23 August 2006
Frequently Asked Questions about Sea Ice
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 so important?
Arctic sea ice keeps the polar regions cool and helps moderate global climate. Sea ice has a bright surface, so 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 summer sea ice extent in the Arctic have shown a dramatic decline over the past thirty years. This is consisistent 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.
For more information about the importance of sea ice, including its importance for global ocean circulation, people, and animals, see All About Sea Ice: Environment.
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 where the ice ends; 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 and 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.
For more information on last year’s record sea ice minimum, see the 2005 Sea Ice Minimum Press Release.
What is the Arctic sea ice maximum?
The Arctic sea ice maximum marks the day of the year when the sea ice covers the largest area of the Arctic. The sea ice maximum occurs at the end of the winter cold season.
The winter cold season usually begins in September and ends in March. Monitoring winter sea ice is also 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.
To learn more about the recent sea ice extent maximum, see the 2006 Sea Ice Maximum Press Release.
How do scientists monitor the Arctic sea ice?
Obtaining reliable measurements of sea ice as it changes was difficult until the satellite era began in the early 1970s. To monitor Arctic sea ice, NSIDC primarily uses the Earth Observing System (EOS) Advanced Microwave Scanning Radiometer (AMSR-E) instrument on the NASA Terra satellite and the Special Sensor Microwave/Imager (SSM/I) instrument on the Defense Meteorological Satellite Program (DMSP) satellite. The satellites pass over the polar region several times each day to gather data; researchers can then form the data into usable information, such as the Sea Ice Index. Scientists can then analyze what they see in the data and report on their findings.
Useful satellite data concerning sea ice began in late 1978 with the launch of NASA’s Scanning Multichannel Microwave Radiometer (SMMR) satellite. When scientists compare average sea ice conditions between years, they often use a reference period of 1979–2000. This reference period allows a consistent comparison of changes in extent over individual years. Scientists generally do not include data from 2000 forward because that period has seen especially sharp declines in sea ice extent.
Is Antarctic sea ice important, too? Is it shrinking?
Scientists monitor both Arctic and Antarctic sea ice, but Arctic sea ice has been more significant to understanding global climate because much more Arctic ice remains through the summer months, reflecting sunlight and cooling the planet.
However, sea ice near the Antarctic Peninsula, south of the tip of South America, has recently experienced a significant decline. The rest of Antarctica has experienced a small increase in Antarctic sea ice.
For more information about Antarctic sea ice, see All About Sea Ice: Arctic vs. Antarctic.
Where can I learn more?
See our All About Sea Ice pages for in-depth information about sea ice, from how it forms to the lives that depend on it.