Polar Ice Packs - Extent and Volume of Sea Ice and Their Trends

Extent and Volume of Sea Ice and Their Trends

Records of Arctic Sea ice from the United Kingdom’s Hadley Centre for Climate Prediction and Research go back to the turn of the 20th century, although the quality of the data before 1950 is debatable. Still, these records show a persistent decline in Arctic Sea ice over the last 50 years.

Reliable measurements of sea ice edge begin within the satellite era. From the late 1970s, the Scanning Multichannel Microwave Radiometer (SMMR) on Seasat (1978) and Nimbus 7 (1978–87) satellites provided information that was independent of solar illumination or meteorological conditions. The frequency and accuracy of passive microwave measurements improved with the launch of the DMSP F8 Special Sensor Microwave/Imager SSMI in 1987. Both the sea ice area and extent are estimated, with the latter being larger, as it is defined as the area of ocean with at least 15% sea ice.

A modeling study of the 52-year period from 1948 to 1999 found a statistically significant trend in Arctic ice volume of −3% per decade; splitting this into wind-forced and temperature forced components shows it to be essentially all caused by the temperature forcing. A computer-based, time-resolved calculation of sea ice volume, fitted to various measurements, revealed that monitoring the ice volume is much more significant for evaluating sea ice loss than pure area considerations.

The trends from 1979 to 2002 have been a statistically significant Arctic decrease and an Antarctic increase that is probably not significant, depending exactly on which time period is used. The Arctic trends of −2.5% ± 0.9% per decade; or about 3% per decade. Climate models simulated this trend in 2002, and attributed it to anthropogenic forcing.

The September minimum ice extent trend for 1979–2011 declined by 12.0% per decade.

In 2007 the ice melt accelerated. The minimum extent fell by more than a million square kilometers, the biggest decline ever, to 4,140,000 km2 (1,600,000 sq mi), then by far the lowest ever. New research shows the Arctic Sea ice to be melting faster than predicted by any of the 18 computer models used by the Intergovernmental Panel on Climate Change in preparing its 2007 assessments. In 2012, a new record low of about 3,500,000 km2 (1,400,000 sq mi) was reached.

While the Northern Hemisphere sea ice reached new record lows, on September 12, 2007 the Southern Hemisphere sea ice area reached 15,910,000 km2 (6,143,000 sq mi), close to the maximum recorded of 16,020,000 km2 (6,185,000 sq mi).

The Antarctic increase is 0.8% per decade although this depends on the period being considered. Vinnikov et al. find the NH reduction to be statistically significant but the SH trend is not.

In the overall mass balance, the volume of sea ice depends on the thickness of the ice as well as the areal extent. While the satellite era has enabled better measurement of trends in areal extent, accurate ice thickness measurements remain a challenge. "Nonetheless, the extreme loss of this summer’s sea ice cover and the slow onset of freeze-up portends lower than normal ice extent throughout autumn and winter, and the ice that grows back is likely to be fairly thin".

As more and more of the sea ice is thinner first-year ice the greater effect storms have on its stability with turbulence resulting from major extratropical cyclones resulting in extensive fractures of sea ice.