Sea Ice

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.

Sea ice begins as thin sheets of smooth nilas in calm water (top) or disks of pancake ice in choppy water (2^nd from top). Individual pieces pile up on top of one another to form rafts and eventually solidify (3^rd from top). Over time, large sheets of ice collide, forming thick pressure ridges along the margins (bottom). (Nilas, pancake, and ice raft photographs courtesy Don Perovich, Cold Regions Research and Engineering Laboratory. Pressure ridge photograph courtesy Ted Scambos, National Snow and Ice Data Center.)

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.

Fast ice is anchored to the shore or the sea bottom, while pack ice floats freely. As it drifts, leads continually open and close between ice floes. Persistent openings, polynyas, are maintained by strong winds or ocean currents. (NASA satellite image courtesy Jacques Descloitres, MODIS Rapid Response Team.)

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.

Satellite Monitoring

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.

Reliable records of Arctic sea ice begin in 1953. Satellites provide a near-continuous record of sea ice beginning in 1979. Monthly (light blue) and annual (dark blue) sea ice anomalies vary from year to year. Scientists describe the range of variability with statistics (the number of standard deviations above or below the mean). Up until the 1970s, Arctic sea ice extent was relatively constant, but it has been decreasing since the 1980s. (Graph by Walt Meier, National Snow and Ice Data Center.)

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.

Satellites measure sea ice concentration on a coarse grid of pixels as large as 25 by 25 kilometers. This image illustrates the area covered by each pixel of the low-resolution microwave instruments used to measure sea ice, superimposed on a higher-resolution color satellite image. Sea ice concentration is the percentage of each pixel that is covered by ice. Sea ice extent is calculated by adding up the area of all the pixels with an ice concentration of at least 15 percent. (NASA image by Robert Simmon, based on MODIS data.)

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.

The minimum Arctic sea ice extent occurs in September. The maximum is in late February or early March. In these maps, dark blue represents open water, and increasingly paler blues indicate higher concentrations of sea ice. Extents historically ranged from 16 million square kilometers to 7 million square kilometers, but 2007 and 2008 were much lower - near 4.5 million square kilometers. (NASA maps by Jesse Allen, based on AMSR-E data from NSIDC.)

Natural Variability

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.

The satellite record shows a clear decrease of average September sea ice extent in the Arctic. Since 1979, sea ice has decreased more than 7.5 percent per decade. (NASA graph by Robert Simmon, based on data from the National Snow and Ice Data Center.)

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.

Arctic ice extent has dropped steeply since 2002. In 2007, summer ice extent was almost 40 percent below the 1979-2000 average. Table based on data from the National Snow and Ice Data Center.

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.

Arctic sea ice cover peaks each year in March, and melts until the minimum is reached in September. In 2007 (red line) and 2008 (gray line), Arctic ice reached the lowest extents ever recorded, well below the historical average (blue line). Light blue regions show the range of natural variability, without the effects of global warming. (NASA graph by Robert Simmon, based on data from the National Snow and Ice Data Center.)

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.

Sea ice extent in the waters surrounding Antarctica peak in September and reach a minimum in February. Roughly 15 million square kilometers of ice alternately melt and freeze during the annual cycle. (Graph by Robert Simmon, based on data from the National Snow and Ice Data Center.)

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.

In the Southern Ocean, sea ice fringes the entire Antarctic continent. Researchers typically subdivide Antarctic sea ice into 5 sectors, each influenced by different geography and weather conditions. Because of the geographic and climatic diversity, Antarctic sea ice is more variable from year to year than Arctic sea ice. (NASA map by Robert Simmon.)

Natural Variability

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).

Antarctic sea ice peaks in September, and reaches a minimum in February. In some places, sea ice melts completely in the summer. (NASA maps by Jesse Allen, based on AMSR-E data from the National Snow and Ice Data Center.)

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.

Antarctic sea ice does not plainly show the effects of global warming. There is little evidence of long-term change in either the maximum (September) or minimum (February) ice extent. (Graph by Robert Simmon, based on data from the National Snow and Ice Data Center )

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.

Credit: NASA Earth Observatory by Michon Scott design by Robert Simmon April 20, 2009