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.