Abstract
1 Introduction
Many recent studies have focused on vegetation feedback to climate systems in sensitive areas like the Qinghai-Tibetan Plateau. Providing allowances and awards to households engaged in animal husbandry that practice grazing exclusion to restore degraded grassland is an important eco-compensation policy effort in China. Grazing exclusion influences grassland variations (
The earth’s climate system tends to be in equilibrium between the absorption of solar radiation and the reflection of surface radiation (
The ecological environment of the northern Tibetan Plateau is very fragile, and the surface heat balance in this region has great significance for changes in the climate of Asia and even globally (
2 Methods and datasets
2.1 Study area
The northern Tibetan Plateau is located in the northwestern hinterlands of the Qinghai-Tibet Plateau, covers approximately 5.95 × 105 km2, and accounts for nearly 25% of the Qinghai-Tibetan Plateau and 50% of the Tibet Autonomous Region in China. The area is surrounded by the Kunlun, Gangdisi, Tanglha, and Nyainqntanglha mountains. Mean temperature is -11.3 ℃ in the coldest month and 8.4 ℃ in the warmest month in most parts of the northern Tibetan Plateau. Alpine grasslands, including meadows dominated by
Figure 1.
2.2 Datasets
The MODIS data for LST, ET and albedo products was taken from LP DAAC (Land Processes Distributed Active Archive Center) (
2.3 Data analysis
In each dataset, a temporal trend across the entire five year study period was calculated with Eqs. (1) and (2) (
The land surface heat balance was calculated with Eq. (3):
where
The land surface short-wave radiation was calculated with Eq. (4):
where
The latent heat flux was calculated with Eq. (5):
where
The sensible heat flux was calculated with Eq. (6):
where
The longwave radiation downward($L\downarrow $) was calculated with Eq. (7):
where
The longwave radiation upward($L\uparrow $) was calculated with Eq. (8):
where $\varepsilon s$is the surface reflectance,
Through the analysis and integration of Eqs (3) to (8), the surface energy balance was calculated with Eq. (9):
The main factors that vegetation can cause to change were $\alpha $,
where
The change of land surface temperature was calculated with Eq. (12):
where
3 Results
3.1 Changes in the LST (Day and Night) of the fenced patches
Fig. 2 illustrates the changes of daytime and nighttime LST at the fenced patches on the northern Tibetan Plateau. Daytime LST increased noticeably across the northern Tibetan Plateau except in the southwestern area. The warming magnitude of daytime LST in the central region (【-逻*辑*与-】gt;1 ℃ per decade) was the greatest, and that in the eastern region (0.2- 0.5 ℃ per decade) was lower. There were large differences in nighttime LST changes, with cooling in the eastern region (-0.2--0.1 ℃ per decade) and warming in the western region (0.2-0.5 ℃ per decade). In general, the daytime and nighttime LST trends showed an asymmetric diurnal variation, with a larger magnitude of warming during the day than cooling during the night.
Figure 2.
3.2 Changes in the albedo and evapotranspiration in the fenced patches
Fig. 3 summarizes the changes of albedo and evapotranspiration on the northern Tibetan Plateau. The albedo noticeably increased by 10% per decade across the northern Tibetan Plateau. The greatest increase (【-逻*辑*与-】gt;10% per decade) occurred in the central region. There were large differences in the amounts of ET changes, with ET noticeably decreasing across the northern Tibetan Plateau, except in the southwestern area.
Figure 3.
3.3 Changes of LST caused by the absorption of shortwave radiation
The effects of albedo changes on surface energy absorption, and especially on the absorption of shortwave radiation, are very important to the surface heat balance. Due to increases of albedo, the absorption of shortwave radiation decreased, and LST decreased on the northern Tibetan Plateau. According to the results of Equation (10), absorption of shortwave radiation led to a maximum LST decrease of -0.5 - -0.4 ℃ per decade in the central region. The minimum decrease (-0.2 - -0.1 ℃ per decade) occurred in the eastern region. The spatial distribution pattern was characterized by a gradual decrease from the center of the region to the peripheries.
Figure 4.
3.4 Changes of LST caused by latent heat
Due to decreases of ET, the LST increased across the northern Tibetan Plateau, except in the southwestern area. The decrease in ET led to an increase in LST. According to the results of Eq. (10), decreased latent heat led to the central region having the maximum increase of LST (【-逻*辑*与-】gt;1 ℃ per decade). The spatial distribution pattern of increased LST resulting from decreased ET was characterized by a gradual decrease moving from the center of the region to the peripheries. The eastern region increased at a rate of 0.2- 0.5 ℃ per decade, while the minimum increase of 0-0.1 ℃ per decade occurred in the northwestern region. In the southwestern area, LST decreased (-0.2 - -0.1 ℃ per decade).
Figure 5.
4 Discussion
4.1 Effects of land surface characteristics on surface heat balance
The Tibetan government has implemented a grazing exclusion program by enclosing large areas of the northern Tibet Plateau with metal fencing. While many studies have focused on the impact this has on biomass and plant diversity (
4.2 Vegetation feedback to the surface heat balance
Vegetation is a regulator of climate change that can dramatically mitigate factors that change climate. Vegetation affects local climates by altering the surface energy budget and water cycle, more changes push climate factors change in vegetation’s favor.
Vegetation cover is affected by precipitation fluctuations, and changes in vegetation cover affect the surface heat balance and the allocation of available energy from sensible heat and latent heat (
4.3 The characteristics of vegetation feedback on surface energy balance in sensitive areas
The feedback of vegetation on climate was more pronounced and noticeable in sensitive areas. When vegetation replaces a snow-covered or desert surface, albedo decreases, and the increase in the absorption of solar radiation leads to higher surface temperatures (
Climate models that do not take into account vegetation feedback are problematic for regions like the Tibetan Plateau where hydrological environments are sensitive to climate change or climate is sensitive to vegetation change. Slowdowns of warming are largely due to increasing evapotranspiration caused by more vegetation, the effect growing seasons for vegetation that begin earlier have on spring temperatures, and the cooling effect enhanced by the interaction between vegetation and evapotranspiration (
5 Conclusions
In summary, daytime LST noticeably increased across the northern Tibetan Plateau, except in the southwestern area. Nighttime LST changes were different, with cooling in the eastern region (0.1 ℃ per decade), and warming in the western region (0.2 ℃ per decade). In general, the daytime and nighttime LST trends showed an asymmetric diurnal variation, with a larger magnitude of daytime warming than nighttime cooling. The spatial distribution pattern of LST induced by albedo and ET was similar, and was characterized by a gradual decrease moving from the center of the plateau to the peripheries. The absorption of shortwave radiation led to the maximum decrease in LST (-0.5 - -0.4 ℃ per decade) occurring in the central region. The minimum decrease (-0.2 - -0.1 ℃ per decade) occurred in the eastern region. A decrease in latent heat led to an increase in the LST maximum (【-逻*辑*与-】gt;1 ℃ per decade) in the central region. The eastern region increased at a rate of 0.2-0.5 ℃ per decade, while the minimum increase (0-0.1 ℃ per decade) occurred in the northwestern region. LST in the southwestern area decreased (-0.2 - -0.1 ℃ per decade). The relatively higher increasing LST by decreasing ET was on the eastern region. The decrease in energy loss caused by the decrease in ET exceeded the decrease in energy input caused by the increase in albedo.
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