Geology Reference
In-Depth Information
15.2.1. Ground-Thermal Regimes
Measurements in northern Canada, Alaska, Siberia, and Tibet all suggest that global
climate change is already affecting permafrost temperatures and the thickness of the
active layer (Osterkamp and Jorgenson, 2006; Pavlov, 1998; Pavlov and Moskalenko,
2002). A number of permafrost monitoring stations in the sub-arctic regions of Europe
and Russia indicate that, during the period 1970-1990, there was a rise of 0.6-0.7 °C in
the temperature of permafrost at a depth of 3.0 m (Pavlov, 1994). In northeast Siberia,
permafrost at a depth of 10 m increased by 0.03 °C/year between 1980 and 1991. Data from
the Qinghai-Xizang (Tibet) Plateau, China, indicate that temperatures at the 20 m depth
have risen by 0.2-0.3 °C during the last two decades (Table 15.2). Because the Tibet
Plateau is sparsely populated and human disturbances are minimal, it is clear this perma-
frost degradation is not caused by agriculture or human-induced terrain disturbance but
by climatic amelioration. Air temperatures recorded at Wudaoliang indicate that the ten-
year running means for 1971 and 1980 are 0.5 °C and 0.7 °C warmer than those for 1961
and 1970 (Wang, 1993). The lower altitudinal limit of permafrost in the Kunlun Shan and
adjacent mountains has risen.
Table 15.2.
Recent trends in ground thermal regimes, Qinghai-Xizang (Tibet) Plateau.
(a) Rise in mean annual ground temperature at Fenguo Shan, 1962-1989
Year:
1962
1963
1967
1976
1979
1980
1984
1989
T(°C)
−
3.5
−
3.5
−
3.5
−
3.4
−
3.3
−
3.3
−
3.3
−
3.3
(b) Decrease in permafrost thickness along northern boundary of permafrost zone adjacent to
the Qinghai-Xizang Highway
Year:
1974
1979
1985
1989
Average th ick ness (m) :
15
14
12
10
Source: S. Wang (1993); quoted in Wang and French (1994).
Similar trends can be observed in many parts of northern Canada. For example, at
Mayo, central Yukon Territory, the mean annual ground temperature at the depth of zero
annual amplitude (10 m) is
1.3 °C, yet upward projection of the temperature gradient
suggests a former mean temperature at the surface of permafrost of approximately
−
2.0 °C
(Burn, 1992). Geothermal modeling indicates that a period of 20 years is required for such
a temperature change. It is suggested that increased snowfall and warmer winter tempera-
tures are probably the cause of this ground warming. At the same time, some permafrost
bodies along the southern boundaries of the permafrost zone in central Canada have
either thinned or disappeared.
These trends, while widespread, are not universal in all areas of high latitude. For
example, M. Allard et al. (1995) document a cooling trend in permafrost at the 20 m depth
of approximately 0.05 °C/year for the period 1988-1993 at several coastal sites in northern
Québec, Canada. Because the active-layer thickness remained unchanged during the
period of analysis, it is suggested that the change is related to changes in the oceanic
thermo-haline circulation system in the Arctic and North Atlantic oceans. They conclude
that climate cooling takes place principally through longer and colder winters.
Collectively, these data mask considerable regional and site variability.
−
15.2.2. Thickness of the Active Layer
In general, the active layer responds to an increase in air temperature by an increase in
thickness. Obviously, the exact amount will depend upon the magnitude of the thermal
