Measurement of Sea Ice

It is widely accepted that global warming will manifest itself most in the polar regions. Sea ice is therefore an effective indicator of the climate change.

Since late in 1978 measurements of sea ice extent have been available from satellite sensors. The measurements have been made by a series of different satellites. The National Sea Ice Center of the University of Colorado maintains files of ice extent and concentration. The area of sea ice varies throughout the year, with of course, the Arctic and Antarctic being out of phase. There is an important difference between the two: Arctic sea ice is surrounded by land whereas the Antarctic is land surrounded by ice.

Figure 1 and Figure 2 show the changes in summer and winter sea ice for the Arctic and Antarctic, respectively, for the years 1979, the first year with data, and 2008, the most recent complete year. As can be seen there are big differences in summer and winter ice and the main change has been in the summer sea ice in the Arctic.

Change in Sea ice Area

Figure 3 shows the change in area of sea ice in the northern and southern polar regions. This suggests that whilst the area of sea ice has declined in the Arctic it has tended to increase in the Antarctic.

Figure 4 shows the seasonal sea ice for the Arctic only from 1870 to 2008. The plots show the annual average and the maximum and minimum seasonal extent. This shows that arctic sea ice was more or less constant until the latter half of the 20th century.

Figure 5 shows the total sea ice for the Arctic and Antarctic. This shows that overall there has been a decline in sea ice and that the increase in the southern hemisphere is less than the decline in the northern hemisphere. This represents a decline of about 32,000 km2/year. Allowing for permanent land ice this represents an annual loss of 0.08% of polar ice.

Data sources

The data for sea ice for the period 1978 to 2009 was downloaded from ftp://sidads.colorado.edu/DATASETS/NOAA/G02135. (Reference: Fetterer, F., K. Knowles, W. Meier, and M. Savoie. 2002, updated 2009. Sea Ice Index, Boulder, Colorado USA: National Snow and Ice Data Center.)

Data for the period 1870 to 2008 was downloaded from: http://arctic.atmos.uiuc.edu/SEAICE/timeseries.1870-2008 (Reference: William Chapman, University of Illinois Sea Ice Dataset, Northern Hemisphere : 1870 – 2008)