Elevation dependent warming over the Tibetan Plateau: Patterns, mechanisms and perspectives

Abstract

The Tibetan Plateau (TP) is also known as the “Third Pole”. Elevation dependent warming (EDW), the phenomenon that warming rate changes systematically with elevation, is of high significance for realistically estimating warming rates and their impacts over the TP. This review summarizes studies of characteristics and mechanisms behind EDW over the TP based on multiple observed datasets and model simulations. Spatial expression of EDW and explanatory mechanisms are still largely unknown because of the lack of suitable data over the TP. The focus is on the roles played by known mechanisms such as snow/ice-albedo feedback, cloud feedback, atmospheric water vapor feedback, aerosol feedback, and changes in land use, ozone and vegetation. At present, there is limited consensus on the main mechanisms controlling EDW. Finally, new perspectives and unresolved issues are outlined, including quantification of EDW in climate model simulations, explanation of the long-term EDW reconstructed from proxies, interaction between the Asian summer monsoon and EDW, importance of EDW for future environmental changes and water resources, and current gaps in understanding EDW over extremely high elevations. Further progress requires a more comprehensive ground observation network, greater use of remote sensing data, and high-resolution climate modeling with better representation of both atmospheric and cryospheric processes.

Introduction

The Tibetan Plateau (TP hereafter, Fig. 1), mostly lying above 4000 m, is the highest and largest plateau in the world, often called “Third Pole” and “the roof of the world” (Kang et al., 2010; Kang et al., 2019; Pithan, 2010; Qiu, 2008, Qiu, 2016; Yang et al., 2019; Yao et al., 2019). Due to its high terrain, the TP is important for biodiversity conservation, carbon balance and other environmental issues (Ghatak et al., 2014; Kang et al., 2019; Sun et al., 2018; Yang et al., 2010; Yang et al., 2019; Yao et al., 2019). The TP contains the largest fresh water resource stored in cryosphere (snow, glacier and permafrost) outside the polar regions, and is often referred to as the “Asian water tower”. It is the birthplace of the great rivers of Asia, such as the Yangtze, the Yellow River, the Ganges and Indus Rivers, and is also the source of many inland rivers, providing fresh water for more than 1.4 billion people over the region and South/East Asia as a whole (Duan and Xiao, 2015; Immerzeel and Bierkens, 2012; Immerzeel et al., 2010; Kang et al., 2010; Qiu, 2008; Yao et al., 2012b). Understanding climate changes over the TP also has consequences beyond the immediate environment of the plateau itself. The uplift of the TP enhanced the Asian summer monsoon by enlarging thermal differences between land and sea, and future warming may influence the monsoonal system (Kang et al., 2010; Zhang et al., 2015). Further, the TP provides important ecosystem services, with numerous benefits for people including water resources, tourism, varied ecosystems and their contributions to human well-being (Chen et al., 2015b; Chen et al., 2017; Grizzetti et al., 2016; Qin et al., 2002; Yao and Coauthors, 2017a, Yao and Coauthors, 2017b, Yao and Coauthors, 2017c; Zhang et al., 2015). Broader environmental responses to recent rapid warming, including glacier melt, permafrost degradation, desertification in some areas and flooding in others, changing ecological systems and increased natural disasters such as landslides and glacial lake outburst floods, are known to be occurring to a high degree of confidence over the TP (Kang et al., 2010; Kuang and Jiao, 2016; Thakuri et al., 2016; You et al., 2016; You et al., 2013; You et al., 2017; You et al., 2020). The TP is recognized as a potentially vulnerable area in terms of its ecological environment and future development needs to understand environmental issues such as climate change (Barnett et al., 2005; Gao et al., 2019; Immerzeel and Bierkens, 2012; Immerzeel et al., 2010; Yao and Coauthors, 2017b, Yao and Coauthors, 2017c).

In recent decades, climate change over the TP and its surrounding areas has become a source of debate, which makes it being one of the hotspots in the global field of geosciences (Chen et al., 2015b; Chen et al., 2017; Ding and Zhang, 2008; Duan et al., 2016; Liu and Hou, 1998; Liu et al., 1998; Liu et al., 2008; Minder et al., 2016; Qin et al., 2002; Wu et al., 2004; Wu et al., 2013; Xu et al., 2015; Yao and Coauthors, 2017b, Yao and Coauthors, 2017c). Since the turn of the 20th/21st century, a strong warming trend over the TP is evident, synchronizing with the global warming trend (Cuo et al., 2013; Yao et al., 2012a; Yao et al., 2015; Yao et al., 2012b). Significant warming of the TP has been shown through analysis of ground observation stations (Duan and Wu, 2006; Duan and Xiao, 2015; Liu and Hou, 1998; Liu and Chen, 2000; You et al., 2008; You et al., 2013; You et al., 2017), remotely sensed satellite data (Cai et al., 2017; Pepin et al., 2019; Qin et al., 2009), paleoclimate proxy indicators (Kang et al., 2007; Thompson et al., 2018; Yao et al., 2012b) and climate models (Chen et al., 2003; Chen et al., 2015a; Yan et al., 2016; You et al., 2016; You et al., 2020). The warming over the TP has accelerated in the second half of the 20th century (the early 1950s), somewhat earlier than that of the Northern Hemisphere as a whole (the mid-1970s) (Kang et al., 2010; Liu and Hou, 1998; Liu and Chen, 2000). During 1955–1996, the warming rate of the TP was highest in winter and autumn (+0.32 and + 0.17 °C/decade, respectively) (Liu and Chen, 2000). Seasonal variations in warming rate are common, and the warming rate in winter being twice that of the annual average is consistent with findings in other regions (Chen et al., 2003; Kang et al., 2010; Krishnan et al., 2019). A later analysis showed that the annual warming rate ranges from +0.16 to +0.36 °C/decade since the 1950s but +0.50 to +0.67 °C/decade from the 1980s (Kuang and Jiao, 2016). The warming rate has also been spatially heterogeneous across space and time periods. The northern TP experienced more warming than the southern TP in all seasons from 1982 to 1998, while the pattern was reversed in the period from 1998 to 2015 (Liu et al., 2019).

Elevation dependent warming (EDW) refers to the phenomenon that the warming rate changes systematically with elevation. The warming rate does not necessarily increase monotonically with elevation, although it sometimes does (Diaz and Bradley, 1997; IPCC, 2019; Pepin and Coauthors, 2015a; Rangwala and Miller, 2012; Rangwala et al., 2010). Table 1 lists previous studies on EDW including information on time period, elevation range and the number of stations considered in various mountain regions, including examples from the Swiss Alps, Rocky Mountains, Andes Mountains, the Kilimanjaro, and the TP and Himalayas (Fig. 1). EDW could also have a significant impact on the conservation of the cryosphere at high elevations and associated runoff, ecosystem stability and agricultural production. Further, EDW could disproportionately restrict species that are dependent on high elevation habitats for survival. EDW has consequences for changes in the fractional distribution of solid and liquid precipitation (e.g. the elevation of the rain vs snow line) as well as melting rates of glacial deposits, ultimately affecting streamflow and water resources downstream (Bolch et al., 2012; Dimri et al., 2018; Gao et al., 2018; Immerzeel and Bierkens, 2012; Pepin and Coauthors, 2015b). EDW has therefore become an important issue for high mountain areas (Table 1), and studies at global and regional scales show that there are significant spatial differences in its manifestation (Gao et al., 2018; Kotlarski et al., 2012; Liu et al., 2008; Pepin and Coauthors, 2015a; Rangwala and Miller, 2012). Taking the results as a whole, it is clear that the majority of these studies suggest an EDW in both observations and climate models, but with strong seasonality in some regions (Table 1). Existing studies on EDW over the TP have focused on certain aspects of this complex issue (Table 1), due to limitations in surface observations, the relatively recent development of the surface meteorological network over the TP, and uncertainties/limitations associated with methods of quantification.

We review recent studies on EDW over the TP in Section 2. Physical mechanisms are discussed in Section 3. Section 4 summarizes new perspectives and unanswered questions. This review has important theoretical and practical significance, aiming to providing a historical reference for understanding EDW and its consequences over the TP.