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Arctic Sea Ice

Credit:Japan Aerospace Exploration Agency, Earth Observation Research Center

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.

What is sea ice?

Sea ice is frozen ocean water. It forms, grows, and melts in the ocean. In contrast, icebergs, glaciers, and ice shelves float in the ocean but originate on land. For most of the year, sea ice is typically covered with snow.

Why is Arctic sea ice important?

Arctic sea ice keeps the polar regions cool and helps moderate global climate. Sea ice has a bright surface; 80 percent of the sunlight that strikes it is reflected back into space. As sea ice melts in the summer, it exposes the dark ocean surface. Instead of reflecting 80 percent of the sunlight, the ocean absorbs 90 percent of the sunlight. The oceans heat up, and Arctic temperatures rise further.

A small temperature increase at the poles leads to still greater warming over time, making the poles the most sensitive regions to climate change on Earth. According to scientific measurements, both the thickness and extent of summer sea ice in the Arctic have shown a dramatic decline over the past thirty years. This is consistent with observations of a warming Arctic. The loss of sea ice also has the potential to accelerate global warming trends and to change climate patterns.

What is sea ice extent, and why do you monitor that particular aspect of sea ice?

Sea ice extent is a measurement of the area of ocean where there is at least some sea ice. Usually, scientists define a threshold of minimum concentration to mark the ice edge; the most common cutoff is at 15 percent. Scientists use the 15 percent cutoff because it provides the most consistent agreement between satellite and ground observations.

Scientists tend to focus on Arctic sea ice extent more closely than other aspects of sea ice because satellites measure extent more accurately than they do other measurements, such as thickness.

What is the Arctic sea ice minimum?

The Arctic sea ice minimum marks the day, each year, when the sea ice extent is at its lowest. The sea ice minimum occurs at the end of the summer melting season.

The summer melt season usually begins in March and ends sometime during September. The sea ice minimum has been occurring later in recent years because of a longer melting season. However, ice growth and melt are local processes; sea ice in some areas will have already started growing before the date of the sea ice minimum, and ice in other areas will still shrink even after the date of the minimum.

Changes in the timing of the sea ice minimum extent are especially important because more of the sun's energy reaches Earth's surface during the Arctic summer than during the Arctic winter. As explained above, sea ice reflects much of the sun's radiation back into space, whereas dark, ice-free ocean water absorbs more of the sun's energy. So, reduced sea ice during the sunnier summer months has a big impact on the Arctic's overall energy balance.

What is the Arctic sea ice maximum?

The Arctic sea ice maximum marks the day of the year when Arctic sea ice reaches its largest extent. The sea ice maximum occurs at the end of the winter cold season.

The Arctic cold season usually begins in September and ends in March. Monitoring winter sea ice is important to understanding the state of the sea ice. Scientists have found that Arctic sea ice has been recovering less in the winter, meaning the sea ice is already "weak" when the summer melting season arrives. A possible cause is that the underlying ocean is warmer.

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.

Credit: NASA Earth Observatory ,NSIDC