Sea Ice and the Climate System
Sea ice forms and melts in sea water, as opposed to land-based ice such as glaciers, ice sheets or shelves, and grounded icebergs. In today's climate regime, sea ice has been observed as far south as Bohai Bay in China—a latitude comparable to the Mediterranean Sea. Sea ice begins to form when water temperature dips just below freezing, at around -1.8°C (or 28.8°F). It grows into small sheets that look like pancakes, and eventually merge together to form large ice floes which can span miles. As the ice forms, it expels the salt, which increases the density of the surrounding water and thus plays a critical role in global ocean circulation.
Temperature in the Arctic has increased at twice the rate as the rest of the globe, and the region is expected to increase an additional 8°C (14°F) in the 21st century. Winter temperature has increased more than summer temperature, which is a trend that is expected to continue. While some have suggested that these variations in temperature and associated sea ice melt are a natural cycle, recent research tells us that the Arctic was in a 2,000 year cooling trend before the 20th century and its influx of greenhouse gases.
Sea ice is generally moderated by sunlight—it grows in the winter and melts in the summer—but there are other factors at play in the decline of ice in the Arctic Ocean. Warm ocean currents travel north from the equator and usher in warmer and warmer water, making sea ice growth difficult. Weather patterns over the high mid-latitudes and the Arctic can also affect sea ice growth. Under normal climate conditions, cold air is confined to the Arctic by the polar vortex winds, which circle counter-clockwise around the North Pole. As sea ice coverage decreases, the Arctic warms, high pressure builds, and the polar vortex weakens, sending cold air is spilling southward into the mid-latitudes, bringing record cold and fierce snowstorms. At the same time, warm air will flowing into the Arctic to replace the cold air spilling south, which drives more sea ice loss. This reversal could be partially driven by sea ice loss, and so is expected to surface more often in the coming years.
The primary role that sea ice plays in global climate its ability to efficiently reflect the Sun's radiation. This property is called "albedo," the measure of the reflecting power of a surface. The albedo of snow-covered sea ice is 0.90, meaning it reflects 90% of the Sun's radiation. Just like wearing a white shirt will keep you cool when you're out in the Sun, the sea ice covering the Arctic keeps the thermostat low. The ocean surface, however, is almost black, and it only reflects 10%, meaning it absorbs 90%. After something absorbs sunlight, it emits heat. Less sea ice and more ocean surface will lead to a warmer Arctic, and a warmer climate.
Observed Sea Ice Melt
Satellite data show that since the late 1970s, September Arctic sea ice extent has decreased by about 12% per decade. What's especially alarming is the decrease in multi-year ice. Sea ice is classified by age, usually as "new ice" or "multi-year" ice (meaning it survived many summer melting seasons). While new ice is very shallow, multi-year ice can grow to be quite thick, typically between 6 and 12 feet, and is very stable. A remarkable study was published in 2007 which measured the amount of multi-year ice in the Arctic. In 1987, 57% of the observed ice pack was at least 5 years old, and around 25% of it was at least 9 years old. When they surveyed the Arctic again in 2007, only 7% of the ice pack was at least 5 years old, and the ice that was at least 9 years old had all but vanished. Likewise, sea ice thickness and volume have decreased markedly since the beginning of the satellite era.
Figure 2. September Arctic sea ice extent. Sea ice extent measurements are made with satellite instruments, and are given in millions of square kilometers. Sea ice measurements are available within the satellite era, and are shown here from 1979 through the previous month. Data source: National Snow and Ice Data Center.
Recent years have set a number of sea ice records in the Arctic. The summer of 2007 saw a "perfect storm" of weather conditions favorable for ice loss. Unusually strong high pressure over the Arctic led to clear skies and plenty of sunshine. The polar vortex weakened, injecting large amounts of warm air into the Arctic. Sea ice loss doubled to 39% in 2007, according to the National Snow and Ice Data Center. In one year, as much ice was lost as in the previous 28 years combined. In 2011, the University of Bremen reported that sea ice had reached a new all-time low on September 8th, and was 27,000 square kilometers below the previous record set in 2007.
Extraordinary melting of sea ice in the Arctic in 2012 shattered the all-time low sea ice extent record set in September 2007. The new sea ice record was set on August 26, 2012, a full three weeks before the usual end of the melting season, according to the National Snow and Ice Data Center. A comprehensive collection of sea ice graphs shows the full story. Satellite records of sea ice extent date back to 1979, though a 2011 study by Kinnard et al. shows that the Arctic hasn't seen a melt like this for at least 1,450 years (see a more detailed article on this over at skepticalscience.com.) The record minimum extent of 3.41 million square kilometers is approximately a 50% reduction in the area of Arctic covered by sea ice, compared to the average from 1979 - 2000.
These recent low sea ice records have provided new opportunities for the shipping industry, opening both the Northeast and the Northwest Passages in the Arctic Ocean. The Northeast Passage is a shipping route that runs along the northern Russian coast and to the Bering Strait, and is sometimes called the "Northern Sea Route." On the other side of the Arctic Ocean, the Northwest Passage runs along the North American coast through waterways in the Canadian Arctic Archipelago. These passages have been elusive since the early 1900s, although climate change has recently freed up both of the typically ice-choked routes. The Northeast Passage opened for the first time in recorded history in 2005, and the Northwest Passage in 2007. For four years in a row, the Northwest Passage was open for ice-free sailing. It now appears that the opening of one or both of these northern passages is the new norm, and business interests are taking note—commercial shipping in the Arctic is on the increase, and there is increasing interest in oil drilling. The great polar explorers of past centuries would be astounded at how the Arctic has changed in the 21st century.
When was the last time the Arctic was this ice-free?We can be confident that the Arctic did not see the kind of melting observed in 2012 going back over a century, as we have detailed ice edge records from ships (Walsh and Chapman, 2001). It is very unlikely the Northwest Passage was open between 1497 and 1900, since this spanned a cold period in the northern latitudes known as "The Little Ice Age". Ships periodically attempted the Passage and were foiled during this period. Research by Kinnard et al. (2011) show that the Arctic ice melt in the past few decades is unprecedented for at least the past 1,450 years. We may have to go back to at least 4,000 B.C. to find the last time so little summer ice was present in the Arctic. Funder and Kjaer (2007) found extensive systems of wave generated beach ridges along the North Greenland coast, which suggested the Arctic Ocean was ice-free in the summer for over 1,000 years between 6,000 - 8,500 years ago, when Earth's orbital variations brought more sunlight to the Arctic in summer than at present. Prior to that, the next likely time was during the last inter-glacial period, 120,000 years ago. Arctic temperatures then were 2 -3 °C higher than present-day temperatures, and sea levels were 4 - 6 meters higher.
A Manmade Problem
Increased water temperatures and air temperatures due to human-caused global warming are the dominant reasons for the record melting of the Arctic sea ice. A July 2012 study by Day et al. found that the most influential of the possible natural influences on sea ice loss was the Atlantic Multi-decadal Oscillation (AMO). The AMO has two phases, negative (cold) and positive (warm), which impact Arctic sea ice. The negative phase tends to create sea surface temperatures in the far north Atlantic that are colder than average. In this study, the AMO only accounted for 5% - 31% of the observed September sea ice decline since 1979. The scientists concluded that given the lack of evidence that natural forces were controlling sea ice fluctuations, the majority of sea ice decline we've seen during the 1953 - 2010 period was due to human causes.
Scientists use numerical models to predict how fast Arctic sea ice is expected to melt in coming decades. Until recently, these climate models have done a poor job predicting the recent record loss of Arctic sea ice. None of the models used in the 2007 Intergovernmental Panel on Climate Change (IPCC) report have foreseen the recent, remarkable sea ice loss. This is likely because the models have a hard time understanding the transport of heat within the ocean itself, which some argue causes over 50% of Arctic sea ice loss. The NOAA GFDL model paints a similar picture as that of the IPCC models: an ice-free Arctic summer by 2100. However, these forecasts are too conservative, and it's looking more and more like the Arctic will be ice-free in the next few decades.
Does 2012 mark a fundamental change in Arctic ice loss? Possibly. The previous record low extent in 2007 was spurred by not just warming, but also a very unfavorable pattern of storms which aided in breaking up the sea ice, allowing it to melt more and faster. This year, though we did see two strong storms in the Arctic, they weren't like what we saw in 2007. It's arguable that 2012's all-time minimum was due more to the warming Arctic than the minimum we saw in 2007. Mark Serreze, the Director the National Snow and Ice Data Center, said about this year's sea ice minimum, "Apart from one big storm in early August, weather patterns this year were unremarkable. The ice is so thin and weak now, it doesn't matter how the winds blow." It's hard to say exactly when we'll see an ice-free summer Arctic, since there is still some natural variability in the process. A study by Day et al. found that 5 to 31% of the sea ice variability could be due to natural causes. However, sea ice minimum has been declining sharply over the past 30 years, and is far exceeding the worst worst-case scenario predicted by the 2007 IPCC report. Annual minimum sea ice extent is decreasing at a rate of 12% per year. Forecasts of an ice-free Arctic range from 20-30 years from now to much sooner. Just this week Dr. Peter Wadhams of Cambridge University predicted that the Arctic will be ice-free within four years. A recent study by Stroeve et al. using updated IPCC models finds that they are more able to predict the current rate of melting. These updated models suggest that "a seasonally ice-free Arctic Ocean within the next few decades is a distinct possibility."
Impacts of Disappearing Sea Ice
Arctic sea ice is an important component of the global climate system. The polar ice caps help to regulate global temperature by reflecting sunlight back into space. White snow and ice at the poles reflects sunlight, but dark ocean absorbs it. Replacing bright sea ice with dark ocean is a recipe for more and faster global warming. The Autumn air temperature over the Arctic has increased by 4 - 6°F in the past decade, and we could already be seeing the impacts of this warming in the mid-latitudes, by an increase in extreme weather events. Another non-trivial impact of the absence of sea ice is increased melting in Greenland. We already saw an unprecedented melting event in Greenland this year, and as warming continues, the likelihood of these events increase.
The impacts of an ice-free Arctic are far-reaching, and could be a trigger for abrupt, cataclysmic climate change in the future. Although it is difficult to see exactly how sea ice decline will impact the local and global environment, basic understanding of the Arctic as well as recent observations give us a good idea of how things might change.
Sea level rise
Direct effect: The melting of the Arctic sea ice will not change ocean sea levels appreciably, since the ice is already floating in the ocean. Sea ice melting does slightly contribute to sea level rise since the fresh melt water is less dense than the salty ocean water it displaces. According to Dr. Robert Grumbine of NOAA's sea ice group, if all the world's sea ice melted, it would contribute to about 4 millimeters of global sea level rise. This is a tiny figure compared to the 20 feet of potential sea level rise locked up in the ice of the Greenland Ice Sheet, which is on land.
Indirect effect: The biggest concern regarding Arctic sea ice loss is the warmer average temperatures it will bring to the Arctic in coming years. Warmer temperatures will accelerate the melting of the Greenland ice sheet, which holds enough water to raise sea level 20 feet. Although the IPCC's 2007 report predicted only a 0.6-1.9 foot sea level rise by 2100 due to melting of the Greenland ice sheet and other factors, these estimates will probably need to be revised upwards in light of the unexpectedly high sea ice loss in the Arctic.
Continued loss of Arctic sea ice may dramatically alter global weather and precipitation patterns in the decades to come. The jet stream will probably move further north in response to warmer temperatures over the pole, which will bring more precipitation to the Arctic. More frequent and intense droughts over the U.S. and other regions of the mid-latitudes may result from this shift in the jet stream. Changes to the course of the jet stream affect weather patterns for the entire planet, and we can expect impacts on the strength of the monsoons and recurvature likelihood of hurricanes. During 1979 to 2006, years that had unusually low summertime Arctic sea ice also had a 10-20% reduction in the temperature difference between the Equator and the North Pole. This resulted in reduced winter precipitation over all of the U.S., Alaska, and Northern Europe. In contrast, increased precipitation fell over Spain, Italy, and Japan during these winters. Although intense La Niña or El Niño events can have a much stronger influence on wintertime weather patterns, reduced summertime Arctic sea ice should give most of the Northern Hemisphere a delayed start to winter during most years for the foreseeable future.
Global ocean circulation
Surface global ocean currents are driven by the winds, but the vertical ocean circulation is determined by the temperature and salt content of the water (hence, is called the thermohaline circulation). The engine of the thermohaline circulation is in the North Atlantic, where warm surface waters travel north past Greenland and into the Arctic on the Gulf Stream current. As the warm water reaches cold air, evaporation cools the water, and sea ice formation increases the salinity (salt content) of the surrounding water (ice rejects the salt as it freezes). This new cold, salty water is very dense, and sinks in a process called overturning. This sinking motion in the Arctic is a driving force behind the "global conveyor belt," and the formation and maintenance of sea ice is a the heart of it all. Not only could the slowdown of new sea ice formation lead to the abatement of the thermohaline circulation, but as sea ice melts, it injects massive quantities of freshwater into the Arctic Ocean. The freshening of Arctic sea water due to manmade climate change could lead to exceptional changes in the world's ocean circulation and thus Earth's climate as well.
Sea ice is important in marine ecosystems in at least three ways. First, it provides a habitat for algae and invertebrates and fish, and regulates the temperature of the water below it. Although it seems counterintuitive, the sea ice insulates the water beneath it, keeping it from becoming too cold. Second, as the ice melts in the summer, it releases the organisms into the water, providing fuel for Arctic marine food webs. Finally, it provides breeding and hunting grounds for marine mammals and birds that call the chilly North their home.
The impacts of melting ice extend well beyond polar bears (which are one step away from endangered on the Conservation Scale). Birds, seals, and whales also use the ice for hunting. Birds nest in the sea ice and use it for protection while raising their young in the potentially deadly environment of the Arctic. The retreat of sea ice, especially in the warm winter months, has decreased the available platforms that seals, walruses, and polar bears use to rest on and hunt from. Scientists estimate that retreating sea ice will result in a loss of 2/3 of the polar bear population, and force the remaining bears into a smaller, iceless area.
As sea ice disappears, coastlines become more susceptible to battering waves. This is particularly common during the fall season, not only because sea ice extent is usually at its minimum, but fall is when storms tend to be stronger with higher storm surges. Recent coastal destruction has already forced residents of the Alaskan town of Shishmaref to vote to abandon their village. More than half the residents of the nearby village of Kivalina (population 400) were forced to evacuate in September 2007, when 25 - 40 mph winds drove a four foot storm surge into the town. The U.S. Army Corps of Engineers completed a $16 million sea wall and shore fortifications in 2009 to protect the town. As sea ice continues to decrease in coming years, leaving more ocean surface exposed to air, more moisture and heat will be available to power storms. These stronger storms will bringer higher winds and higher storm surges to coastal areas in the Arctic over the remainder of the 21st century, resulting in increased erosion and flooding of low-lying areas. Contributing to the erosion will be sea level rise. Kivalina, which lies on a narrow barrier island in the Chukchi Sea, has been losing up to 8 feet of shore each year due to erosion, and the long-term survival of the island is in serious doubt. Plans have been drawn up by the Army Corps of Engineers to relocate the city to the mainland, but finding funding for the $300+ million dollar move has been problematic. The city of Kivalina and a federally recognized tribe, the Alaska Native Village of Kivalina, sued Exxon Mobil Corporation, eight other oil companies, 14 power companies, and one coal company in a lawsuit filed in federal court on February 26, 2008, claiming that the large amounts of greenhouse gases these companies are responsible for contribute to global warming that threatens the community's existence. The lawsuit estimates the cost of relocation at $400 million.
The Cold, Hard Facts
Arctic sea ice has been melting at break-neck speeds in the past few decades, driven by warming air temperature, warming ocean water temperature, and new, extreme weather patterns, all of which are caused by or accelerated by man-made climate change. Unfortunately, melting sea ice is a slippery slope—once it starts, it's hard to reverse, and even under normal climate conditions would take centuries to reestablish. The lack of bright white ice on the dark ocean surface is leading to a temperature increase that likely extends beyond the borders of the Arctic, and a breakdown of the polar vortex, which is so critical in maintaining a cold, ice-conducive atmosphere at the pole. Models suggest sea ice will disappear by 2100, but most Arctic sea ice experts are calling for an summertime ice-free Arctic by 2030.
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