Northern Hemisphere trends

Despite considerable year-to-year variability, significant negative trends are apparent in both maximum and

Ice extent anomaly (million km2)

1980 1985 1990 1995 2000 2005

Figure 5.4: Time series of the difference in Arctic sea-ice extent in March (maximum) and September (minimum) from the mean values for the time period 1979-2006. Based on a linear least squares regression, the rate of decrease in March and September was 2.5% per decade and 8.9% per decade, respectively. Source: Data courtesy of National Snow and Ice Data Center (NSIDC)

1980 1985 1990 1995 2000 2005

Figure 5.4: Time series of the difference in Arctic sea-ice extent in March (maximum) and September (minimum) from the mean values for the time period 1979-2006. Based on a linear least squares regression, the rate of decrease in March and September was 2.5% per decade and 8.9% per decade, respectively. Source: Data courtesy of National Snow and Ice Data Center (NSIDC)

minimum ice extents, with a rate of decrease of 2.5 per cent per decade for March and 8.9 per cent per decade for September (Figure 5.4).

There are major regional differences (Figure 5.5), with the strongest decline in ice extent observed for the Greenland Sea (10.6 per cent per decade). The smallest decreases of annual mean sea-ice extent were found in the Arctic Ocean, the Canadian Archipelago and the Gulf of St. Lawrence. In the marginal Arctic seas off Siberia (the Kara, Laptev, East Siberian and Chukchi Seas) a slight negative, but not significant, trend in ice extent was observed between 1900 and 2000s.

Figure 5.6 compares the Arctic sea-ice extent in September for the years 1982 (the record maximum since 1979) and 2005 (the record minimum). The ice extent was 7.5

Figure 5.5: Regional changes in Arctic annual mean sea-ice extent (% per decade) for the period 1979-2004.

Source: Data courtesy of NASA 2007a9

Northern Hemisphere Whole N Hemisphere Greenland Sea Baffin Bay Sea of Okhotsk Kara-Barents Sea Hudson Bay Bering Sea Arctic Ocean Gulf of St Lawrence Canadian Archipelago

Figure 5.5: Regional changes in Arctic annual mean sea-ice extent (% per decade) for the period 1979-2004.

Change in annual mean sea ice extent (% per decade)

million km2 in 1982 and only 5.6 million km2 in 2005, a difference of 25 per cent. As has been observed in other recent years, the retreat of the ice cover was particularly pronounced along the Eurasian coast. Indeed, the retreat was so pronounced that at the end of the summer of 2005 the Northern Sea Route across the top of Eurasia was completely ice-free (see section below on shipping and tourism).

Ice extent is only part of the equation. To assess changes in ice cover it is also important to look at ice thickness - however ice thickness is difficult to monitor and measurements are much more limited. Satellite-based techniques have only recently been introduced and there is no comprehensive record of sea-ice thickness. There are many datasets of ice thickness from measurements taken opportunistically, including holes drilled through the ice, observations from ships, upward-looking sonars moored at the sea floor10, and above-ice surveys using laser techniques and electromagnetic sensors11.

The most comprehensive source of ice-thickness observations were the sonar profiles made from submarines cruising under the Arctic ice cover from the 1950s to the 1990s. These observations were made irregularly, but researchers were able to group them for comparison into seven regions and into two time periods. Rothrock and others12 concluded from these records that a substantial thinning of the ice occurred in several regions between the period 1956-1978 and the 1990s, with an overall 40 per cent decrease in thickness from an average of 3.1 m to 1. 8 m. Other later publications dealing with analyses of submarine-based sonar data conclude that the thinning rates may have been less than this13,14.

Figure 5.6: Arctic sea ice minimum extent in September 1982 and 2005. The red line indicates the median minimum extent of the ice cover for the period 1979-2000. The September 2005 extent marked a record minimum for the period 1979-2006.

Source: Data courtesy of National Snow and Ice Data Center (NSIDC)

Figure 5.6: Arctic sea ice minimum extent in September 1982 and 2005. The red line indicates the median minimum extent of the ice cover for the period 1979-2000. The September 2005 extent marked a record minimum for the period 1979-2006.

Source: Data courtesy of National Snow and Ice Data Center (NSIDC)

Thickness of land-fast ice is monitored from coastal sites in Arctic Canada, Svalbard and Siberia8,15,16. Most sites show large variations among years and among decades. Data extending back to 1936 from sites off the coast of Siberia show, in general, no significant trends up to 2000s. Consistent observations at Svalbard do not go that far back in time, but monitoring during the last decade showed that during the warmer-than-normal winters of 2005/2006 and 2006/2007 the land-fast ice in most Svalbard fjords was less extensive, thinner and lasted for a shorter time than normal.

The age of sea ice in the Arctic is also changing. Studies show that in recent years there is a higher proportion of younger ice to older ice than was observed in the late 1980s6 (Figure 5.7).

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