Tibetan Plateau under the Mercy of Climate Change and Modernisation[We (at the Environment and Development Desk at DIIR, CTA)
would like to alert all our readers about the impacts of Climate Change
and Unplanned Urbanization on the Tibetan Plateau in series of short
articles] Permafrost Degradation and Glacial Meltdown Retreating glaciers and Permafrost DegradationMany
glaciers in Tibet are retreating due to the ongoing changes in the
climatic variables over the plateau. The dramatic retreat of all the
glaciers on the Tibetan Plateau in 1990’s is related to the continuous
warming on the plateau since 1980s. In a more recent study about the
sensitivity of cold glaciers, it was indicated that changes (rise)
in the air temperature affects the glacier runoff more sensitively than
other variables. The warmer air coupled with less snow and more rain
would significantly increase the glacial runoff. In other words, less
snow and more rain would mean decrease in the surface albedo (resulting to more solar heat absorption and more glacial runoff).
Chinese scientist also predicts that 2/3rd of the present glacial cover
will disappear by the end of this century. And most of all, to further
increase the current worries, there has been no net accumulation of
snow since 1950 over these mountains. According to Jane Qui (2008), 82%
of the Tibetan Glaciers have already retreated in the past half
century. In the past 40 years, Tibet’s glaciers have shrunk by 6,600 sq
km (as of year 2006). It is estimated that they are currently melting
at a rate of 7% per year (figures 1 a & b). Gou et al (2006), has even developed a tree-ring width chronology (from southern slope of Qiemuqu valley – the first major branch of the Yellow River)
showing that trees in the area recorded glacier variations on the
northeastern Tibetan Plateau. They also indicated that the glaciers on
the edge of the mountain regions are more sensitive to climate change;
as a result a dramatic retreat of Glaciers on the Eastern and Southern
part of the plateau were observed.
Figures 1 a & b: Halong Glacier (1985 & 2007), Image source: John Novis/ Greenpeace |
Recent
studies have shown that glacial melting and thawing of permafrost in
the Tibetan Plateau will lead to a large scale release of GHGs
(greenhouse gases) in the atmosphere and bringing further changes in
the already warming climate. The permafrost layers and the seasonally
frozen soils on the Tibetan Plateau are well preserved over a long time
by low winter air temperatures. Its seasonal thawing and refreezing
have also played a vital role in balancing the delicate alpine
vegetations, until now. With the significant increase in the mean cold
season average temperature, the permafrost layers and seasonally frozen
grounds are slowly degrading leading to increased microbial
decomposition of previously frozen organic carbon. Slope failures are
often seen on the Tibetan Plateau due to permafrost degradation. Permafrost
degradation due to the warming climate has changed the regime of water
retention and regulation by producing more runoff in areas of
permafrost, leading to more evaporation. It has also prolonged the
thawed period significantly leading to many interconnected ecological
changes and technological worries. Locally, this degradation would
result in the gradual desertification of grasslands, resulting to
higher surface albedo and increased ground temperature. The
Tibetan Plateau rests on a seasonally frozen ground and permafrost
layers. Almost 65 percent of its total area (2.5 Million Sq. Km) sits
on permafrost (figures 2 a & b). With the warming climate and the positive feedbacks, these permafrost layers are in a process of continuous degradation. In
general, permafrost plays an important role in regulating water and has
a significant influence on maintaining the plateau’s high-elevation
ecosystems. Active layer of permafrost undergoes thawing in the summer,
helping to drain away water into the wetlands and vast grasslands.
Studies have indicated that the freeze and thaw cycles in the Earth’s
surface intensify the heat and water exchange between the atmosphere
and ground surface. This, in turn, affects the climate – as it does in
East Asia. For instance, with the rise of 0.052°C/ y (air temperature),
the permafrost area on the Tibetan Plateau will reduce about 195, 000
km2 (13%) and over 700,000 km2 (46%) within next 50 and 100 years
respectively. In part, solar radiation is also responsible for the
accelerating the thawing process, the areas below 5000 m are receiving
radiation higher than all part of the china, about (250 – 360) J/ cm2.
This will lead to the rise in the daily maximum temperature, at times
reaching above 0 °C (even
in cold winter months, Nov–Feb). Such instances lead to the frequent
and strong thawing–freezing processes in the active layer.
Figures 2 a & b: Frozen & seasonally frozen grounds on the Tibetan Plateau |
Besides
the hydrological regimes, the permafrost degradation or thawing in many
regions of Tibetan Plateau are affecting the surrounding vegetation as
well as engineered structures. H. Jin et al (2000 & 2008)
in their study, along the SLH (Siling Lhasa Highway) corridor have
found that those areas covered by asphalt road surface showed
significantly higher degradation ‘or’ depths of permafrost layer and
higher mean annual ground temperature (MAGT). They also found that the
heat accumulation under the asphalt road bases resulted in increased
thaw depths too great to be frozen and consequently resulting in the
transformation of vertically connected permafrost into disconnected
phase. In other words, the road construction/ renovation have resulted
to a faster degradation of the permafrost layer, compared to the
natural state. According to U.N. Environment Programme (UNEP),
the frozen soil of the Tibetan plateau has warmed about 0.3°C over the
past 30 years. Where human activity has disturbed the soil, such as
during the construction of the railway, the rate is double, 0.6°C.Recent
studies showed that permafrost thawing in many regions of the Tibetan
Plateau is influencing both hydrological regimes and vegetation, as
well as engineered structures. According to Cheng and Wu (2007), the
permafrost on the Tibetan Plateau is thawing more forceful than in
Alaska. Such thawing of permafrost leads to the formation of thaw
slumps, slope failure and in the process will inject lots of trapped
carbons in the atmosphere, let alone for the lost of vegetations and
carbon fixation. The conditions of permafrost degradation, as observed by Jin et al (in 1999 or earlier) at the Huashixia Permafrost Station (about 70 km from Matoe; Ch: Maduo) was
such that the active layers were oversaturated by water either from
glacial runoff or rainfall during the thawing season (see box 1 for
more detail). It was observed that the increase in air
temperature was more significant in cold season than in warm season
leading to rise in the mean annual ground surface temperature (MAGST).
Increase in the mean cold season air temperature plays a major role in
changing the landscapes dominated by seasonally frozen grounds. In a
separate study by H. Jin et al (2008), they indicated that the
ground temperature at the shallow depths in transition and quasi-stable
permafrost zone have been increasing noticeably, as a result leading to
the shrinkage of permafrost boundaries. For instance, along the SLH
(Siling Lhasa Highway/ Ch: QTH), the southern lower limit of permafrost
have moved 10 km northward and the northern lower limit has moved 3 km
southward. They also reported that the grounds temperature (at the depth of 6- 15 m) at
Kunlun Pass have increased (02 – 04) °C in the past 15 years (1982 –
1997). Some other researchers have pointed out that approximately 30%
of the SLH has to be repaired every year due to damage cause by frost
action (see box 1 for more detail).
Figures 3 a & b: Mass-movement feature, possibly an active-layer detachment slide, developed during summer of 1996 on the south side of the Kunlun Pass (Tibet); Thermokarst pits developed in response to removal of vegetation by humans during the Siling-Lhasa (Ch:Qinghai-Xizang) railroad construction near Fenghuo Shan.
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The
maximum depth of the permafrost table under the asphalt road surface
(near Kunlun Mountains) was recorded 5.2 m compared to 2.8 m under the
normal state; and the MAGT ranging from (-1.0 to -0.2)°C under the
asphalt road and (-2.5 to -3.5) for the normal state. In a separate
study by Wei et al (2006), they found (in 2003) an arc shaped thaw
slumping area near Fenghuaoshan Mountain measuring up to 73 m wide and
103 m long with the total volume failure about 10,000 m3 caused by the
repairing of SLH. Their survey results showed that the thaw slum has
been active for more than 10 years compared to the general thaw
slumping life year of 3- 5 years and it will further remain active
until the ice rich permafrost has completely disappeared. Similar
thawing of permafrost was observed in an excavation in the Kunlun Pass
(elevation of 4715m in Amdo province) in September 1997. There, an oil
pipeline, with a diameter of 168 mm (built in 1973 along the SLH), has
induced a summer thaw depth of much higher magnitude (140–150 cm)
compared to the nearby areas (90 cm). In consistent with that, due to
the thaw settlement, the bridges and water conduits have also been
damaged considerably. In a separate study in Amdo province, a
permafrost drilling sample (at a depth of 3.5 m) taken in 1975 recorded
the permafrost depth/ thickness to be 6.5 m. In July 1989, no frozen
layer was detected in the same site, it had completely thawed.Permafrost
temperature at the source areas of the Drichu (Yangtze) and Machu
(Yellow) Rivers rose by (0.11–0.14) °C between 1980 and 1998 and the
active layer thickness increased at rates ranging from 2 to 10 cm/yr.
The base of sporadically distributed permafrost in the source region of
the Machu (Yellow) Rivers climbed around 50-70m in elevation between
the 1970s and 1990s. The severe grassland degradation in the Nagchu
area is also related to the degradation of permafrost. These changes in
permafrost affect moisture contents in the soil, carbon exchange
between the underground and the atmosphere. It was also found that the
groundwater level at the head regions of Drichu (Yangtze) and Machu are
dropping annually at the rate of 10 cm. These changes in
permafrost affect moisture contents in the soil, carbon exchange
between the underground and the atmosphere. Such degradation also leads
to the lowering of the water table, lake water levels and the shrinking
of wetland and grazing grassland. This greatly affects farmers, for
whom the growing season begins well before the rains of summer. The
snow cover and frozen ground in winter melt sooner as climate change
causes the plateau to warm sooner and stay unfrozen longer. Data
collected by Cheng and Wu (2007), indicated that there is a close
relationship between the occurrence of sandstorms in China and the
minimum freezing depth (of Permafrost) on the Tibetan Plateau. (The
lesser the depth of permafrost, drier the top soil becomes and gets
blown away in the form of sandstorms towards the windward side) The
sands of the Tibetan Plateau can be transported by westerly winds to
eastern China and even places as far away as the North Pacific.
According to some other researchers, by 2040 (with the average increase in the mean annual ground temperature -MAGT by 0.4 to 0.5°C) most
of the permafrost on the Tibetan Plateau will disappear based on the
analyses of air temperature and fluctuation in the precipitation over
East Asia.We (EDD) argue that, the degradation of permafrost in
the head regions of Machu and Drichu and the annual drop of water table
(due to the climate change) has triggered a serious threat to the water
security of China in the coming years. These natural factors have also
led to the forceful removal of Tibetan nomads from their ancestral pasturelands. Permafrost: The subsurface earth materials remaining below 0°C for two or more yearsActive layer:
The top layer of permafrost soil that thaws during the summer and
freezes during winter. The temperature in the lower levels of the soil
will remain more stable than that at the surface, where the influence
of the ambient temperature is greatest. This means that, over many
years, the influence of cooling in winter and heating in summer will
decrease as depth increases
Box 1. The onset Permafrost degradation: Construction of SLH- Siling Lhasa HighwayThe construction of Siling Lhasa Highway- SLH (Ch: Qinghai-Tibet Highway – QTH) about References: 1.Abrahm |