A similar lack of temperature trend is found for warm and In environments containing permafrost, the top layer
thin permafrost in the southern Yukon Territory
19,20
. (active layer) of soil thaws during the summer and
freezes again in the autumn and winter. Trends in the
Warming of permafrost is also observed in the eastern depth of this active layer are less conclusive than trends
and high Canadian Arctic but this appears to have main- in permafrost temperature. In the North American Arc-
ly occurred in the late 1990s. At Alert, Nunavut, a warm- tic, the depth of the active layer varies strongly from
ing of 0.15°C per year occurred between 1995 and 2001 year to year
24–26
. An increase in active-layer thickness
at a depth of 15 m and warming of about 0.06°C per year was reported for the Mackenzie Valley in Canada
27
.
has occurred since 1996 at a depth of about 30 m
8
. At an- However, after 1998 the active layer began decreasing
other high Arctic site, shallow permafrost (upper 2.5 m) in thickness at most of the same sites
28
. An increase in
temperatures increased by 1°C between 1994 and 2000
21
. thickness of more than 20 cm between the mid-1950s
At Iqaluit in the eastern Arctic, permafrost cooled be- and 1990 was reported for the continuous permafrost
tween the late 1980s to the early 1990s at a depth of 5 m regions of the Russian Arctic
29,30
. At the same time, re-
and warmed by 0.4°C per year between 1993 and 2000
7
. ports from central Yakutia show no significant changes
A similar trend was observed in northern Quebec
22,23
. in active-layer thickness
31,32
.
Thawing permafrost along the bank
of the Kolyma River in Siberia.
Photo: V. Romanovsky
CHAPTER 7 FROZEN GROUND 185