Temperature (˚C)
0.5
5 m depth
0
8 m depth
-0.5
-1
-1.5
1974 1975 1976 1977 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000
-4
-4.25
-4.5
-4.75
Seasonal thawing
Seasonal freezing
-5
Thickness (m)
Figure 7.9: Permafrost temperatures and active-layer thickness during 1974–1977 and 1990–2004 measured
in a borehole at the “Cosmostation” permafrost observatory, 3300 m above sea level, Northern Tien Shan
Mountains in Central Asia.
Source: Based on Marchenko and others 2007
82
China 35.6 per cent between 1974 and 1996
84
, while the perma-
frost area of the northern Qinghai-Tibet highway decreased
Permafrost in China has degraded significantly during the by 12 per cent between 1975 and 2003
85
. In the Xing’anling
past 40 years
79,83
, with changes observed in both permafrost Mountains in northeast China, many patches of permafrost
extent and temperature. The area of the southern Qinghai- have disappeared
86
. The permafrost area in China is project-
Tibet highway with underlying permafrost decreased by ed to decrease by 30-50 per cent during the 21st

century
79,87
.
CHAPTER 7 FROZEN GROUND 195