million km
2
in 1982 and only 5.6 million km
2
in 2005, a the ice, observations from ships, upward-looking sonars
difference of 25 per cent. As has been observed in other moored at the sea floor
10
, and above-ice surveys using
recent years, the retreat of the ice cover was particularly laser techniques and electromagnetic sensors
11
.
pronounced along the Eurasian coast. Indeed, the re-
treat was so pronounced that at the end of the summer The most comprehensive source of ice-thickness obser-
of 2005 the Northern Sea Route across the top of Eurasia vations were the sonar profiles made from submarines
was completely ice-free (see section below on shipping cruising under the Arctic ice cover from the 1950s to the
and tourism). 1990s. These observations were made irregularly, but re-
searchers were able to group them for comparison into
Ice extent is only part of the equation. To assess chang- seven regions and into two time periods. Rothrock and
es in ice cover it is also important to look at ice thick- others
12
concluded from these records that a substantial
ness – however ice thickness is difficult to monitor and thinning of the ice occurred in several regions between
measurements are much more limited. Satellite-based the period 1956–1978 and the 1990s, with an overall 40
techniques have only recently been introduced and there per cent decrease in thickness from an average of 3.1 m
is no comprehensive record of sea-ice thickness. There to 1. 8 m. Other later publications dealing with analyses
are many datasets of ice thickness from measurements of submarine-based sonar data conclude that the thin-
taken opportunistically, including holes drilled through ning rates may have been less than this
13,14
.
1982 2005
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)
CHAPTER 5 ICE IN THE SEA 69