|Working Group I: The Scientific Basis|
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Sea-ice extent (the area within the ice-ocean margin) was observed from space from 1973 to 1976 using the ESMR (Electrically Scanning Microwave Radiometer) satellite-based instrument, and then continuously from 1978 using the SSMR (Scanning Multichannel Microwave Radiometer) (1978 to 1987) and SSM/I (Special Sensor Microwave/Imager) (1987 to present). By inter-calibrating data from different satellites, Bjørgo et al. (1997) and subsequently Cavalieri et al. (1997) obtained uniform monthly estimates of sea-ice extent for both hemispheres from November 1978 through to December 1996. Over this period, the sea-ice extent over the Northern Hemisphere showed a decrease of -2.8 ± 0.3%/decade (Parkinson et al., 1999), consistent with Johannessen et al. (1995) (Figure 2.14). The Arctic decrease was strongest in the Eastern Hemisphere and most apparent in summer (Maslanik et al., 1996; Parkinson et al., 1999).
Hemispheric and regional data sets for the Arctic enable the satellite-derived trends in Figure 2.14 to be placed into a century scale context. Figure 2.15 shows annual time-series of the Northern Hemisphere ice extent by season from 1901 to 1999 using in situ data before the satellite era (Vinnikov et al., 1999a). It should be emphasised that the spatial coverage of earlier data is not complete, with the largest data voids in the autumn and winter. Because few data were available, the variability of the autumn and wintertime series in Figure 2.15 is smaller during the early decades of the century. Essentially no data for summer and autumn are available for the World War II period. The summer decrease that is largely responsible for the overall downward trend during the satellite era is present during the entire second half of the 20th century (Figure 2.15). This decrease represents about 15% of the average summer extent in the first half of the 20th century. Spring values show a somewhat weaker negative trend over the same period with a total reduction of near 8%, but there is only a slight and uncertain downward trend in autumn and winter since about 1970.
The overall recent decrease of Arctic ice extent is, at first sight, consistent with the recent pattern of high latitude temperature change, which includes a warming over most of the sub-arctic land areas (Section 126.96.36.199). Some of this pattern of warming has been attributed to recent trends in the atmospheric circulation of the North Atlantic Oscillation and its Arctic-wide manifestation, the Arctic Oscillation (Section 2.6).
Related to the decline in sea-ice extent is a decrease in the length of the sea-ice season (Parkinson, 2000) and an increase in the length of the Arctic summer melting season between 1979 and 1998, also derived from satellite data. The shortest season was 1979 (57 days) and the longest was in 1998 (81 days) with an increasing trend of 5 days per decade (Smith, 1998, updated). The 7% per decade reduction in the multi-year ice area during 1978 to 1998 is relatively large compared with an approximately 2%/decade decrease in the total ice area in winter (Johannessen et al., 1999). This reflects greater summer melting, consistent with the results of Smith (1998).
Over the period 1979 to 1996, the Antarctic (Cavalieri et al., 1997; Parkinson et al., 1999) shows a weak increase of 1.3 ± 0.2%/decade. Figure 2.16 (for 1973 to 1998) shows a new integrated data set of Antarctic sea-ice extent that was put together for the new European Centre for Medium-range Weather Forecasts (ECMWF) 40-year reanalysis that extends the record back to 1973. While showing the same weak increase after 1979, it also suggests greater ice extents in the mid-1970s. Although century scale time-series cannot be constructed for the Antarctic, de la Mare (1997) has used whaling ship logs to infer significantly greater ice extent in the Southern Ocean during the 1930s and 1940s than during recent decades. The indirect nature of the earlier evidence, however, introduces substantial uncertainty into this conclusion.
Antarctic Peninsula ice shelves
Wadhams and Davis (2000) have compared ice thickness changes between October 1976 and September 1996 between 81°N and 90°N near the 0° meridian. The overall decline in mean sea-ice thickness between 1976 and 1996 was 43%. Over every one degree of latitude, both a significant decline in ice thickness and some completely open water were observed. Despite these dramatic results, it is not known whether these changes reflect long-term change or a major mode of multi-decadal variability. Vinje et al. (1998) measured the thickness of ice exiting the Arctic Ocean through Fram Strait from 1990 to 1996 using moored upward looking sonars and reported a rather different result. The mean annual ice thickness in Fram Strait varied from 2.64 to 3.41 m. These observations were consistent with Arctic Ocean-wide ice thickness estimates made by drilling from Soviet Ice Stations from 1972 to 1981 and from submarine transects from 1960 to 1982, suggesting little change in ice thickness from the 1960s and 1970s to the 1990s.Nagurnyi et al. (1994, 1999) used measurements of long surface gravity waves in the Arctic ice pack to estimate the mean ice thickness from wave attenuation. These measurements are available for the winters of 1978/79 to 1990/91. Johannessen et al. (1999) demonstrated a strong correlation between these ice thickness estimates and the area of multi-year (MY) ice in the Arctic Ocean as obtained from the SSMR and SSM/I. Both the area of MY ice and the ice thickness (winter) estimates show a decrease of 5 to 7%/decade, considerably less than the submarine estimates (late summer). Even though the satellite measurements have continued for more than twenty years, they are inadequate to distinguish between changes due to long-term trends or interannual/inter-decadal variability (Johannessen et al., 1999).
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