arctic has declined considerably in recent decades. In 1988, ice that was at least 4 years old accounted for 26% of the Arctic's sea ice. By 2013, ice that age was only 7% of all Arctic sea ice.
Scientists recently measured sixteen-foot (five-meter) wave heights during a storm in the Beaufort Sea in mid-August until late October 2012. This is a new phenomenon for the region, since a permanent sea ice cover normally prevents wave formation. Wave action breaks up sea ice, and thus could become a feedback mechanism, driving sea ice decline.
For January 2016, the satellite based data showed the lowest overall Arctic sea ice extent of any January since records begun in 1979. Bob Henson from Wunderground noted:
Hand in hand with the skimpy ice cover, temperatures across the Arctic have been extraordinarily warm for midwinter. Just before New Year’s, a slug of mild air pushed temperatures above freezing to within 200 miles of the North Pole. That warm pulse quickly dissipated, but it was followed by a series of intense North Atlantic cyclones that sent very mild air poleward, in tandem with a strongly negative Arctic oscillation during the first three weeks of the month.
The January 2016's remarkable phase transition of Arctic oscillation was driven by a rapid tropospheric warming in the Arctic, a pattern that appears to have increased surpassing the so-called stratospheric sudden warming. The previous record of the lowest area of the Arctic Ocean covered by ice in 2012 saw a low of 1.58 million square miles (4.1 million square kilometers). This replaced the previous record set on September 28, 2007 at 1.61 million miles (2.59 million kilometers).
An "ice-free" Arctic Ocean is often defined as "having less than 1 million square kilometers of sea ice", because it is very difficult to melt the thick ice around the Canadian Arctic Archipelago. The IPCC AR5 defines "nearly ice-free conditions" as sea ice extent less than 106 km2 for at least five consecutive years.
Many scientists have attempted to estimate when the Arctic will be "ice-free". Professor Peter Wadhams of the University of Cambridge is among these scientists. Wadhams and several others have noted that climate model predictions have been overly conservative regarding sea ice decline. A 2013 paper suggested that models commonly underestimate the solar radiation absorption characteristics of wildfire soot. A 2006 paper predicted "near ice-free September conditions by 2040". Overland and Wang (2013) investigated three different ways of predicting future sea ice levels. From sea ice models and recent satellite images it can be expected that a sea ice free summer will come before 2020. The IPCC AR5 (for at least one scenario) estimates an ice-free summer might occur around 2050. The Third U.S. National Climate Assessment (NCA), released May 6, 2014, reports that the Arctic Ocean is expected to be ice free in summer before mid-century. Models that best match historical trends project a nearly ice-free Arctic in the summer by the 2030s. However, these models do tend to underestimate the rate of sea ice loss since 2007. Based on the outcomes of several different models, Overland and Wang (2013) put the early limit for a sea ice free summer Arctic near 2040.
Warming Arctic temperatures provide a powerful forcing toward lessened sea ice coverage
There is an ongoing debate if the Arctic Ocean has already passed a "tipping point", defined as a threshold for abrupt and irreversible change. A 2013 study identified an abrupt transition to increased seasonal ice cover variability in 2007, which has persisted in following years. The researchers made a distinction between a bifurcation and a non-bifurcation 'tipping point'. The IPCC AR5 report stated with medium confidence that precise levels of climate change sufficient to trigger a tipping point remain uncertain, and that the risk associated with crossing multiple tippingpoints increases with rising temperature.
Amplified Arctic warming
Dark, open water left behind as sea ice melts absorbs vastly more heat than ice covered water, leading to physical implications that include the ice-albedo feedback or warmer sea surface temperatures which increase ocean heat content. As Peter Wadhams, a polar researcher writes "once summer ice yields to open water, the albedo ... drops from 0.6 to 0.1, which will further accelerate warming of the Arctic and of the whole planet." This warming has increased to such an extent that the poles are heating approximately twice as fast as the global average, according to Rutgers University climate scientist Jennifer Francis. Economical implications of ice free summers and the decline in Arctic ice volumes include a greater number of journeys across the Arctic Ocean Shipping lanes during the year. This number has grown from 0, in 1979 to 400–500 along the Bering strait and >40 along the Northern Sea Route, in 2013.
Polar vortex disruption
The polar vortex is a whirlwind of especially cold, dense air forming near the poles that is contained by the jet stream, a belt of fast-flowing winds that serves as a boundary between cold polar air and the warmer air of other hemispheres. Because the power of the polar vortex and jet stream is derived partly from the temperature contrast between cold polar air and warmer tropical air, it is at risk of becoming severely diminished as this contrast is eroded by the effects of melting sea ice. According to the Journal of the Atmospheric Sciences "there [has been] a significant change in the vortex mean state over the twenty-first century, resulting in a weaker, more disturbed vortex." As the vortex becomes weaker, it is more likely to allow cold arctic air to escape from the confines of the jet stream and spill over into other hemispheres. This disruption has already begun to affect global temperatures. In a 2017 study conducted by climatologist Dr. Judah Cohen and several of his research associates, Cohen wrote that "[the] shift in polar vortex states can account for most of the recent winter cooling trends over Eurasian midlatitudes".
A 2015 study concluded that Arctic sea ice decline accelerates methane emissions from the Arctic tundra. One of the study researchers noted, "The expectation is that with further sea ice decline, temperatures in the Arctic will continue to rise, and so will methane emissions from northern wetlands."
A link has been proposed between reduced Barents-Kara sea ice and cold winter extremes over northern continents. Model simulation suggest diminished Arctic sea ice may have been a contributing driver of recent wet summers over northern Europe, because of a weakened jet stream, which dives further south. Extreme summer weather in northern mid-latitudes has been linked to a vanishing cryosphere. Evidence suggest that the continued loss of Arctic sea-ice and snow cover may influence weather at lower latitudes. Correlations have been identified between high-latitude cryosphere changes, hemispheric wind patterns and mid-latitude extreme weather events for the Northern Hemisphere. A study from 2004, connected the disappearing sea ice with a reduction of available water in the American west.
Based on effects of Arctic amplification (warming) and ice loss, a study in 2015 concluded that highly amplified jet-stream patterns are occurring more frequently in the past two decades, and that such patterns can not be tied to certain seasons. Additionally it was found that these jet-stream patterns often lead to persistent weather patterns that result in extreme weather events. Hence, continued heat trapping emissions favour increased formation of extreme events caused by prolonged weather conditions.
In 2018, climate scientist Michael E. Mann explained that the west Antarctic ice sheet may lose twice as much ice by the end of the century as previously thought, which also doubles the projected rise in sea level from three feet to more than six feet.
Plant and animal life
Sea ice decline has been linked to boreal forest decline in North America and is assumed to culminate with an intensifying wildfire regime in this region. The annual net primary production of the Eastern Bering Sea was enhanced by 40–50% through phytoplankton blooms during warm years of early sea ice retreat.
Polar bears are turning to alternative food sources because Arctic sea ice melts earlier and freezes later each year. As a result, they have less time to hunt their historically preferred prey of seal pups, and must spend more time on land and hunt other animals. As a result, the diet is less nutritional, which leads to reduced body size and reproduction, thus indicating population decline in polar bears.
Melting Arctic ice caps are likely to increase traffic through the Arctic Ocean. An early study by James Hansen and colleagues suggested in 1981 that a warming of 5 to 10 °C, which they expected as the range of Arctic temperature change corresponding to doubled CO
2 concentrations, could open the Northwest Passage. A 2016 study concludes that Arctic warming and sea ice decline will lead to "remarkable shifts in trade flows between Asia and Europe, diversion of trade within Europe, heavy shipping traffic in the Arctic and a substantial drop in Suez traffic. Projected shifts in trade also imply substantial pressure on an already threatened Arctic ecosystem." In August 2017, the first ship traversed the Northern Sea Route without the use of ice-breakers. Also in 2017, the Finnish icebreaker MSV Nordica, set a record for the earliest crossing of the Northwest Passage. According to the New York Times, this forebodes more shipping through the Arctic, as the sea ice melts and makes shipping easier. A 2016 report by the Copenhagen Business School found that large-scale trans-Arctic shipping will become economically viable by 2040.
The decline of arctic sea ice will provide humans with access to previously remote coastal zones. As a result, this will lead to an undesirable effect on terrestrial ecosystems and increase the risk of marine species survival. 
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