沙漠研究 22-1, 119 -122 (2012 )"ICAL 1 / DT X"

Journal of Arid Land Studies－Refereed Paper－

Climatic Warming due to Overgrazing on the Tibetan Plateau- An Example at Damxung in the Central Part of the Tibetan Plateau Mingyuan DU*1), Seiichiro YONEMURA1), Xianzhou ZHANG2), Yongtao HE2),
Jingshi LIU3) and Shigeto KAWASHIMA4)
Abstract: Many studies have shown that the increase in air temperature on the Tibetan Plateau is greater than that for the NorthernHemisphere and the same latitudinal zone. Recent paper has pointed that this warming; especially warming in wintertime is clearer atlower altitude than at higher altitude on the Tibetan Plateau and the local environmental changes such as urbanization and land coverchange may play very important role for the geographical changes of recent climate warming. However, there is no any concrete
evidence to support this hypothesis. This paper identifies the relationship between overgrazing and climate warming at Damxung in
the central part of the Tibetan Plateau by using recent 47 years data of Damxung meteorological observatory station and 5 years
intensive observations data on a pasture on a mountain slope where overgrazing is occurred in the lower part of the slope.
Meteorological data reconstruction of the pasture is carried out by using the relationships between the observation data on a pastureand the meteorological observatory’s data. Annual air temperature both at the meteorological observatory and at the overgrazed
pasture has increased about 2 degrees during past 47 years and this extreme air temperature increasing at Damxung is mainly causedby the degradation of grassland due to overgrazing. It provides evidence that degradation of grassland due to overgrazing promote
the climate warming and thereby the land and vegetation degradation process.
Key Words: Climatic warming, Data reconstruction, Land degradation, Overgrazing, Tibetan Plateau.

1. Introduction
It is well known that the thermal and dynamical processesof Tibetan Plateau (TP) have a profound influence both toclimate and to ecosystems of the Asian continent and even thewhole world. Research on the climate and cryosphericchange in the TP has received increasing attention since
mid-1970s as Kang et al. (2010) reviewed. Many works
showed that the main portion of the TP has experienced
statistically significant warming since the mid-1950s,
especially in winter, which exceed that of its surrounding
regions (e.g. Liu and Chen, 2000, Du et al., 2004, Lu and Liu,2010). Most of the works assumed this is the global warming
effects. However, Frauenfeld et al. (2005) pointed out that
“The significant trends in station data may reflect the extensive
land use change and industrialization that has occurred on theTibetan Plateau” because no trends are observed in ERA-40
reanalysis data. Du et al. (2004, 2010) pointed that
degradation of grassland on the TP should have been affected
the climate change on the TP and may be one of the reasons
why climate warming on the TP is greater than on other places
and why recent warming is clearer at lower altitude than at
higher altitude on the TP. However, there is no any

confidential evidence to support this hypothesis due to lack ofdata. This paper is an attempt to clarify the relationshipbetween overgrazing and climate warming at Damxung in thecentral part of the TP by using recent 47 years data of
Damxung meteorological observatory station and 5 years
intensive observations data on the mountain slope pasture,
where overgrazing is occurred in the lower part of the slope.

2. Data and Methods
2.1. Observation site description
In order to clarify the local climate features, we haveselected a pasture on a south facing slope in the central part of
the TP. Figure 1 shows the main topography over the TP and
the observation sites. Damxung meteorological observatory
station is located near the small town Damxung in the valley(about 4280m a.s.l.). The low and middle part of themountain slope (between 4300 m and 5200 m a.s.l.) is used aspastures, where overgrazing is occurred in the lower part (4300m to 4600 m) of the slope (Ohtsuka et al., 2008). We set up
11 observation stations on a south slope of Mount
Nyainqentanglha as shown in Figure 1 (down) at Damxung(30º28-32’N，91º02-03’E) in 2005. One of the 11 stations is abasic automatic station at about 4300 m in the valley. All the* Corresponding Author: dumy@affrc.go.jp
3-1-3 Kannondai, Tsukuba, Ibaraki, 305-8604 Japan1) Department of Agro-Meteorology, National Institute for Agro-Environmental Sciences, Japan3) Institute of Tibetan Plateau, Chinese Academy of Sciences, China

2) Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, China

4) Kyoto University, Japan

Fig. 2. Observed Air temperature variation during 1962 to 2010 atDamxung.

Fig. 1. The main topography over the Tibetan Plateau (up) andobservation stations (down). The Tibetan Plateau is shown as
the shaded region where elevation is higher than 3000 m a.s.l..
Black contour lines show topography with an interval of 1000 m(up) and 100 m (down). The triangles indicate the observation
stations.

meteorological elements including short and long wave
radiations, wind speed and direction, air temperature andhumidity, soil temperature and water content etc. weremeasured there by using measurement system of Campbell
Scientific, Inc. Data were recorded with a data-logger(CR10X; CSI) at 30 min intervals. Measurement system on
the other 10 stations are Hobo weather stations (Onset
Computer Corporation, Bourne, MA, USA) settled at 4300 m,4400 m, 4500 m, 4650 m, 4800 m, 4950 m, 5100 m, 5200 m,5300 m and 5530 m on a south slope. Stations at 4300 m arelocated on a wet land. Stations at 5300 m and 5530 m arelocated on a bare land and others are located in the pasture.
Air temperature and humidity at 2 m and soil temperature andsoil water content at 5 cm, 20 cm and 50 cm are sampled at 1
min intervals and the mean value were recorded with adata-logger at 30 min intervals. The air temperature datafrom August, 2005 to July, 2010 are used in this study.
2.2. Data and method
Daily mean of air temperature (T), minimum andmaximum air temperature (Tmin and Tmax), air pressure (P),absolute and relative humidity (e and RH), low and total cloudamount (Cl and C), wind speed (V), surface temperature (Ts),precipitation (Rain), sunshine hour (S) and pan evaporation(Et) of the Damxung meteorological observatory station from

August 1, 1962 to June 30, 2009 are used.
Stepwise linear regression method was used for getting
relationship between daily mean air temperature on the slope
and the meteorological elements of the Damxung
meteorological observatory station by using SAS software(SAS institute inc., Cary, NC, USA) with t-test at statisticalsignificance determined at the P < 0.05 level. Due that themechanism of local temperature varies for different seasons,
we get the relationships for each month. Therefore,k =mTcalij = aij + ∑ bijk Ek

(1)

k =1

where, Tcalij is estimated air temperature for station i month j(i=1, 10; j=1, 12) . aij and bijk are regression coefficients ofstation i month j for meteorological elements Ek (Ek=T, Tmax,Tmin, P, e, RH, Cl, C, V, Ts, Rain, S, Et).
We assume that the relationship between daily mean airtemperature on the slope and the meteorological elements of
the Damxung meteorological observatory should reflect theeffects of land degradation by overgrazing and may be thesame in the past 47 years. Therefore, we can use therelationship to reconstruct the daily mean air temperatureduring 1962 to 2004 on the slope and then to calculate the 18years mean temperature for 1962-1980 due to there was almost
no temperature increase during this period as shown in Figure
2. Therefore, we can compare the temperature differences
between the observed mean of 2005 to 2010 and the estimatedmean of 1962-1980.

3. Results and Discussion
3.1. Climate warming at Damxung
As shown in Figure 2, observed air temperature atDamxung meteorological observatory shows statisticallysignificant increase trend for each month, especially in wintertime (0.02°C /year in July and 0.087°C /year in January).
Increase in daily minimum air temperature is clearer (0.022°C/year in July and 0.114°C /year in January). Annual air

Fig. 3. Vertical distribution of vegetation coverage and root biomassalong the south slope (redraw from Ohtsuka et al., 2008 and data
at 4300m is with personal communication)

Fig. 4. Observed and estimated air temperature variation along theslope during 2005 to 2010 and 1962 to 1980.

temperature at the meteorological observatory has increased
about 2 degrees during past 47 years. This increase in air
temperature is higher than other places on the TP at similar
altitudes and surrounding area.

described. However, air temperature at the lower part of theslope is little bit higher than the lapse rate predicted (right up
the lapse line shown in Figure 4) in summertime and higher inwinter partly due to the temperature inversion.
Figure 4 also shows the estimated mean temperature (1962-1980) distribution along the slope. It shows a similar
distribution pattern as observed one (mean of 2005 to 2010)
with significant increase of air temperature throughout theslope, especially in January. However, there is evidently highvalue of the temperature in the overgrazed lower part of theslope. Therefore, the lapse rate in summer changes from
0.72ºC/100m to 0.69ºC/100 m. Although this value is stilllarger than that of averaged over the TP, this value can betreated as a natural environmental, which do not induced landdegradation. Lapse rate at higher part of the slope changes
from 0.79ºC/100m to 0.72ºC/100m in wintertime.

3.2. Overgrazing and vegetation cover change
Figure 3 shows the distribution of vegetation coverage andand relation between root biomass and elevation along thesouth slope in 2005. It can be seen that both vegetation
coverage and root biomass at the lower part of the slope(between 4400-4650 m a.s.l.) was smaller than that at themiddle part of the slope and that in the valley (4300 m). Thisis due to the overgrazing there. The middle part of the slope
has been used as summer pasture and the valley wet land has
been used as winter pasture, while the lower part of the slope isbeen used throughout a year because most of herdsmen areliving at the lower part of the slope. Interviews with theherdsmen let us know that this overgrazing was occurred
around 1980 and becoming more and more remarkable with
the increase in livestock numbers as shown by Du et al. (2004).
3.3. Air temperature distribution along the slope and itschanges
As shown in Figure 4, observation data (mean of 2005 to2010) along the slope shows a uniform lapse rate about 0.72
ºC/100m along the slope, in summer season. This lapse rateis greater than that of averaged over the TP (Du et al., 2010).
However, there is an inversion layer at lower part of slope(below 4800m) and a relatively larger lapse rate about 0.79
ºC/100m at higher part of the slope in wintertime. This is due
to strong radiation cooling based on lower humidity and cold
air run-off from a higher mountain region into the valley by alocal circulation, the air temperature in the valley becomes
lower and lower during night in wintertime. This process
occurred and a temperature inversion layer exited almost
occurred everyday during wintertime as Du et al. (2009)

3.4. Relationship between overgrazing and climate
warming at DamxungLand degradation by overgrazing can affect climate change(e.g. Jackson and Idso, 1975; Balling, 1998; Du, 1996).
Many researchers have found that decrease in vegetation cover
reduces evapotranspiration thereby allowing an increase inlocal temperature levels. Balling (1998) has revealed that
overgrazing and consequently land degradation in semiarid
areas of northern Mexico resulted in significantly airtemperatures increase both in northern Mexico and in its
adjacent Arizona, USA.
Damxung meteorologicalobservatory is located very near to the overgrazing pasture.
Therefore, air temperature increase has coincided with theprogress of overgrazing since 1980 as shown in Figure 2.
Moreover, the land degradation effect were reflected upon
not only the air temperature but also all the observed
meteorological elements at Damxung meteorologicalobservatory. For example, humidity (e and RH) and cloudamount (C and Cl) have decreased and sunshine hour (S) has

Table 1. Comparison of regression coefficients of station 4400 m and 5300 m.
b ij

Station &montha ij

TT max4400m Jan.

-1.798

0.829

0.342

5300m Jan.

-13.593

0.341

0.804

P

4400m Jul.

-58.451

0.094

0.768

0.196

5300m Jul.

-182.873

0.287

0.833

0.187

T mineRH
0.036-0.337(Continue of Table 1.)
Station &monthb ijCClVTs

4400m Jan.

-0.1825300m Jan.

-0.2664400m Jul.
5300m Jul.

RainSEt-0.086-0.0180.015

increased slightly. Air temperature at the pasture andsurrounding area has been also affected by the land degradation.
These variations have effects on air temperature at different
area of the slope pasture with the relationship obtained by thestepwise linear regression. Table 1 shows the comparison ofregression coefficients for July and January of station 4400 mand station 5300 m. As shown in Table 1, despite theregression coefficients differences, there are two moreelements (RH and S in January, Tmin and C in July) selectedfor station 5300 m. Since RH and C are decreased and Tmin
and S is increased and air temperature at 5300 m is directly
proportional to RH and C and inversely proportional to S andTmin, this is one of the reasons of increase amount of airtemperature at 5300 m is smaller than that at 4400 m.

4. Conclusion (or Recommendation)
Both observed air temperature at the meteorologicalobservatory and estimated air temperature at the overgrazed
pasture at Damxung has increased about 2 degrees during past
47 years and this extreme air temperature increase is mainly
caused by the land degradation due to overgrazing byfollowing reasons.
(1) 47 years observed air temperature at the meteorologicalobservatory has increased since 1980 which coincides
with the progress of the overgrazing.
(2) 5 year observed air temperature at the pasture shows
relative higher value at the overgrazed area.
(3) Estimated air temperature at the pasture shows moreincrease amount at the overgrazed area than other area.

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