Recent permafrost warming on the Qinghai-Tibetan Plateau

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113,, doi: /2007jd009539, 2008 Recent permafrost warming on the Qinghai-Tibetan Plateau Qingbai Wu 1,2 and Tingjun Zhang 2,3 Received 25 October 2007; revised 6 December 2007; accepted 1 February 2008; published 9 July [1] Permafrost temperature monitoring through 10 boreholes up to 10.7 m depth has been conducted half-monthly from 1996 through 2006 along the Qinghai-Tibetan Highway. The primary results show that the long-term mean annual permafrost temperatures at 6.0 m depth vary from 0.19 C at the Touerjiu Mountains (TM1) site to 3.43 C at Fenghuo Mountain (FH1) site, with an average of about 1.55 C from all 10 sites over the period of their records, indicating permafrost is relatively warm on the Plateau. Mean annual permafrost temperatures at 6.0 m depth have increased 0.12 C to 0.67 C with an average increase of about 0.43 C during the past decade. Over the same period, mean annual air temperatures from four National Weather Service Stations show an increase of about 0.6 C to 1.6 C, generally sufficient to account for the permafrost warming although other factors, such as changes in snow cover and soil moisture conditions, may also play important roles in permafrost warming. Increase in summer rainfall and decrease in winter snowfall may be cooling factors to the underlying soils, offsetting less degree of permafrost warming compared with the magnitude of air temperature increase. Permafrost temperatures at 6.0 m depth increased year-around with most of the increase happened in spring and summer. Winter air temperature has increased 2.9 C to 4.2 C from 1995 through 2005, which may account for significant spring and summer permafrost warming at 6.0 m depth due to three to six month time lag. However, there were no significant trends of air temperature change in other seasons. Further investigation, especially comprehensive monitoring, is needed to better comprehend the physical processes governing the thermal regime of the active layer and permafrost on the Qinghai-Tibetan Plateau. Citation: Wu, Q., and T. Zhang (2008), Recent permafrost warming on the Qinghai-Tibetan Plateau, J. Geophys. Res., 113,, doi: /2007jd Introduction [2] Permafrost regions occupy approximately 24% of the land area in the Northern Hemisphere [Brown et al., 1997; Zhang et al., 1999, 2000], while areal extent of seasonally frozen ground (including the active layer over permafrost) covers, on average, approximately 56% of the northern hemisphere landmass [Zhang et al., 2003]. During the past several decades, studies of permafrost and global change have received great attention worldwide. These studies indicate that significant changes are occurring in permafrost and seasonally frozen ground. Such changes can have large impacts on the land surface energy and moisture balance and hence on weather and climate, surface and subsurface hydrology, carbon exchange between the land and the 1 State Key Laboratory of Frozen Soil Engineering, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou, China. 2 National Snow and Ice Data Center, Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA. 3 Chinese Academy of Meteorological Sciences, China Meteorological Administration, Beijing, China. Copyright 2008 by the American Geophysical Union /08/2007JD atmosphere, ecosystems in cold seasons/cold regions, landscape and geomorphological processes. Permafrost thaw can result in ground surface subsidence, which can disrupt infrastructure stability and operations in cold regions [Smith and Burgess, 1999; Nelson et al., 2001]. As a consequence, the understanding, evaluation, and anticipation of changes in permafrost and seasonally frozen ground are of great scientific interest and bear a high relevance to society [IPCC, 2007; Lemke et al., 2007]. [3] Permafrost is one of the key components of the terrestrial system in cold regions and is extremely sensitive to climate change. Approximately 970 Gt of carbon are contained in permafrost [Zimov et al., 2006]. Warming and thawing of permafrost will release the trapped carbon into the atmosphere, further exacerbating global warming. The changing properties of permafrost play an important role in terrestrial hydrologic cycles [Serreze et al., 2000; Hinzman et al., 2005; Walsh et al., 2005; White et al., 2007]. Thawing of ice-rich permafrost and thermokarst processes can partly or completely destroy the existing Arctic landscape and ecosystems, and change the fluxes of energy, water, and carbon between the land surface and the atmosphere [Lewkowicz, 1992; Osterkamp et al., 2000; Jorgenson et al., 2001; Hinzman et al., 2005]. Thawing of ice-rich permafrost can severely damage any engineering constructions due mainly to sub- 1of22

2 stantial thaw settlement [Esch and Osterkamp, 1990; US Arctic Research Commission Permafrost Task Force, 2003]. [4] The study of permafrost and changing climate received much attention during the recent decades, especially since the work by Lachenbruch and Marshall [1986] and Lachenbruch et al. [1988]. On the basis of temperature profiles measured in permafrost in the northernmost Alaska, Lachenbruch and Marshall [1986] demonstrated that there was a variable but widespread secular warming of the permafrost surface, generally in the range of 2 C to 4 C over the period from the beginning of the 20th century to the early 1980s. Changes in air temperature alone may not be sufficient to explain such a magnitude of change in permafrost surface temperature; changes in seasonal snow cover may play an important role [Zhang and Osterkamp, 1993]. Osterkamp [2003, 2005, 2007] reported that since the early 1980s, the permafrost surface has warmed an additional 3 C to 4 C along the Alaskan Arctic Coastal Plain, 1 C to2 C over the Brooks Range including its northern and southern foothills, and 0.3 C to 1 C over the region south of the Yukon River. All of these studies suggest a total warming of the permafrost surface by up to 6 C during the 20th century in northern Alaska. Osterkamp [2005, 2007] further described the warming as seasonal, primarily occurring in winter. [5] Measurements [Osterkamp, 2003, 2007] and modeling results [see Hinzman et al., 2005; Walsh et al., 2005] indicate that permafrost temperature has increased up to 2 3 C in northern Alaska since the 1980s. Data from the northern Mackenzie Valley in the continuous permafrost zone show that permafrost temperature between depths of 20 to 30 m has increased about 1 C in the 1990s [Smith et al., 2005], with smaller changes in the central Mackenzie Valley. There is no significant trend in temperatures at the top of permafrost in the southern Mackenzie Valley, where permafrost is thin (less than 10 to 15 m thick) and warmer than -0.3 C [Smith et al., 2005; Couture et al., 2003]. The absence of a trend is likely due to the absorption of latent heat required to melt ice. Similar results are reported for warm permafrost in the southern Yukon Territory [Haeberli and Burn, 2002]. Warming of permafrost at depths of 15 to 30 m since the mid 1990s has also been observed in the Canadian High Arctic [Smith et al., 2003]. [6] There is also evidence of permafrost warming in the Russian Arctic. Permafrost temperature increased approximately 1 C at depths between 1.6 m to 3.2 m from the 1960s to the 1990s in East Siberia, about 0.3 to 0.7 C at depth of 10 m in northern West Siberia [Pavlov, 1996], and about 1.2 to 2.8 C at depth of 6.0 m from 1973 through 1992 in northern European Russia [Oberman and Mazhitova, 2001]. Fedorov and Konstantinov [2003] reported that permafrost temperatures from three central Siberian stations did not show an apparent trend between 1991 and Mean annual temperature in Central Mongolia at depth from 10 to 90 m increased 0.05 to 0.15 C/10 a over 30 years [Sharkhuu, 2003]. Permafrost temperatures increased significantly at the Murtèl-Corvatsch borehole in the Swiss Alps at a depth of 11.5m in ice-rich frozen debris since 1987 [Vonder Mühll et al., 2004], due mainly to changes not only summer air temperature, but also the depth and duration of snow cover, particularly in early winter [Harris and Haeberli, 2003]. Results from 6 years of ground temperature monitoring at Janssonhaugen, Svalbard, indicate that the permafrost has warmed at a rate of about 0.5 C/10a at a depth of 20 m [Isaksen et al., 2001]. Results from Juvvasshøe, in southern Norway, indicate that ground temperature has increased by 0.3 C at 15 m depth from 1999 to 2006 [Isaksen et al., 2007]. [7] In Chinese permafrost classification, permafrost is divided into continuous and discontinuous zones [Zhou et al., 2000; Qiu et al., 2000]. Continuous permafrost zone refers to regions where permafrost underlies greater than 90% of the area, which is consistent with the International Permafrost Association permafrost classification [Brown et al., 1997]. Discontinuous permafrost zone refers to regions where permafrost underlies less than 90% of the area. The discontinuous permafrost zone is further divided into predominantly continuous permafrost (70% to 90%), predominantly continuous and island permafrost zone (30% to 70%), and sparsely island permafrost zone (less than 30%). These categories differ substantially from the percentage of permafrost zones designated by the International Permafrost Association and the other published permafrost classifications [Brown et al., 1997]. Zhang [2005] provided detailed reviews about permafrost classification in China. [8] Observed evidence shows that permafrost is warming, thawing, and degrading during the past few decades on the Qinghai-Tibetan Plateau [Cheng and Wu, 2007]. From the 1970s to the 1990s, ground temperature of seasonally frozen soil and sporadic permafrost has increased by C, while mean annual ground temperature of the predominantly continuous permafrost at depth of the zero annual amplitude has increased by C [Wang, 1993; Wang et al., 2000; Cheng and Wu, 2007]. Soil temperature near the permafrost table has increased by 0.1 to 0.7 C from 1996 to 2001 [Wu and Liu, 2004; Wu et al., 2006]. Active layer thickness has deepened by 10 to 40 cm in permafrost regions along Qinghai-Tibetan Highway [Wu and Liu, 2004; Yang et al., 2004; Wu et al., 2006]. Depth of seasonally frozen ground has reduced on average by 22 cm, with an average decreasing rate of about 0.71 cm/a since the 1980s over the eastern Qinghai-Tibetan Plateau [Zhao et al., 2004; Wang et al., 2005]. The lower boundary of permafrost distribution has moved up by 50 to 70 m in the northern Qinghai-Tibetan Plateau [Nan et al., 2003; Zhang et al., 2004; Yang et al., 2004; Wu et al., 2005, 2006]. The northern boundary of permafrost was retreated km southwards, while the southern boundary moved 1 2 km northward from 1975 to 1995 [Tong and Wu, 1996; Nan et al., 2003]. Recent study shows that permafrost areas reduced by approximately 20 km 2 in Xidatan area over the northern Plateau since 1975 [Nan et al., 2003; Wu et al., 2006]. Results from numerical simulation [Li et al., 1996] and GIS-aided regional modeling [Li and Cheng, 1999; Wu et al., 2000] demonstrate that permafrost areas on the Qinghai-Tibet Plateau have decreased by about 10,000 km 2 from 1975 through [9] During the Qinghai-Tibetan Highway expansion, a series of monitoring sites on the active layer and permafrost temperature were established in 1995 and Groundbased measurements from these sites continued until present, which are the longest continuous records of the active layer and permafrost temperature monitoring on the Qinghai-Tibetan Plateau. With the construction of the Qinghai- 2of22

3 Figure 1. Geographical locations of 10 soil temperature mentoring sites (solid black circle) and National Weather Service Stations (larger grey circles) along the Qinghai-Tibetan Highway. Tibetan railway, additional monitoring sites of the active layer and permafrost temperature were established [Wu et al., 2006]. These sites along both the Qinghai-Tibetan Highway and Railway have become a monitoring network (Figure 1) of the active layer and permafrost temperatures on the Qinghai-Tibetan Plateau. [10] The main objective of this article is to investigate the long-term means, trends, and variability of soil temperatures using data and information from this longest continuous record of permafrost temperature monitoring network (Figure 1) along the Qinghai-Tibetan Highway and Railway corridor. We will firstly focus on studies of monthly, seasonal, and annual means and variability of soil temperatures within the active layer and upper permafrost (up to 10.7 m depth) over the period from 1996 through We will further investigate trends of soil temperature change and the forcing that drove the changes in soil temperatures. Finally, we will provide assessment on the impact of changes in permafrost conditions on ecosystem and engineering infrastructures, mainly the Qinghai-Tibetan Highway and Railway. 2. Data and Methods [11] The data used in this study include soil temperature, air temperature, and precipitation. Soil temperature data were obtained from 10 monitoring sites along the Qinghai- Tibetan Highway (Figure 1). Monthly air temperature and precipitation data were obtained from the standard China National Weather Service stations along the transact (Figure 1) Site Description [12] The soil temperature monitoring sites were established along the Qinghai Tibetan Highway (Figure 1). These 10 monitoring sites span from Xidatan near the northern boundary of permafrost occurrence to Anduo of the southern boundary of permafrost along the Qinghai- Tibetan Highway, roughly 550 km long. Table 1 summarizes geographical locations and observation period, while Table 2 provides detailed information on climatic conditions, soil type, vegetation cover, and permafrost conditions at each site. Overall, the 10 soil temperature monitoring sites span about 3.2 latitudinal degrees within about 2.4 longitudinal degrees on the eastern Plateau (Figure 1). Site elevation varies from 4482 m a.s.l. at CM2 site to 4997 m a.s.l. at TG1 site with an average elevation of about 4747 m a.s.l. The observation period among these 10 sites varies from 6 to 11 years. Near surface soil type at these sites varies from fine materials as clay to silt/sand/gravel soils (Table 2). Vegetation cover is relatively poor except sites over Fenghou Mountains and south of the Tanggula Mountains (Figure 2) Soil Temperature Measurements [13] All sites north of the Tanggula Mountains were established in 1995 and sites south of the Tanggula Mountains were established in 1998 (Table 1). Soil temperature measurements from CM2, WD2 and FM1 sites are 3of22

4 Table 1. Geographical Data and Information of 10 Monitoring Sites on the Qinghai-Tibetan Plateau Location Sites Number Areas Latitude Longitude Altitude, m Observation Period KM1 Kunlun Mts N E KM N E CM1 Chumaer River N E CM N E WD1 Wudaoliang N E WD N E FH1 Fenghuo Mts N E TG1 Tanggula Mts N E TM1 Touerjiu Mts N E AD1 Near Amdo N E continued until present, while the other sites were either terminated or relocated in 2003 due to highway reconstruction. Soil temperature was measured at depths from 0.5 to 6.0 m for KM1, KM2, WD1, and FH1 sites, from 0.5 through 8.0 m for CM1, CM2, and WD2 sites, from 0.5 through 10.0 m for TG1 site, and from 1.0 to 10.7 m for TM1 and AD1 sites. All measurements were made by a string of thermistors with an increment of 0.5 m with depth. These thermistors were made by the State Key Laboratory of Frozen Soil Engineering (SKLFSE). Temperature sensitivity of these thermistor sensors in the laboratory is ±0.05 C. The in situ measurements were conducted by well-trained technicians and professionals with standard guideline, manually measured on the 5th and 20th day of each month, respectively. The data set is the longest continuous record of soil temperature within the active layer and upper permafrost on the Qinghai-Tibetan Plateau, suitable for investigating seasonal and interannual changes and variations of permafrost thermal conditions. [14] The half-monthly measured soil temperatures were arithmetically averaged into monthly mean throughout this study. To justify the validity or errors of monthly mean soil temperature estimated using half-monthly measurements from this study and conventionally daily measurements, we conducted a detailed comparison study between the two methods. Daily soil temperatures were measured near CM2 and WD2 sites (Figure 1) using a string of thermistor sensors and recorded into a data logger system from October 2005 through October The bias of mean annual soil temperature estimated from half-monthly measurements is generally less than 0.08 C at 0.5 m depth comparing with conventionally daily average (Figure 3). As moving to deeper layer, the bias becomes even smaller. These results indicate that we can use the mean monthly soil temperature averaged over half-monthly measurements with confidence to study the long-term changes in soil temperature on the Qinghai-Tibetan Plateau Meteorological Data [15] Mean monthly air temperature and monthly precipitation along the Qinghai-Tibetan Highway were used in this study. Data from all stations except the Fenghuo Mountain stations are provided by the China National Meteorological Stations. Data from the Fenghuo Mountain were provided by the Northwest Institute of Railway in Lanzhou, China. Monthly mean air temperature were averaged using daily measurements, monthly precipitation is the summation of daily precipitation within the month. [16] The observational sites of meteorological stations at Wudaoliang, Fenghuo Mountain, Tuotuohe, and Anduo are all in permafrost regions (Figure 1). Although records of meteorological data from these stations are much longer, we only use data covering roughly the same period ( ) as soil temperature records to investigate the potential response of changes in soil temperature to climate conditions. Unfortunately, meteorological data for 2006 from the National Weather Service stations are not available at this time. 3. Results 3.1. Means and Variability of Soil Temperatures Within the Active Layer and Upper Permafrost Monthly Mean and Variability [17] Based on soil temperature measurements from 10 sites over the period of their records, we firstly present the Table 2. Climatic and Environmental Parameters at 10 Monitoring Sites on the Qinghai-Tibetan Plateau a Climatic onditions Permafrost Conditions Sites Number MAAT, C Precipitation, mm Soil Types Vegetation Cover, % ALT, m MAGT, C PT, m FST KM1 6.0 to to 300 clay 10 to to to to 120 H, B KM2 CM1 5.0 to to 250 sandy clay 20 to to to to 30 F CM2 WD1 5.5 to to 350 sandy clay 15 to to to to 80 F WD2 clay 60 to 80 H, B FH1 6.0 to to 300 clay 70 to to to to 120 H, B TG1 6.0 to to 300 clay 20 to to to to 120 F TM1 4.0 to to 400 clay 50 to to to to 60 B AD1 2.0 to to 450 clay to to to 20 H, B a MAAT: mean annual air temperature; ALT: active layer thickness; MAGT: mean annual ground temperature at depth of zero annual amplitude, usually at 10 to 15 m depth below the ground surface on the Plateau; PT: permafrost thickness; FST: frozen soil types, where H stands for frozen soils with ice, B for saturated frozen soils, and F for saturated frozen soils with excess ground ice. 4of22

5 Figure 2. In situ surface conditions at selected monitoring sites on the Qinghai-Tibetan Plateau. long-term means of monthly soil temperatures within the active layer and upper permafrost. Monthly soil temperature at each depth was averaged from two measurements each month, while long-term mean was averaged using the monthly value for each month over the period of their records. Long-term mean soil temperature up to 8.0 m in depth is only within a few tenths of a degree Centigrade below the freezing point at CM2 site (Figure 4b). Permafrost in this region is extremely sensible to changes in climate and any surface disturbance. Further down to the south at FH1 site, long-term mean permafrost temperature at 6.0 m depth is about 3.43 C (Figure 4d). At both CM2 and FH1 (Figure 4) sites, the average annual amplitude of soil temperature at 0.5 m depth is about 8.0 C to8.5 C. Thermal conditions from these two sites essentially cover the range of long-term mean soil temperatures within the active layer and upper permafrost along the Qinghai-Tibetan Highway. [18] Because of relatively little snow on ground in winter and lack of vegetation in summer (Figure 2), soil temperatures on the Qinghai-Tibetan Plateau in general are strongly linked with air temperature. Seasonal soil temperature variations follow the similar patterns as air temperatures except soil temperature time lag with depth. Annual amplitude of monthly air temperature ranges from greater than 12 C to about 10 C with an average of 11.5 C from 1995 to 2005 at the Wudaoliang National Weather station and from greater than 12 C to about 10 C with an average of 11.8 C from 1995 to 2005 at the Fenghuo Mountains Weather station. Annual amplitude of soil temperature is about 8 C at 0.5 m depth, while it is reduced to about 0.4 C just below the permafrost table at CM2 site (Figure 4a). The rapid amplitude reduction of seasonal soil temperature with depth is mainly due to energy consumption for soil temperature change and soil water phase change within the active layer. [19] Surprisingly, there is no thermal offset observed at CM2 site even though active layer thickness is greater than 3.5 m. At FH1 site, active layer thickness is about 1.4 m (Figure 4d), long-term mean soil temperature at 0.5 m depth is about 0.23 C higher than that at 1.0 m depth (Figure 4d), indicating thermal offset occurrence. Soil thermal offset is mainly produced due to the difference in soil thermal conductivity between soil frozen and thaw status [Goodrich, 1982; Burn and Smith, 1988]. No observational soil thermal offset at CM2 site is probably mainly due to the dry site condition since changes in soil thermal conductivity without Figure 3. Mean annual soil temperatures with depths estimated from half-monthly measurements (5th and 20th day of each month) and conventional daily measurements near CM2 site (circles) and WD2 site (triangles). Solid circles and triangles represent mean annual soil temperature calculated using half-monthly measurements, while open circles and triangles represent daily measurements. 5of22

6 Figure 4. Long-term means of monthly soil temperatures at various depths over the period from 1996 through 2006 at CM2 site and FH1 site. Seasonal variations of soil temperatures within the active layer and upper permafrost at (a) CM2 and (c) FH1, averaged maximum, mean, and minimum soil temperatures with depth at (b) CM2 and (d) FH1. soil water phase change is extremely small. Although we do not have continuous soil moisture measurements at CM2 site, in situ visit show that the site is extremely dry dominated with coarse materials (Figure 2). On the other hand, soil moisture content at FH1 site is much higher than that at CM2 site. There is obvious evidence of thermal offset observation at FH1 site (Figure 4d) Seasonal Mean and Variability [20] Study of seasonal means and variability in soil temperatures within the active layer and permafrost is important for understanding the response of changes in permafrost to climate forcing. Over the period of records, soil temperatures at 1.0 m depth from all sites were below the freezing point during spring (circles in Figure 5a) and winter (squares in Figure 5a) with an average of 3.07 C and 5.03 C, respectively. Soil temperatures were generally above the freezing point in summer (triangles in Figure 5a) and autumn (diamonds in Figure 5a) with an average of 1.92 C and 1.36 C, respectively. Geographically, average spring soil temperature increased from below 4.0 C over the Kunlun Mountains (KM1 and KM2) to about 2.0 C over the Chumaer River basin (CM1 and CM2) (Figure 5a). Average spring soil temperature decreased and reached the minimum of about 5.36 C at FH1 site. Then, it increased again to 0.80 C at AD1 site. Over summer, the average soil temperature increased from KM1 and KM2 sites and reached the highest at WD1 site, decreased and reached the minimum at FH1 site. The summer average soil temperature increased again at TG1 and TM1 except at AD1 site where soil temperature decreased substantially compared with others sites. The geographical pattern of the autumn average soil temperature was almost the same as that in spring except the overall range was small and lower values at AD1 site (Figure 5a). Over winter, soil temperature decreased significantly at WD1 site, while it was exceptional high at AD1 site when they are compared with other sites (Figure 5a). Soil temperature at FH1 site was the lowest all yearlong compared with others. [21] Surprisingly, soil temperature at AD1 site is the highest in spring and winter, the lowest one in summer, and on average in autumn among 10 monitoring sites. Higher soil temperatures during winter and spring months are due to higher winter air temperatures at this southernmost site. The lower soil temperatures during summer months can be explained by wet soil surface at AD1 site. Frequent site visit show that soil surface at AD1 site is always saturated, often with standing water (Figure 2). Wet soil surface consumes more energy due to heavy soil water evaporation, resulting in lower soil surface temperatures, hence lower soil temperatures under the ground surface. Zhang et al. [2001] reported that summer air temperature at Irkutsk, Russia, increased several degrees during the 20th century, while near-surface soil temperatures decreased by up to 4 C, mainly due to the increase in summer precipitation, thus soil surface wetness and consumption of surface energy. [22] The long-term mean seasonal permafrost temperatures at 6.0 m depth were more consistent spatially and temporally among these 10 sites (Figure 5b). Overall, interannual variations of permafrost temperatures at 6.0 m depth for each season were very small, usually within ±0.2 C except at WD2 and FH1 sites but still within 6of22

7 Figure 5. Long-term mean soil temperatures and its range within the (a) active layer at 1.0 m depth and (b) upper permafrost at 6.0 m depth along the Qinghai-Tibetan Highway. Solid circles stand for spring (MAM), solid triangles for summer (JJA), solid diamonds for autumn (SON), and solid squares for winter (DJF). (c) Open and light solid circles represent long-term mean annual temperature within the active layer and the upper permafrost, respectively, over the period of their records. ±0.4 C (Figure 5b). Geographically, permafrost temperature at 6.0 m depth was about 3.0 C at two Kunlun Mountain sites (KM1 and KM2) where elevation is relatively high (Table 1 and Figure 5b). As moving over the Kunlun Mountains pass to the Chumaer River basin, permafrost temperature increased by more than 2.0 C at sites CM1 and CM2. Permafrost temperatures, then, decreased sharply as moving southward, reaching to the minimum at Fenghuo Mountains (FH1 site) at about 3.43 C. Permafrost temperature increased again at Tanggula Mountains (TG1) and the two sites south of the Tanggula Mountains (TM1 and AD1). Geographically, patterns of permafrost temperature change do not change significantly with seasons (Figure 5b). In other words, there is no observable spatial variation in permafrost temperature over different seasons along the Qinghai-Tibetan Highway (Figure 5b) Annual Mean and Variability [23] Based on average soil temperature data at 1 m depth from all observation sites, the long-term mean annual soil temperature on the Qinghai-Tibetan Plateau is about 1.20 C from the period of their records (Figure 5c). Long-term mean annual soil temperature at 1.0 m depth varied from about 3.33 C at FH1 site to about 0.34 C at TM1 site. The range (maximum minus minimum values) of mean annual soil temperature over the period of their records varied from less than 0.4 C at TG1 site to greater 7of22

8 Figure 6. Soil temperatures within the active layer at 1.0 m depth from 10 monitoring sites on the Qinghai-Tibetan Plateau. (a) Time series of mean annual soil temperatures, (b) departures from its mean over the period of their records, and (c) rate of soil temperature change over the period of their records. than 1.2 C at WD2 site. Maximum values of mean annual soil temperature at 1 m depth over the period of their records were about 0.1 C at CM1 and CM2 sites, 0.79 C at TM1 site, and 0.22 C at AD1 site (Figure 5c). [24] The long-term mean annual permafrost temperature at 6.0 m depth is about 1.55 C (Figure 5c), indicating overall permafrost temperature over the Plateau is relatively high. The long-term mean annual permafrost temperature at 6.0 m depth varied from 3.43 C at FH1 site to 0.19 C at TM1 site. The range of mean annual permafrost temperature over the period of their records varied from 0.07 C at TM1 site to 0.59 C at FH1 site. Geographically, permafrost temperature at 6 m depth was about 3 C at two Kunlun Mountain sites (KM1 and KM2) where elevation is relatively high (Table 1 and Figure 5c). As moving over the Kunlun Mountains pass to the Chumaer River basin, permafrost temperature increased by more than 2 C at sites CM1 and CM2. Permafrost temperature, then, decreased sharply as moving southward, reaching to the minimum at Fenghuo Mountains (FH1) at 3.43 C. Permafrost temperature increased again at Tanggula Mountains (TG1) and the two sites south of the Tanggula Mountains (TM1 and AD1). [25] Generally, sites with higher annual permafrost temperature have smaller interannual variations, and vice versa. For example, the long-term mean annual permafrost temperature at 6 m depth is about 0.87 C at CM1 site with interannual variation less than ±0.05 C (Figure 5c), while at FH1 site, the long-term mean annual permafrost tempera- 8of22

9 Table 3. Rate of Changes in Mean Annual Soil Temperature Within the Active Layer at 1.0 m Depth and the Upper Permafrost at 6.0 m Depth From 10 Monitoring Sites on the Qinghai-Tibetan Plateau Rising Rate Soil Temperature, C/10 a Sites Number Areas Active Layer (1.0 m) Permafrost (6.0 m) Observed Period KM1 Kunlun Mts a 0.59 a KM a 0.59 a CM1 Chumaer River a CM a WD1 Wudaoliang WD a 0.61 a FH1 Fenghuo Mts a 0.59 a TG1 Tanggula Mts a TM1 Touerjiu Mts a AD1 Near Amdo 0.57 a 0.24 a Average 0.83 b 0.39 b a P < b Excluding TG1 site. ture is about 3.43 C with interannual variation of about ±0.30 C. The extremely small interannual variations of soil temperatures in the upper permafrost at most of these 10 sites imply that there is very little energy flow in or out of permafrost over a year. There are two explanations for this phenomenon. First, soils experience maximum potential seasonal freezing in winter and thawing in summer within the active layer at these sites. The long-term average active layer thickness at CM2 site is greater than 3.5 m (Figure 4b). The active layer starts to develop at the beginning of May and will not completely freezeup until the middle of February [see Wu et al., 2003]. In this case, there exists a 0 C upper boundary above permafrost for almost 10 months per year. Permafrost is essentially decoupled from the atmosphere during this period of time. Soon after the active layer freezeup in the middle of February, air temperature starts to rise and cooling effect of the overlying atmosphere on underlying permafrost is reduced to minimum. Temperature gradient between the 0 C boundary and the permafrost surface is close to 0 C isothermal or very small, resulting in a very small heat flux through the permafrost surface, thus very little seasonal change in permafrost temperatures below the permafrost surface (Figure 5c). Second, if there is any heat flow across the permafrost surface, much of the energy would be consumed for phase change due to latent heat consumption for changes in unfrozen water content mainly because of the warm permafrost. Permafrost temperature at FH1 site is almost 3 C lower than that at CM2 site (Figure 4a) with average active layer thickness of about 1.4 m (Figure 4d). The annual amplitude of soil temperature at 0.5 m depth is about 8.5 C at FH1 site (Figure 4c), approximately the same magnitude as at CM2 site (Figure 4b). However, the annual amplitude of permafrost temperature below the permafrost table is about 4 C (Figure 4b). This is mainly due to thinner active layer depth and much shorter active-layer freezeup period at FH1 site than those at CM2 site Trends of Soil Temperature Within the Active Layer and Upper Permafrost [26] Significant changes in soil temperatures within the active layer and upper permafrost have been observed from the 10 monitoring sites along the Qinghai-Tibetan Highway. Although there were strong interannual variations in soil temperature at 1 m depth, overall increasing trend in soil temperatures is significant (Figure 6a). To better demonstrate the changes in soil temperature, we removed the mean over the period of their records (Figure 6b). We further conduct the linear least squares correlation analysis for each site. Finally, we demonstrate the mean rate of soil temperature increase per decade within the active layer and upper permafrost although some records are less than 10 years. [27] The results show that mean annual soil temperatures at 1 m depth increased with variable magnitude, ranging from less than 0.4 C at AD1 site to greater than 1 C at WD1 site over the period of their records (Figure 6b). At CM2 site, mean annual soil temperature was generally in decreasing trend from 1998 through Soil temperature at TG1 shows clearly a decreasing trend over the period of record. At KM1 site, mean annual soil temperature experienced largest interannual variations with an overall increasing trend of about 1.5 C/10a. Recent study indicates that precipitation, especially rainfall, has decreased in the regions north of the Tanggula Mountains and increased in the regions south of the Tanggula Mountains in the past few decades [Wang et al., 2007]. Increase in rainfall in the south of the Tanggula Mountains may contribute the cooling of soil temperatures, while decrease in rainfall in the north of the Tanggula Mountains may reduce the cooling effect in summer, partly explain the warming of soils in northern Plateau. Excluding TG1 site, the average increasing rate of soil temperature within the active layer was about 0.83 C/10a (Table 3 and Figure 6c). [28] Changes in permafrost temperatures at 6 m depth demonstrate the similar pattern (Figure 7) but with smaller magnitude comparing with changes in active layer temperature at 1 m depth (Figure 6). Permafrost temperature increased by 0.1 C at CM1 and TM1 sites within the period of their records to 0.67 C at FH1 site from 1996 through 2006 (Figure 7b). Interestingly, mean annual soil temperature at 1 m depth at CM2 site shows a decreasing trend since 1998, while permafrost temperature at 6 m depth at CM2 site demonstrate a strongly increasing trend over the entire period of record. Average active layer thickness at CM2 site is about 3.56 m. Further investigation is needed to explain this situation. Except TG1 site, the average permafrost temperature increasing rate is about 0.39 C/10a (Table 3 and Figure 7c). [29] Generally, the rate of soil temperature increase within the active layer and upper permafrost was greater at sites 9of22

10 Figure 7. Permafrost temperatures at 6.0 m depth from 10 monitoring sites on the Qinghai-Tibetan Plateau. (a) Time series of mean annual permafrost temperature at 6.0 m depth, (b) departures from its mean over the period of their records, and (c) rate of permafrost temperature change over the period of their records. 10 of 22

11 Figure 8. Soil temperature departures from their means over the period of their records within the active layer at 1.0 m depth from 10 monitoring sites along the Qinghai-Tibetan Highway, (a) spring (MAM), (b) summer (JJA), (c) autumn (SON), and (d) winter (DJF). with lower long-term average soil temperatures, and vice versa (Figures 6c and 7c). For example, at sites with longterm mean permafrost temperature at or higher than 1 C (such as AD1, CM1, CM2, and TM1 sites), the rate of permafrost temperature increase is usually less than 0.3 C/ 10 a (Figure 7c), while at sites with long-term mean permafrost temperature lower than 2 C (such as KM1, KM2, and FH1 sites), the rate of permafrost temperature increase is greater than 0.5 C/10a (Figure 7c). At WD1 and WD2 sites where long-term mean permafrost temperature ranges from 1.4 C through 1.6 C, the rate of permafrost temperature increase varies from 0.4 through 0.6 C/10a (Figure 7c). Osterkamp [2003, 2007] observed the similar phenomena in Alaska where permafrost temperature increased by 2 to 3 C in northern Alaska and by less than 1 C in the Interior of Alaska since the early 1980s. Smith et al. [2005] also reported that the magnitude of permafrost warming in southern Machazine Valley is much smaller than in the northern Canada. Besides local climatic and microclimatic conditions, changes in unfrozen water content in permafrost due to permafrost warming may play a leading role to explain such phenomena. [30] Soil temperatures within the active layer and upper permafrost show a slight decreasing trend at TG1 site over a period from 1999 through 2006 (Figures 6 and 7). The TG1 site has an elevation of about 4997 m on the south side of 11 of 22

12 Figure 9. Permafrost temperature departures from their means over the period of their records at 6.0 m depth from 10 monitoring sites along the Qinghai-Tibetan Highway, (a) spring (MAM), (b) summer (JJA), (c) autumn (SON), and (d) winter (DJF). the Tanggula Mountains. Winter precipitation is relatively low with no or little snowfall. During summer and autumn months, area surrounding the TG1 site is always wet and often with standing water (Figure 2). In this case, when wet or saturated active layer freezes up in the late autumn or early winter, thermal conductivity of the saturated frozen active layer is high, a very favorable condition for heat flow from frozen soils to the atmosphere. Little or no snow cover at the site in winter and spring probably also produce no or little insulating impact between the soil surface and the atmosphere. During summer months, wet or saturated soil surface at the TG1 site consume substantial energy for soil surface water evaporation, a cooling factor for soil surface. The combination of (1) relatively high thermal conductivity in winter, (2) no or little snow cover insulation effect, and (3) high energy consumption of surface water evaporation in summer may result in cooling the active layer and upper permafrost at TG1 site. Further monitoring at this site is needed to better explain the cooling trend Seasonality of Changes in Soil Temperatures Within the Active Layer and Upper Permafrost [31] Although there was an overall warming of the active layer and permafrost on the Qinghai-Tibetan Plateau in the past decade as discussed above, seasonality of changes in soil temperatures within the active layer and upper permafrost is more critical for ecological, biological, and surface/subsurface hydrological processes. Again, we removed the monthly mean over the period of their records (Figures 8 and 9). [32] During spring, soil temperatures at 1 m depth experienced a mixed variation in terms of long-term trend. Soil 12 of 22

13 Figure 10. (a) Time series of mean annual air temperature, (b) departure from its mean from 1995 through 2005, and (c) rate of air temperature increase at four national meteorological stations along the Qinghai-Tibetan Highway. temperatures increased significantly at WD2 and FH1 sites, while at WD1 site, soil temperatures experienced a significant decreasing trend (Figure 8a). For the remaining sites, soil temperature had large interannual variations but with little or no significant trend observed. During the summer months (Figure 8b), soil temperatures at 1 m depth had experienced significant increase except the three southern sites (namely TG1, TM1, and AD1). Soil temperatures at most of the sites increased in autumn except TG1 and CM2 sites where there were no obvious trends (Figure 8c). Surprisingly, the active layer at many sites experienced cooling during winter months, especially at KM1, CM1, CM2, TG1, and TM1 sites where the cooling trend is significant. Soils experienced warming only at KM2 and WD1 sites. There were no detectable trends at WD2 and AD1 during winter months. Overall, because of the extremely variable site specific conditions and complex terrain on the Qinghai-Tibetan Plateau, there was no consistent trend of changes in soil temperature within the active layer among all monitoring sites over each season. Soils at most sites at 1 m depth experienced significant warming during summer and autumn, cooling during winter (except at WD1 and KM2 sites), and mixed (warming, cooling, and no change with large interannual variation) in spring. 13 of 22

14 [33] Permafrost temperatures at 6 m depth experienced increasing trend each season at all sites with variable magnitudes except TG1 site where permafrost cooling was observed (Figure 9). Relatively cold permafrost experienced most warming, such as at KM1, KM2, FH1, WD1, and WD2 sites. At all warming sites, the magnitude of permafrost temperature increase were greater in spring and summer than in autumn and winter (Figure 9). The year-around permafrost warming on the Qinghai-Tibetan Plateau is in a sharp contrast with permafrost warming in Alaska where cold season warming is dominant [Zhang and Osterkamp, 1993; Zhang et al., 1997; Osterkamp, 2003, 2007]. [34] An interesting feature is that in a few cases, active layer was cooling while the underlying permafrost was experiencing warming. For example, at CM2 site, winter (DJF) soil temperatures at 1 m depth experienced positive anomaly from 1996 through 2003 and then negative anomaly from 2004 through 2006 (Figure 8d), an overall decreasing trend. Winter permafrost temperatures at 6 m depth had negative anomaly from 1996 through 1999 and then positive anomaly from 2000 through 2006 (Figure 9d), an overall increasing trend. Considering the time lag of the soil temperature propagation from 1 m depth to 6 m depth, which roughly requires 3 to 6 months, permafrost temperatures at 6 m depth in spring and summer should follow the negative temperature anomaly at 1 m depth. However, permafrost temperatures at 6 m depth still show strong positive anomalies in 2005 and 2006 (Figure 9). The decoupling of soil temperatures between the active layer and the upper permafrost can also be found in a few other sites along the Qinghai-Tibetan Highway. The possible explanation is the talik formation between seasonal freeze/ thaw depth and the permafrost surface. However, due to the nature data collection (half-monthly soil temperature measurements) in this project, we are unable to detect such processes using the current data. Further work is needed to document the talik formation processes at selective sites along the Qinghai-Tibetan Plateau. 4. Response of Changes in Soil Temperature to Climate Forcing 4.1. Response of Changes in Soil Temperature to Climatic Change [35] Based on data from four stations from 1995 through 2005, mean annual air temperature generally had an increasing trend along the Qinghai-Tibetan Highway (Figure 10). Linear least squares analysis shows that the rate of air temperature increase was at about 1.2 to 1.4 C/10 a in relatively warmer regions, such as in Anduo and Tuotuohe; while in relatively colder regions, such as in Fenghuo Mts and Wudaoliang, the rate of air temperature increase was only about 0.5 to 0.7 C/10 a (Figure 10c). These rates may be questionable since 1997 was an extremely cold year over the whole study period with 1 C to 2 C anomaly below the average (Figure 10b). Without considering the low point from 1997, the rate of air temperature increase was well below 1.0 C/10 a at Anduo, air temperature even show a decreasing trend since 1999 at Fenghuo Mts, and almost no trend with large interannual variations at Wudaoliang station (Figure 10). [36] Annual precipitation show an increasing trend at Anduo and Tuotuohe stations, while at Wudaoliang station, there is no significant trend of changes in annual total precipitation (Figure 11). Summer rainfall had increased more than 100 mm from 1995 through 2005 at Tuotuohe and Anduo meteorological stations (Figures 12b and 12c). Increase in summer rainfall had about the same magnitude at the other two stations based on available data (Figure 12b). Although summer rainfall also shows a slight increase trend at Wudaoliang station (Figure 12c), the trend is not statistically significant. Snowfall at Tuotuohe and Anduo stations shows significant decreasing trends from 1995 through 2005 (Figures 13b and 13c). On the basis of available data, the decreasing trend of snowfall at Fenghuo Mts. station is also significant (Figure 13b). Again, changes in snowfall at Wudaoliang station are not significant (Figures 13b and 13c). [37] Based on data and information from changes in air temperature and precipitation, we can conclude that overall increase in soil temperatures within the active layer and upper permafrost was mainly driven by changes in air temperature on the Qinghai-Tibetan Plateau. Increase in summer rainfall (Figure 12) and decrease in winter snowfall (Figure 13) are cooling factors for soil thermal regime since more summer precipitation consumes more energy for soil water evaporation and less snow on ground in winter months results in less snow insulation effect. [38] Further analysis indicates that changes in mean annual air temperature alone may not be sufficient to explain the changes in soil temperature within the active layer and upper permafrost. For example, mean annual soil temperature at WD2 site increased at a rate of about 0.91 C/ 10 a at 1 m depth (Figure 6c) and about 0.61 C/10 a at 6 m depth (Figure 7c), while mean annual air temperature at Wudaoliang station increased at a rate of about 0.65 C/10 a (Figure 10c). Again, Wudaoliang National Weather Station is about 10 km away from the WD2 site. To fully explain the response of changes in soil temperature to changes in climate at WD2 site, additional data and information on snow cover and soil moisture conditions are needed. Mean annual soil temperature at AD1 site increased 0.57 C/10 a at 1 m depth (Figure 6c) and almost 0.24 C/ 10a at 6 m depth (Figure 7c), while air temperature increased by up to 1.35 C/10a at Anduo National Weather Station (Figure 10c), which is sufficient to fully explain changes in soil temperatures. Reduced magnitude in soil temperature increase may be due to the cooling effect of increased summer precipitation (Figure 10c) Seasonal Response of Changes in Soil Temperature to Climatic Change [39] Mean seasonal air temperature and precipitation show large interannual fluctuations but with no significant trends except winter months (Figures 14 and 15). Linear least squares analysis (P < 0.05) indicates that winter air temperature increased by 2.9 C to4.2 C from 1995 through 2005 at four meteorological stations (Figure 14d). At the same time, winter precipitation, primarily snowfall, decreased significantly with P < 0.05 at Wudaoliang and Anduo stations (Figure 15d). Increase in winter air temperature at both Wudaoliang and Anduo results in increase in 14 of 22

15 Figure 11. (a) Time series of annual precipitation, (b) departures from its mean from 1995 through 2005, and (c) rate of annual precipitation change from four meteorological stations along the Qinghai- Tibetan Highway. soil temperatures within the active layer and upper permafrost at WD2 and AD1 sites. Observed permafrost temperature increase at 6 m depth was greatest in spring and summer (Figure 7) which can be explained by substantial increase in winter air temperature (Figure 14d) due to the time lag. However, soil temperatures within the active layer (Figure 8) and upper permafrost (Figure 9) show a yeararound increase at Anduo, Wudaoliang, and other sites. On the basis of available seasonal air temperature and precipitation data and information, we are unable to explain the year-around warming of permafrost at these sites. Additional data and information such as snow conditions, vegetation, surface wetness, soil moisture, and even surface energy fluxes are needed for better understanding the governing physical processes that drive the soil thermal regime on the Qinghai-Tibetan Plateau. 5. Discussions [40] Overall, permafrost on the Qinghai-Tibetan Plateau is relatively warm with the majority of permafrost temperature is within 2 C below the freezing point of water [Cheng and Wu, 2007]. Thickness of the active layer varies from about 1.2 m at KM1 site to about 4 m at CM2 site. Permafrost thickness ranges from about 10 m in the south to about 120 m in the mountain areas. The majority of permafrost is ice-rich. The combination of warm and icerich permafrost over the Qinghai-Tibetan Plateau could 15 of 22