The extent and influence of polar sea ice

Sea ice influences the rest of the polar climate system in multiple ways: limiting ocean/atmosphere exchanges of heat, mass and momentum; reflecting solar radiation back toward space; and exerting a wide range of influences on polar weather and polar ecosystems (see, for example, Parkinson, 2004; Serreze and Barry, 2005). Sea ice spreads over a significant area of each polar region, globally exceeding the 17.8 million km2 area of South America throughout the year (see Figures 4.1 and 4.2 and Plates 1 and 2). Because of this large spatial expanse and the absence of permanent settlements on the ice, there was no feasible means of

(a) Northern Hemisphere (b) Southern Hemisphere

(a) Northern Hemisphere (b) Southern Hemisphere

Figure 4.1 February sea ice coverage averaged over the 26 years 1979-2004, for (a) the Northern Hemisphere, and (b) the Southern Hemisphere (see Plate 1 for color version)

Note: Mid-winter in the Northern Hemisphere and mid-summer in the Southern Hemisphere, February, averaged over 1979—2004, had sea ice covering 15.2 million km2 of the north polar region and 3.0 million km2 of the south polar region, for a global total of 18.2 million km2, exceeding the 17.8 million km2 area of South America. The sea ice concentration (per cent areal ice coverage) is derived from data from the satellite-borne SMMR and SSMI passive-microwave instruments. No data are available poleward of 87.6°N from either sensor, and only the SSMI data are available between 84.6°N and 87.6°N; hence the former region is presented in black, representing missing data, and the latter region only incorporates data for the 17 years 1988-2004.

obtaining frequent, large-scale records of sea ice coverage before the satellite era. As a result, our consistent large-scale sea ice records extend back only to the 1970s, greatly limiting the database available for climate studies.

On the positive side, the satellite record of sea ice is particularly robust. This is because sea ice is well observed from space with a technology, specifically passive-microwave radiometry, that allows near-global data coverage almost daily, irrespective of sunlight conditions and of most cloud conditions. The multichannel satellite passive-microwave data record extends back to late October 1978, with some lesser-quality, single-channel data also available for much of the period 1973-1976.

The key satellite passive-microwave instruments used for examining sea ice changes in the Arctic and Antarctic since the late 1970s are the Scanning

Changes in Polar Sea Ice Coverage 51 (a) Northern Hemisphere (b) Southern Hemisphere

Changes in Polar Sea Ice Coverage 51 (a) Northern Hemisphere (b) Southern Hemisphere

Figure 4.2 August sea ice coverage averaged over the years 1979-2004, for (a) the Northern Hemisphere, and (b) the Southern Hemisphere (see Plate 2 for color version)

Note: Mid-summer in the Northern Hemisphere and mid-winter in the Southern Hemisphere, August, averaged over 1979—2004, had sea ice covering 7.5 million km2 of the north polar region and 17.5 million km2 of the south polar region, for a global total of 25.0 million km2, exceeding the 24.3 million km2 area of North America. The sea ice concentration is derived from data from the satellite-borne SMMR and SSMI passive-microwave instruments. No data are available poleward of 87.6°N from either sensor, and only the SSMI data are available between 84.6°N and 87.6°N; hence the former region is presented in black, representing missing data, and the latter region only incorporates data for the 17 years 1988—2004.

Multichannel Microwave Radiometer (SMMR) and the Special Sensor Microwave Imager (SSMI). The SMMR was launched in October 1978 on NASA's Nimbus 7 satellite and collected data through mid-August 1987, and the first SSMI was launched in June 1987 on a satellite of the Defense Meteorological Satellite Program (DMSP), with subsequent SSMIs launched on other DMSP satellites. The SMMR/SSMI combination provides a sea ice record from late 1978 until at least 2007. The earlier, single-channel passive-microwave data are primarily from the Electrically Scanning Microwave Radiometer (ESMR), which was launched in December 1972 on the Nimbus 5 satellite and collected data for much of the period 1973-1976. Technical details regarding the ESMR record can be found in Zwally et al (1983) for the Antarctic and in Parkinson et al (1987) for the Arctic; details regarding the SMMR record can be found in Gloersen et al (1992); and details regarding the SMMR/SSMI record and the linking of the SMMR and SSMI time series can be found in Cavalieri et al (1999).

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