Elevation-dependent Climate Change in the Tibetan Plateau
Summary and Keywords
As a unique and high gigantic plateau, the Tibetan Plateau (TP) is sensitive and vulnerable to global climate change, and its climate change tendencies and the corresponding impact on regional ecosystems and water resources can provide an early alarm for global and mid-latitude climate changes. Growing evidence suggests that the TP has experienced more significant warming than its surrounding areas during past decades, especially at elevations higher than 4 km. Greater warming at higher elevations than at lower elevations has been reported in several major mountainous regions on earth, and this interesting phenomenon is known as elevation-dependent climate change, or elevation-dependent warming (EDW).
At the beginning of the 21st century, Chinese scholars first noticed that the TP had experienced significant warming since the mid-1950s, especially in winter, and that the latest warming period in the TP occurred earlier than enhanced global warming since the 1970s. The Chinese also first reported that the warming rates increased with the elevation in the TP and its neighborhood, and the TP was one of the most sensitive areas to global climate change. Later, additional studies, using more and longer observations from meteorological stations and satellites, shed light on the detailed characteristics of EDW in terms of mean, minimum, and maximum temperatures and in different seasons. For example, it was found that the daily minimum temperature showed the most evident EDW in comparison to the mean and daily maximum temperatures, and EDW is more significant in winter than in other seasons. The mean daily minimum and maximum temperatures also maintained increasing trends in the context of EDW. Despite a global warming hiatus since the turn of the 21st century, the TP exhibited persistent warming from 2001 to 2012.
Although EDW has been demonstrated by more and more observations and modeling studies, the underlying mechanisms for EDW are not entirely clear owing to sparse, discontinuous, and insufficient observations of climate change processes. Based on limited observations and model simulations, several factors and their combinations have been proposed to be responsible for EDW, including the snow-albedo feedback, cloud-radiation effects, water vapor and radiative fluxes, and aerosols forcing. At present, however, various explanations of the mechanisms for EDW are mainly derived from model-based research, lacking more solid observational evidence. Therefore, to comprehensively understand the mechanisms of EDW, a more extensive and multiple-perspective climate monitoring system is urgently needed in the areas of the TP with high elevations and complex terrains.
High-elevation climate change may have resulted in a series of environmental consequences, such as vegetation changes, permafrost melting, and glacier shrinkage, in mountainous areas. In particular, the glacial retreat could alter the headwater environments on the TP and the hydrometeorological characteristics of several major rivers in Asia, threatening the water supply for the people living in the adjacent countries. Taking into account the climate-model projections that the warming trend will continue over the TP in the coming decades, this region’s climate change and the relevant environmental consequences should be of great concern to both scientists and the general public.
The Tibetan Plateau (TP), a unique plateau and the highest gigantic plateau, with an average elevation of more than 4,000 meters and an area of approximately 2.5 × 106 km2, is sensitive and vulnerable to global climate change (Messerli & Ives, 1997), and hence its warming rates and effects on ecosystems and water resources can provide an early alarm for large-scale or even global climate change. During the early 21st century, growing evidence has revealed recent climate change in the TP, especially climate warming. Significant warming in the TP has been observed in terms of mean, minimum, and maximum surface air temperatures (Duan & Wu, 2006; Liu & Chen, 2000; Liu et al., 2009; Pepin et al., 2015; Yan et al., 2016).
One interesting manifestation of climate warming in the TP and its neighborhoods is elevation-dependent climate change, or elevation-dependent warming (EDW), i.e., greater warming at higher elevations than at lower elevations (Pepin et al., 2015). Based on observational data from weather stations in the TP and its neighboring areas, Liu and Chen (2000) reported that the TP had experienced statistically significant warming since the mid-1950s, especially in winter, and they first discovered a tendency for the warming trend to increase with elevation, suggesting that the TP could be one of the earth’s areas most sensitive to global climate change. An observational analysis based on land surface temperature retrieved from Moderate Resolution Imaging Spectroradiometer (MODIS) data also indicated that the warming rates in the TP were dependent on elevation (Qin et al., 2009). Climate-model projections also showed that EDW would continue in the future (Liu et al., 2009), and even become stronger along with increased global warming (Guo et al., 2016; Rangwala et al., 2013).
Recently, it was discovered that hiatus in global warming seemed to exist from 1999 to 2008, with a trend of only 0.07°C/decade (Knight et al., 2009), much lower than the trend of 0.18°C/decade recorded between 1979 and 2005 (Solomon, 2007). Although this so-called hiatus of warming is still controversial (e.g., Karl et al., 2015; Rajaratnam et al., 2015; Trenberth & Fasullo, 2013), observational data from weather stations showed a persistent warming over the TP from 2001 to 2012 at a warming rate of 0.316°C/decade, especially at elevations higher than 4 km (Yan & Liu, 2014), implying the existence of a continuous increase in the surface air temperature over and around the TP even during the background of the reduced global warming rate. Such observations led to the question whether the continued warming of the TP is related to EDW.
Elevation-dependent climate change in mountainous regions on earth has been demonstrated by growing observations, and the most striking evidence for EDW came from the TP (Pepin et al., 2015), although previous studies have also reported that no apparent relationship between elevation and warming rate was seen in some regions (Pepin & Lundquist, 2008; You et al., 2010) or weaker EDW in the 1990s (Rangwala et al., 2009). Scientists also found stronger warming during 1991–2012 than during 1961–1990 in the TP, with possible implications for EDW amplification in the context of global warming (Pepin et al., 2015).
Climate Warming in the Tibetan Plateau
Due to its high mountainous environment and sparse population, there are only 139 weather stations with relatively long and continuous observational records in the TP and its surrounding areas, as shown in Figure 1a. Most stations are located in the eastern and central TP, and hardly any stations are present in the southwestern TP. The TP elevation was divided into 10 zones from 0 to 5 km, with an interval of 0.5 km (Figure 1b); a minimum of five stations, and as many as 30 stations, exist in each elevation zone. Of the 139 stations, with elevations ranging from below 0.5 km to above 4.5 km, 73 stations are above the elevation of 2 km (TP_2km), 46 stations are above 3 km (TP_3km), and 16 stations are above 4 km (TP_4km).
Monthly mean, minimum, and maximum temperature records of the 139 weather stations were collected from January 1961 to February 2013. The annual mean surface air temperature (TA) for the TP_2km, TP_3km, and TP_4km groups were derived from the simple arithmetic means of 73, 46, and 16 stations, respectively. A global annual temperature series from the Climatic Research Unit at the University of East Anglia (CRUTEM4 dataset; Brohan et al., 2006) and an eastern China temperature series obtained from weather stations east of the TP with the same latitudinal range but lower elevations, were used in comparisons with the TP temperature records. The rates of temperature changes were calculated using the least-square linear regression.
During the 52 years from 1961 to 2012, the TP experienced significant warming, especially in areas at elevations of more than 4 km. The rates of increase of annual mean TA for the TP_2km, TP_3km, and TP_4km groups from 1961 to 2012 were 0.316 (Figure 2c), 0.331, and 0.359 (Figure 2d) °C/decade, respectively, suggesting more warming at higher elevations. Meanwhile, during the same period, the warming rates of annual mean TA for the globe and eastern China were 0.222 (Figure 2a) and 0.253 (Figure 2b) °C/decade, respectively, much lower than those for the TP, implying that the TP’s warming is more significant than that of the neighboring regions and global average.
Figure 3 shows rates of temperature changes above 2 km during 1961–2012 by season. At the seasonal scale, winter features the strongest warming, with a rate of 0.468°C/decade (Figure 3d), followed by autumn (0.316°C/decade, Figure 3c) and summer (0.259°C/decade, Figure 3b), while spring has the weakest warming (0.237°C/decade, Figure 3a).
There are distinct asymmetric trends of daily minimum and maximum temperature in the TP area. Figure 4 shows the increasing rates of annual mean daily minimum temperature (TN) and mean daily maximum temperature (TX) for the TP_2km from 1961 to 2012. The increasing rate of TN (Figure 4a) is 0.426°C/decade, approximately 1.6 times of the increasing rate of TX (Figure 4b), implying greater warming in nighttime temperature than in daytime temperature.
Elevation Dependency of Climate Warming in the Tibetan Plateau and its Surroundings
The above observational data analysis shows that the TP has experienced faster warming than the global average or the regions at lower elevations in recent decades, especially at elevations higher than 4 km (Figure 2), suggesting greater warming at higher elevations. To further investigate the elevation dependency of warming, the 139 weather stations in the TP and its surroundings were divided into ten elevation zones, with a 0.5-km interval (Figure 1b). After averaging the linear trends of annual and seasonal TA, TN, and TX records within each elevation zone, the results clearly indicate that warming is more pronounced at higher elevations than at lower elevations (Figure 5). The EDW is most evident for TN, followed by TA, and weakest for TX. The increasing rates of TN (Figure 5b) showed a nearly linear trend with elevation, while the increasing rates of TX for elevations above 2.5 km exhibited no clear trend with elevation (Figure 5c).
Taking into account the fact that the EDW is most robust for TN, the EDW of TN was analyzed at the seasonal scale, as shown in Figure 6. The EDW of TN is most notable in winter (Figure 6d), followed by spring (Figure 6a) and autumn (Figure 6c), and is weakest in summer (Figure 6b). For example, the increasing rate for TN in winter (Figure 6d) is about 0.83°C/decade in the 4.5 to 5 km elevation zone, while it is only 0.16°C/decade in the 0 to 0.5 km elevation zone.
Mechanisms of Elevation-Dependent Warming in the Tibetan Plateau
Temperature change is primarily a response to energy balance and, therefore, factors that increase the net flux of energy to the surface along an elevation gradient would lead to enhanced warming as a function of elevation. Up to the early 21st century, the mechanisms of EDW may be a few possible factors, such as snow-albedo feedbacks, cloud cover change, water vapor modulation of longwave heating, wind stilling, and increased absorbing aerosols. For example, the annual 1% CO2 increase experiment using CCSM3 for 100 simulation years shows that decreases in cloud amount and snow depth at higher elevations could lead to increases in absorbed solar radiation (Liu et al., 2009), and the snow-albedo feedback was reported in quadrupled CO2 experiments (Yan et al., 2016). In the future climate projection experiments, the elevation at which peak EDW occurs becomes higher when the warming intensifies, and the decrease in the reflected shortwave radiation due to the depletion of snow plays an important role in causing the EDW (Guo et al., 2016; Palazzi et al., 2016). Besides, observational analysis from weather stations showed that the cloud amount during daytime displayed decreasing trends, leading to more absorbed solar radiation at the surface and the associated surface warming on the TP (Duan & Wu, 2006). At the same time, the increases in the observed surface water vapor and the related changes in downward longwave radiation (DLR) can partially explain the evident winter warming during 1961–2000 in the TP (Rangwala et al., 2009). Additionally, the significant warming of minimum temperature at higher elevations in the projection experiments of the RCP 8.5 emissions scenario showed the strongest elevation-dependent increases in surface water vapor and elevation-dependent decreases in surface albedo (Rangwala et al., 2016), implying amplification of the EDW with continued warming. On the other hand, observed wind slowdown over the TP since the beginning of the 1980s led to less surface sensible heat exchange and hence more heat storage at the surface (Yang et al., 2014). Last, it was also found that a thick aerosol layer (atmospheric brown clouds) over the Himalayan mountains and the TP caused by increasing aerosol emissions over India and China may strengthen atmospheric heating over the TP, exerting an influence on the temperature variations and trends over the TP (Lau et al., 2010; Ramanathan & Carmichael, 2008).
Recently, the EDW working group of the Mountain Research Initiative (Pepin et al., 2015) reviewed existing studies related to the EDW phenomenon across the global mountainous regions, and pointed out that various combinations of the aforementioned mechanisms may account for regional patterns of EDW. It should be emphasized that the mechanisms of EDW in the TP and its surroundings are not fully understood at present owing to the scarcity of available observational data in the western TP. In particular, there are very limited systematic observations of surface energy budgets on the TP, which play an important role in causing greater warming at higher elevations.
Environmental Consequences of the Elevation-Dependent Climate Change
Elevation-dependent climate change, especially the enhanced warming at high elevations, may have resulted in a series of environmental consequences, such as vegetation changes (Piao et al., 2011; Xu & Liu, 2007), permafrost melting (Cheng & Wu, 2007), and glacier shrinkage (Su & Shi, 2002; Yao et al., 2004) in the mountainous areas of the TP. In particular, glacial retreat on the TP could alter the headwater environments of several major rivers in Asia and their hydrometeorological characteristics, threatening the water supply for millions of people living in the adjacent countries. For example, permafrost degradation could cause a dropping groundwater table in the source regions of the Yangtze River and Yellow River, which in turn leads to lower lake water levels, drying of wetlands, and shrinking of grasslands (Cheng & Wu, 2007). Glacial retreat in the High Asia regions in China in the 1990s caused an increase of 5.5% in river runoff in Northwestern China (Yao et al., 2004). In the long run, however, the depletion of the mountainous snow and ice storage could mean serious water shortages for these regions (Hijioka et al., 2014).
The regional climate and environment over the Tibetan Plateau (TP) are sensitive and vulnerable to global climate change. Enhanced climate warming, as well as its elevation dependency, formation mechanisms, and environmental consequences, in the TP and surroundings during recent decades have revealed:
(1) The TP has experienced significant warming from 1961 to 2012, especially at elevations higher than 4,000 meters. The increasing rate of annual mean surface air temperature (TA) averaged for the TP area above 2 km from 1961 to 2012 is 0.316°C/decade, much greater than that of eastern China (0.253°C/decade), at the same latitudes but lower elevations. At the seasonal scale, winter temperature shows the greatest warming rate, followed by autumn, summer, and spring. The increasing rate of annual mean daily minimum temperature (TN) is greater than that of annual mean daily maximum temperature (TX).
(2) The elevation-dependent warming (EDW) is strongest for TN, followed by TA, and weakest for TX around the TP. At the seasonal scale, the EDW of TN is most evident in winter, followed by spring, autumn, and summer.
(3) The mechanisms of EDW may be one or several influential factors, including elevation-dependent snow-albedo feedbacks, cloud cover, water vapor, wind speed, absorbing aerosols, or a combination of these factors working together, causing greater warming at higher elevations.
(4) The elevation-dependent climate change may have resulted in significant environmental consequences, such as vegetation changes, permafrost melting, and glacier shrinkage, in the mountainous areas. Taking into account climate-model projections that the warming trend will continue over the TP in the coming decades (Liu et al., 2009; Rangwala et al., 2013), this region’s climate change and the associated eco-environmental effects should be a major concern for the scientific community.
It is worth noting that the analyses so far are based on observations from only 139 weather stations and there are no weather stations above 5 km on the TP, which rises as high as more than 8 km. In order to fully investigate the extent of EDW of the TP, more satellite-based data and climate-modeling studies are needed to further clarify what has been and is occurring on the TP (Pepin et al., 2015).
The work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB03020601) and the National Natural Science Foundation of China (41420104008, 41572150, 41690115).
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