Sea ice is frozen seawater that floats on the ocean surface. It forms in both the Arctic and the Antarctic in each hemisphere’s winter, and it retreats, but does not completely disappear, in the summer.
The Importance of Sea Ice
Sea ice has a profound influence on the polar physical environment, including ocean circulation, weather, and regional climate. As ice crystals form, they expel salt, which increases the salinity of the underlying ocean waters. This cold, salty water is dense, and it can sink deep to the ocean floor, where it flows back toward the equator. The sea ice layer also restricts wind and wave action near coastlines, lessening coastal erosion and protecting ice shelves. And sea ice creates an insulating cap across the ocean surface, which reduces evaporation and prevents heat loss to the atmosphere from the ocean surface. As a result, ice-covered areas are colder and drier than they would be without ice.
Sea ice also has a fundamental role in polar ecosystems. When sea ice melts in the summer, it releases nutrients into the water, which stimulate the growth of phytoplankton, which are the base of the marine food web. As the ice melts, it exposes ocean water to sunlight, spurring photosynthesis in phytoplankton.When ice freezes, the underlying water gets saltier and sinks, mixing the water column and bringing nutrients to the surface. The ice itself is habitat for animals such as seals, Arctic foxes, polar bears, and penguins.
Sea ice’s influence on the Earth is not just regional; it’s global. The white surface of sea ice reflects far more sunlight back to space than ocean water does. (In scientific terms, ice has a high albedo.) Once sea ice begins to melt, a self-reinforcing cycle often begins. As more ice melts and exposes more dark water, the water absorbs more sunlight. The sun-warmed water then melts more ice. Over several years, this positive feedback cycle (the “ice-albedo feedback”) can influence global climate.
Sea ice plays many important roles in the Earth system, but influencing sea level is not one of them. Because it is already floating on the ocean surface, sea ice is already displacing its own weight. Melting sea ice won’t raise ocean level any more than melting ice cubes will cause a glass of iced tea to overflow.
The Sea Ice Life Cycle
When seawater begins to freeze, it forms tiny crystals just millimeters wide, called frazil. How the crystals coalesce into larger masses of ice depends on whether the seas are calm or rough. In calm seas, the crystals form thin sheets of ice, nilas, so smooth they have an oily or greasy appearance. These wafer-thin sheets of ice slide over each other forming rafts of thicker ice. In rough seas, ice crystals converge into slushy pancakes. These pancakes can slide over each other to form smooth rafts, or they can collide into each other, creating ridges on the surface and keels on the bottom.
Some sea ice is fast ice that holds fast to a coastline or the sea floor, and some sea ice is pack ice that drifts with winds and currents. Because pack ice is dynamic, pieces of ice can collide and form much thicker ice. Leads—narrow, linear openings in the ice ranging in size from meters to kilometers—continually form and disappear.
Larger and more persistent openings, polynyas, are sustained by upwelling currents of warm water or steady winds that blow the sea ice away from a spot as quickly as it forms. Polynyas often occur along coastlines where offshore winds maintain their presence.
As the water and air temperatures rise each summer, some sea ice melts. Because of differences in geography and climate, it’s normal for Antarctic sea ice to melt more completely in the summer than Arctic sea ice. Ice that escapes summer melting may last for years, often growing to a thickness of 2 to 4 meters (roughly 6.5 to 13 feet) or more in the Arctic.
For ice to thicken, the ocean must lose heat to the atmosphere. But the ice insulates the ocean like a blanket. Eventually, the ice gets so thick that no more heat can escape. Once the ice reaches this thickness—3 to 4 meters (10 to 13 feet)—further thickening isn’t possible except through collisions and ridge-building.
Ice that survives the summer melt season is called multi-year ice. Multi-year ice increasingly loses salt and hardens each year it survives the summer melt. In contrast to multi-year ice, first-year ice—ice that has grown just since the previous summer—is thinner, saltier, and more prone to melt in the subsequent summer.
Monitoring Sea Ice
Records assembled by Vikings showing the number of weeks per year that ice occurred along the north coast of Iceland date back to A.D. 870, but a more complete record exists since 1600. More extensive written records of Arctic sea ice date back to the mid-1700s. The earliest of those records relate to Northern Hemisphere shipping lanes, but records from that period are sparse. Air temperature records dating back to the 1880s can serve as a stand-in (proxy) for Arctic sea ice, but such temperature records were initially collected at only 11 locations. Russia’s Arctic and Antarctic Research Institute has compiled ice charts dating back to 1933. Today, scientists studying Arctic sea ice trends can rely on a fairly comprehensive record dating back to 1953, using a combination of satellite records, shipping records, and ice charts from several countries.
In the Antarctic, data prior to the satellite record are even more sparse. To try to extend the historical record of Southern Hemisphere sea ice extent further back in time, scientists have been investigating two types of proxies for sea ice extent. One is records kept by Antarctic whalers since the 1930s that document the location of all whales caught. Because whales tend to congregate near the sea ice edge to feed, their locations could be a proxy for the ice extent. A second possible proxy is the presence of a phytoplankton-derived organic compound in Antarctic ice cores. Since phytoplankton grow most abundantly along the edges of the ice pack, the concentration of this sulfur-containing organic compound has been proposed as an indicator of how far the ice edge extended from the continent. Currently, however, only the satellite record is considered sufficiently reliable for studying Antarctic sea ice trends.
Since 1979, satellites have provided a continuous, nearly complete record of Earth’s sea ice. The most valuable data sets come from satellite sensors that observe microwaves emitted by the ice surface because, unlike visible light, the microwave energy radiated by the sea ice surface passes through clouds and can be measured even at night. The continuous sea ice record began with the Nimbus-7 Scanning Multichannel Microwave Radiometer (October 1978-August 1987) and continued with the Defense Meteorological Satellite Program Special Sensor Microwave Imager (1987 to present). The Advanced Microwave Scanning Radiometer–for EOS on NASA’s Aqua satellite has been observing sea ice since 2002.
Ice Area Versus Ice Extent
Satellite images of sea ice are made from observations of microwave energy radiated from the Earth’s surface. Because ocean water emits microwaves differently than sea ice, ice “looks” different to the satellite sensor. The observations are processed into digital picture elements, or pixels. Each pixel represents a square surface area on Earth, often 25 kilometers by 25 kilometers. Scientists estimate the amount of sea ice in each pixel.
There are two ways to express the total polar ice cover: ice area and ice extent. To estimate ice area, scientists calculate the percentage of sea ice in each pixel, multiply by the pixel area, and total the amounts. To estimate ice extent, scientists set a threshold percentage, and count every pixel meeting or exceeding that threshold as “ice-covered.” The National Snow and Ice Data Center, one of NASA’s Distributed Active Archive Centers, monitors sea ice extent using a threshold of 15 percent.
The threshold–based approach may seem less accurate, but it has the advantage of being more consistent. When scientists are analyzing satellite data, it is easier to say whether there is or isn’t at least 15 percent ice cover in a pixel than it is to say, for example, whether the ice cover is 70 percent or 75 percent. By reducing the uncertainty in the amount of ice, scientists can be more certain that changes in sea ice cover over time are real.
Arctic Sea Ice
Most Arctic sea ice occupies an ocean basin largely enclosed by land. Because there is no landmass at the North Pole, sea ice extends all the way to the pole, making the ice subject to the most extreme oscillations between wintertime darkness and summertime sunlight. Likewise, because the ocean basin is surrounded by land, ice has less freedom of movement to drift into lower latitudes and melt. Sea ice also forms in areas south of the Arctic Ocean in winter, including the Sea of Okhotsk, the Bering Sea, Baffin Bay, Hudson Bay, the Greenland Sea, and the Labrador Sea.
Arctic sea ice reaches its maximum extent each March and its minimum extent each September. This ice has historically ranged from roughly 16 million square kilometers (about 6 million square miles) each March to roughly 7 million square kilometers (about 2.7 million square miles) each September.
On time scales of years to decades, the dominant cause of atmospheric variability in the northern polar region is the Arctic Oscillation (AO). (There is still debate among scientists whether the North Atlantic Oscillation and the Arctic Oscillation are the same phenomenon or different but related patterns.) The Arctic Oscillation is an atmospheric seesaw in which atmospheric mass shifts between the polar regions and the mid-latitudes. The shifting can intensify, weaken, or shift the location of semi-permanent low and high-pressure systems. These changes influence the strength of the prevailing westerly winds and the track that storms tend to follow.
During the “positive” phase of the Arctic Oscillation, winds intensify, which increases the size of leads in the ice pack. The thin, young ice that forms in these leads is more likely to melt in the summer. The strong winds also tend to flush ice out of the Arctic through the Fram Strait. During “negative” phases of the oscillation, winds are weaker. Multiyear ice is less likely to be swept out of the Arctic basin and into the warmer waters of the Atlantic. The Arctic Oscillation was in a strong positive phase between 1989 and 1995, but since the late 1990s, it has been in a neutral state.
Current Status and Trends
In September 2008, Arctic sea ice dropped to its second-lowest extent since satellite records began in 1979: 4.67 million square kilometers (1.8 million square miles). Between 1979 and 2006, the annual average decline was 45,100 square kilometers per year, which is about 3.7 percent per decade. But the September minimum ice extent dropped by an average of nearly 57,000 square kilometers per year, which is just over 7.5 percent per decade. In every geographic area, in every month, and every season, current ice extent is lower today than it was during the 1980s and 1990s.
Natural variability and rising temperatures linked to global warming both appear to have played a role in this decline. The Arctic Oscillation’s strongly positive mode through the mid-1990s flushed thicker, older ice out of the Arctic, replacing multiyear ice with first-year ice that is more prone to melting. After the mid-1990s, the AO assumed a more neutral phase, but sea ice failed to recover. Instead, a pattern of steep Arctic sea ice decline began in 2002. The AO likely triggered a phase of accelerated melt that continued into the next decade thanks to unusually warm Arctic air temperatures.
|Year||Average Minimum Extent (million square kilometers)||Compared to 1979-2000 Average (million square kilometers)||Compared to 1979-2000 Average (percent)|
The sea ice minimum was especially dramatic in 2007, when Arctic sea ice extent broke all previous records by mid-August, more than a month before the end of melt season. Both the southern and northern routes through the Northwest Passage opened in mid-September. Ice also became particularly prone to melting in the Beaufort Gyre that summer. The Beaufort Gyre is a clockwise-moving ocean and ice circulation pattern in the Beaufort Sea, and starting in the late 1990s, ice began to melt in the southernmost stretch of the gyre. In the summer of 2007, sea ice retreat was especially pronounced in the region encompassing the Beaufort, Chukchi, East Siberian, Laptev, and Kara Seas.
Many global climate models predict that the Arctic will be ice free for at least part of the year before the end of the century. Some models predict an ice-free Arctic by mid-century, and some even sooner. Depending on how much Arctic sea ice continues to melt, the ice could become extremely vulnerable to natural variability. In the future, the ice might respond even more dramatically than it has in the past to natural cycles such as the Arctic Oscillation.
Impacts of Arctic Sea Ice Loss
Projected effects of declining sea ice include loss of habitat for seals and polar bears, as well as movement of polar bears onto land where bear-human encounters may increase. Indigenous peoples in the Arctic who rely on Arctic animals for food have already described changes in the health and numbers of polar bears.
As sea ice retreats from coastlines, wind-driven waves—combined with permafrost thaw—can lead to rapid coastal erosion. Alaskan and Siberian coastlines have already experienced coastal erosion.
Other potential impacts of Arctic sea ice loss include changed weather patterns: less precipitation in the American West, a weaker storm track that has shifted south over the Atlantic, or (according to one simulation) an intensified storm track.
Some researchers have hypothesized that melting sea ice could interfere with ocean circulation. In the Arctic, ocean circulation is driven by the sinking of dense, salty water. A cap of freshwater resulting from rapid, extensive sea ice melt could interfere with ocean circulation at high latitudes. Although a study published in 2005 suggested that the Atlantic meridional (north-south) overturning circulation had slowed by about 30 percent between 1957 and 2004, that conclusion was not based on comprehensive measurements. Subsequent modeling analyses indicated that the freshwater from melting sea ice was not likely to affect ocean circulation for decades.
Antarctic Sea Ice
The Antarctic is in some ways the precise opposite of the Arctic. The Arctic is an ocean basin surrounded by land, which means that the sea ice is corralled in the coldest, darkest part of the Northern Hemisphere. The Antarctic is land surrounded by ocean. Whereas Northern Hemisphere sea ice can extend to roughly 40 degrees north, Southern Hemisphere sea ice can extend to roughly 50 degrees south. Moreover, Antarctic sea ice does not extend southward to the pole; it can only fringe the continent.
Because of this geography, the Antarctic’s sea ice coverage is larger than the Arctic’s in winter, but smaller in the summer. Total Antarctic sea ice peaks in September—the end of Southern Hemisphere winter—historically rising to an extent of roughly 18 million square kilometers (about 6.9 million square miles). Ice extent reaches its minimum in February, when it dips to roughly 3 million square kilometers (about 1.2 million square miles).
To study patterns and trends in Antarctic sea ice, scientists commonly divide the sea ice pack into five sectors: the Weddell Sea, the Indian Ocean, the western Pacific Ocean, the Ross Sea, and the Bellingshausen/Amundsen seas. In some sectors, it is common for nearly all the sea ice to melt in the summer.
Antarctic sea ice is distributed around the entire fringe of the continent—a much broader area than the Arctic—and it is exposed to a broader range of land, ocean, and atmospheric influences. Because of the geographic and climatic diversity, Antarctic sea ice is more variable from year to year than Arctic sea ice. In addition, climate oscillations don’t affect ice in all sectors the same way, so it is more difficult to generalize the influence of climate patterns to the entire Southern Hemisphere ice pack.
Similar to the Arctic, the Antarctic experiences atmospheric oscillations and recurring weather patterns that influence sea ice extent. The primary variation in atmospheric circulation in the Antarctic is the Antarctic Oscillation, also called the Southern Annular Mode. Like the Arctic Oscillation, the Antarctic Oscillation involves a large-scale seesawing of atmospheric mass between the pole and the mid-latitudes. This oscillation can intensify, weaken, or shift the location of semi-permanent low- and high-pressure weather systems. These changes influence wind speeds, temperature, and the track that storms follow, any of which may influence sea ice extent.
For example, during positive phases of the Antarctic Oscillation, the prevailing westerly winds that circle Antarctica strengthen and move southward. The change in winds can change the way ice is distributed among the various sectors. In addition, the strengthening of the westerlies isolates much of the continent and tends to have an overall cooling effect, but it causes dramatic warming on the Antarctic Peninsula, as warmer air from over the oceans to the north is drawn southward. The winds may drive the ice away from the coast in some areas and toward the coast in others. Thus, the same climate influence may lessen sea ice in some sectors and increase it in others.
Changes in the El Niño-Southern Oscillation Index (ENSO), an oscillation of ocean temperatures and surface air pressure in the tropical Pacific, can lead to a delayed response (three to four seasons later) in Antarctic sea ice extent. In general, El Niño leads to more ice in the Weddell Sea and less ice on the other side of the Antarctic Peninsula, while La Niña causes the opposite conditions.
Another atmospheric pattern of natural variability that appears to influence Antarctic sea ice is the periodic strengthening and weakening of something that meteorologists call “zonal wave three,” or ZW3. This pattern alternately strengthens winds that blow cold air away from Antarctica (toward the equator) and winds that bring warmer air from lower latitudes toward Antarctica. When southerly winds intensify, more cold air is pushed to lower latitudes, and sea ice tends to increase. The effect is most apparent in the Ross and Weddell Seas and near the Amery Ice Shelf.
As in the Arctic, the interaction of natural cycles is complex, and researchers continue to study how these forces work together to control the Antarctic sea ice extent.
Current Status and Trends
In September 2008, Antarctic sea ice peaked at 18.5 million square kilometers (7.14 million square miles), slightly below the monthly average for 1979-2000. The February 2009 minimum of Antarctic sea ice was also slightly below average, at 2.9 million square kilometers (about 1.1 million square miles).
Since 1979, the total annual Antarctic sea ice extent has increased about 1 percent per decade. Compared to the Arctic, the signal has been a “noisy” one, with wide year–to-year fluctuations relative to the trend. The largest summer minimum in the satellite record occurred in February 2003. The largest winter maximum occurred in September 2006. The 2006 maximum was interesting because it followed a February minimum that was the third lowest on record.
Unlike the Arctic, where the downward trend is consistent in all sectors, in all months, and in all seasons, the Antarctic picture is more complex. Based on data from 1979-2006, the annual trend for four of the five individual sectors was a very small positive one, but only in the Ross Sea was the increase statistically significant (greater than the natural year-to-year variability). On the other hand, ice extent decreased in the Bellingshausen/Amundsen Sea sector during the same period.
The variability in Antarctic sea ice patterns in different sectors and from year to year makes it difficult to predict how Antarctic sea ice extent could change as global warming from greenhouse gases continues to warm the Earth. Climate models predict that Antarctic sea ice will respond more slowly than Arctic sea ice to warming, but as temperatures continue to rise, a long-term decline is expected.
You might wonder why the negative trends in Arctic sea ice seem to be more important to climate scientists than the smaller increase in Antarctic ice. Part of the reason, of course, is simply that the size of the increase is much smaller and slightly less certain than the Arctic trend. Another reason, however, is that the complete summertime disappearance of Northern Hemisphere ice would be a dramatic departure from what has occurred throughout the satellite record and likely throughout recorded history. In the Antarctic, however, sea ice already melts almost completely each summer. Even if it completely disappeared in the summer, the impact on the Earth’s climate system would likely be much smaller than a similar disappearance of Arctic ice.
You might also wonder how Antarctic sea ice could be increasing, even a little bit, while global warming from greenhouse gases is raising the planet’s average surface temperature. It’s a question scientists are asking, too. One reason may be that other atmospheric changes are softening the influence of global warming on Antarctica. For example, the ozone hole that develops over Antarctica each spring actually intensifies a perpetual vortex of winds that circles the South Pole. The stronger this vortex becomes, the more isolated the Antarctic atmosphere becomes from the rest of the planet. In addition, ocean circulation in the Antarctic behaves differently than it does in the Arctic. Around Antarctica, warm water moves downward in the ocean’s water column, making sea ice melt from warm water less likely.
Impacts of Antarctic Sea Ice Loss
A study on warming of West Antarctica since the 1957 geophysical year correlates widespread warming in West Antarctica and sea ice decline. Whether sea ice decline has led to warming temperatures on the continent, or whether both phenomena are caused by something else is not currently known.
One concern related to potential Antarctic sea ice loss is that sea ice may stabilize Antarctic ice shelves. Ice shelves are slabs of ice that partly rest on land and partly float. Ice shelves frequently calve icebergs, and this is a natural process, not necessarily a sign of climate change. But the rapid disintegration and retreat of an ice shelf (such as the collapse of the Larsen B shelf in 2001) is a warming signal. Although sea ice is too thin to physically buttress an ice shelf, intact sea ice may preserve cool conditions that stabilize an ice shelf because air currents passing over sea ice are cooler than air currents passing over open ocean. Sea ice may also suppress ocean waves that would otherwise flex the shelf and speed ice shelf breakup.
The interaction between sea ice loss and ice shelf retreat merits careful study because many ice shelves are fed by glaciers. When an ice shelf disintegrates, the glacier feeding it often accelerates. Because glacier acceleration introduces a new ice mass into the ocean, it can raise ocean level. So while sea ice melt does not directly lead to sea level rise, it could contribute to other processes that do, both in the Arctic and the Antarctic. Glacier acceleration has already been observed on the Antarctic Peninsula, although the accelerating glaciers in that region have so far had a negligible effect on ocean level.
Because of differences in geography and climate, the amount, location, and natural variability of sea ice in the Arctic and the Antarctic are different. Global warming and natural climate patterns may affect each hemisphere’s sea ice in different ways or at different rates. Within each hemisphere, sea ice can change substantially from day to day, month to month, and even over the course of a few years. Comparing conditions at only two points in time or examining trends over a short period is not sufficient to understand the impact of long-term climate change on sea ice. Scientists can only understand how sea ice is changing by comparing current conditions to long-term averages.
Since 1979, satellites have provided a consistent continuous record of sea ice. Through 2008, annual average sea ice extent in the Arctic fell by about 4.1 percent per decade relative to the 1979–2000 average. The amount of ice remaining at the end of summer declined even more dramatically—over 11.1 percent per decade. Declines are occurring in every geographic area, in every month, and every season. Natural variability and rising temperatures linked to global warming appear to have played a role in this decline. The Arctic may be ice-free in summer before the end of this century.
Antarctic sea ice trends are smaller and more complex. Through 2008, the total annual Antarctic sea ice extent increased about 1 percent per decade, but the trends were not consistent for all areas or all seasons. The variability in Antarctic sea ice patterns makes it harder for scientists to explain Antarctic sea ice trends and to predict how Southern Hemisphere sea ice may change as greenhouse gases continue to warm the Earth. Climate models do predict that Antarctic sea ice will respond more slowly than Arctic sea ice to warming, but as temperatures continue to rise, a long-term decline is expected.
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