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 2007 study found the decline to be "faster than forecasted" by model simulations. A 2011 study suggested that this could be reconciled by internal variability enhancing the greenhouse gas-forced sea ice decline over the last few decades. A 2012 study with a newer set of simulations also projected rates of retreat which were somewhat less than that actually observed.

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 4,000 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.

Definitions
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 maintains 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 sun is being absorbed by the ocean. 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
An article in Popular Mechanics, published in March 1912, described open waters in the Arctic regions in 1911, a year with exceptional above average temperatures.

Satellite era
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 (4.09 million square kilometers). This replaced the previous record set on September 28, 2007 at 1.61 million square miles (4.17 million square kilometers).

A 2018 study of the thickness of sea ice found a decrease of 66% or 2.0 m over the last six decades and a shift from permanent ice to largely seasonal ice cover.

Ice-free summer


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 in 2014 predicted that by 2020 "summer sea ice to disappear,"  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. In 2007, Professor Wieslaw Maslowski from the Naval Postgraduate School, California, predicted removal of summer ice by 2013; subsequently, in 2013, Maslowski predicted 2016 ±3 years. 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. Professor James Anderson of Harvard University envisions the Arctic Ice gone by the early 2020s. "The chance that there will be any permanent ice left in the Arctic after 2022 is essentially zero," he said in June of 2019.



Warming Arctic temperatures provide a powerful forcing toward lessened sea ice coverage.

Predictions by non-scientists
Former U.S. Vice President Al Gore, when speaking at the UN's COP 15 meeting in December 2009 said "Some of the models suggest that there is a 75 percent chance that the entire north polar ice cap during some of the summer months will be completely ice-free within the next five to seven years." Dr Wieslav Maslowski, of the Naval Postgraduate School in Monterey, California, whom Gore apparently used as source, disagreed with Gore's forecast and told The Times: “It’s unclear to me how this figure was arrived at," and instead clarified his forecast called for "a six-year projection for the melting of 80 percent of the ice, but he said he expects some ice to remain beyond 2020."  It was not the first time Gore had used "most aggressive statements" when predicting ice cap melting.

Tipping point
There has been debate whether the Arctic Ocean will pass a "tipping point", defined as a threshold for abrupt and irreversible change, as the amount of ice cover declines. Although some earlier studies supported the presence of a tipping point, the IPCC AR5 concluded that there is little evidence for such a tipping point based on more recent studies that used global climate models  and low-order sea ice models. However, a 2013 study identified an abrupt transition to increased seasonal ice cover variability in 2007 which persisted in following years, which the researchers considered a non-bifurcation 'tipping point', with no implications of irreversible change. The IPCC AR5 WGII report stated with medium confidence that precise levels of climate change sufficient to trigger a tipping point remain uncertain, but that the risk associated with crossing multiple tipping points 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. This also increases pressure and decrease wind speeds. These feedback effects are stronger in the lower atmosphere. 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. Arctic amplified warming is observed as stronger in lower atmospheric areas because of the expanding process of warmer air increases pressure levels which decreases poleward geopotential height gradients. These gradients are the reason that cause west to east winds through the thermal wind relationship, declining speeds are usually found south of the areas with geopotential increases. This relationship has been documented through observation evidence and model responses to sea ice loss according to the 2017 Wiley Periodicals Article, Amplified Arctic warming and mid latitude weather: new perspectives on emerging connections.

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".

Atmospheric chemistry
Cracks in sea ice can expose the food chain to greater amounts of atmospheric mercury.

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."

Atmospheric regime


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. The arctic refuge is where polar bears main habitat is to den and the melting arctic sea ice is causing a loss of species. There are only about 900 bears in the arctic refuge national conservation area.

Shipping
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 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.

Human Impact
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 put marine species at risk.

Maps

 * NSIDC | Arctic Sea Ice News
 * Cryosphere Today
 * Daily AMSR2 Sea Ice Maps

Video

 * Annual Arctic sea ice minimum 1979–2016 with area graph
 * The Arctic Meltdown & Extreme Weather – Jennifer Francis (2017)