4.4.2.2 Hemispheric, Regional and Seasonal Time Series from Passive Microwave

Most analyses of variability and trend in ice extent using the satellite record have focussed on the period after 1978 when the satellite sensors have been relatively constant. Different estimates, obtained using different retrieval algorithms, produce very similar results for hemispheric extent, and all show an asymmetry between changes in the Arctic and Antarctic. As an example, an updated version of the analysis done by Comiso (2003), spanning the period from November 1978 through December 2005, is shown in Figure 4.8. The annual mean ice extent anomalies are shown. There is a significant decreasing trend in arctic sea ice extent of –33 ± 7.4 × 10³ km² yr^–1 (equivalent to –2.7 ± 0.6% per decade), whereas the antarctic results show a small positive trend of 5.6 ± 9.2 × 10³ km² yr^–1 (0.47 ± 0.8% per decade), which is not statistically significant. The uncertainties represent the 90% confidence interval around the trend estimate and the percentages are based on the 1978 to 2005 mean. In both hemispheres, the trends are larger in summer and smaller in winter. In addition, there is considerable variation in the magnitude, and even the sign, of the trend from region to region within each hemisphere.

Figure 4.8. Sea ice extent anomalies (computed relative to the mean of the entire period) for (a) the NH and (b) the SH, based on passive microwave satellite data. Symbols indicate annual mean values while the smooth blue curves show decadal variations (see Appendix 3.A). Linear trend lines are indicated for each hemisphere. For the Arctic, the trend is –33 ± 7.4 × 10³ km² yr^–1 (equivalent to approximately –2.7% per decade), whereas the Antarctic results show a small positive trend of 5.6 ± 9.2 × 10³ km² yr^–1. The negative trend in the NH is significant at the 90% confidence level whereas the small positive trend in the SH is not significant (updated from Comiso, 2003).

The most remarkable change observed in the arctic ice cover has been the decrease in ice that survives the summer, shown in Figure 4.9. The trend in the minimum arctic sea ice extent, between 1979 and 2005, was –60 ± 20 × 10³ km² yr^–1 (–7.4 ± 2.4% per decade). These trends are superimposed on substantial interannual to decadal variability, which is associated with variability in atmospheric circulation (Belchansky et al., 2005).

Figure 4.9. Summer minimum arctic sea ice extent from 1979 to 2005. Symbols indicate annual mean values while the smooth blue curve shows decadal variations (see Appendix 3.A). The dashed line indicates the linear trend, which is –60 ± 20 × 10³ km² yr^–1, or approximately –7.4% per decade (updated from Comiso, 2002).