Recent changes in ground surface thermal regimes in the context of air temperatu

QingFeng Wang ,TingJun Zhang ,XiaoQing Peng ,Bin Cao

1.State Key Laboratory of Frozen Soil Engineering,Cold and Arid Regions Environmental and Engineering Research Institute,Chinese Academy of Sciences,Lanzhou,Gansu 730000,China

2.College of Earth and Environmental Sciences,Lanzhou University,Lanzhou,Gansu 730000,China

3.National Snow and Ice Data Center,Cooperative Institute for Research in Environmental Sciences,University of Colorado at Boulder,Boulder,80309,USA

1 Introduction

Global warming is a non-controversial scientific fact (Qinet al.,2007).The Fourth Assessment Report(AR4) of the Intergovernmental Panel on Climate Change (IPCC) points out that,the earth surface temperature has warmed by 0.74 °C in the last four decades (Solomonet al.,2007).Global surface temperature has generally warmed for the entire instrumental record,and the most significant and strongest warming occurred in the most recent 40 years (Cohenet al.,2012).Moreover,changes in ground surface thermal regimes play a vital role in surface and subsurface hydrology,ecosystem diversity and productivity(Frauenfeldet al.,2007),and global thermal,water and carbon budgets (Jinet al.,2009) as well as climate change.Numerous studies on climate warming in the Heihe River Basin,the Qilian Mountains region,and the Hexi Corridor were published in the past decade.They mainly focused on the spatiotemporal distribution characteristics,especially the regional difference,and the catastrophe of climate change (e.g.,Gong and Li,2001;Lanet al.,2004;Jiaet al.,2008a,b),as well as the impacts of climate change on surface runoff(e.g.,Dinget al.,2000;Lanet al.,2001;Wang and Li,2005;Zhang,2009).But the ground surface thermal regimes in the context of air temperature warming are still poorly understood over the Heihe River Basin.

There are two main indices that could directly react to changes in ground surface thermal regimes,including ground surface temperature and freezing/thawing index.Soil temperature,especially ground surface temperature,is a sensitive climate indicator,which integrates meteorological elements and all processes occurring at/above the ground surface.Ground surface temperature is of great importance for monitoring of the surface-to-air energy budget by affecting sensible and latent heat fluxes (Okuet al.,2006;Wuet al.,2013).In addition,it could influence the physical and biogeochemistry processes occurring in the soil.Generally speaking,the freezing/thawing index is defined as the cumulative number of degree days for air or surface temperatures which are below/above 0 °C (Associate Committee on Geotechnical Research,1988;van Everdingen,2005).Therefore,it is a measure of the duration and temperature in cold/warm season during a year(Johnson and Hartman,1971).It can be used to predict and map permafrost and seasonally frozen ground distribution (Nelson and Outcalt,1987),estimate active layer thickness (ALT) (Nelsonet al.,1997;Shiklomanov and Nelson,1999,2002;Zhanget al.,2005),depth of ground frost penetration(Zhanget al.,2001),provide information for engineering designs in cold regions (US Army/Air Force 1966;Lunardini,1981;Steurer,1996;Steurer and Crandell,1995),as well as a possible useful indicator of climate change (Frauenfeldet al.,2007;Wuet al.,2011b).These indices can also be used to estimate the maximum thickness of sea,river,and lake ice (Johnson and Hartman,1971).

The main purpose of this study is to investigate the observed trends of air temperature warming and ground surface thermal regimes response to air temperature warming in recent years over the Heihe River Basin in Northwest China.

2 Study area

The Heihe River Basin is located in the central part of the Eurasian continent,far from the oceans and surrounded by high mountains.It is the second largest inland river basin in Northwest China,located in the central part of Hexi Corridor and roughy between 98°00'E–101°30'E,38°N–42°N (Gao and Li,1991).The basin originates in the watershed between the Zoulangnanshan and Tuolainanshan Mts.in Qilian Mts..Geographically,the basin is divided into three parts from south to north,the upper reaches (from Yingluoxia valley upstream),the middle reaches (between Yingluoxia and Zhengyixia valleys),and the lower reaches (from Zhengyixia valley downstream) (Cheng,2009) (Figure 1).

The upper reaches,with elevation ranging from 2,000 m a.s.l.to 5,500 m a.s.l.,belong to the cold semi-arid mountain zone dominated by shrubs and trees.The vegetation displays obviously altitudinal zonation from bottom upwards,i.e.,desert steppe,alpine steppe,alpine frost-steppe,alpine shrub-meadow,alpine meadow,and alpine cushion vegetation zones.Mean annual air temperature (MAAT) in the area is lower than 2 °C,and mean annual precipitation (MAP) increases from about 250 mm in the low-mountain or hill zone to about 500 mm in the high-mountain zone (Chen and Qu,1992).It is calculated that average decreasing rate of MAAT and increasing rate of annual precipitation with elevation from Yingluoxia valley to Yeniugou is 0.80 °C/(100m) and 15 mm/(100m),respectively (Chenget al.,2010).The middle reaches belong to the temperate zone controlled by crops like wheat and corn,with MAAT less than 6–8 °C.With the elevation in the middle reaches decreases from<2,000 m a.s.l.to 1,000 m a.s.l.,the MAP decreases from 250 mm to <100 mm from south to north (Liet al.,2001).Mainly occupied by gobi with mean elevation of about 1,000 m a.s.l.,the lower reaches pertain to the downstream warm temperate zone,with MAAT of 8–10 °C and MAP of <50 mm (Qi and Luo,2005;Liet al.,2012).

3 Data source and method

In this study,we collected daily surface temperatures (including mean,maximum and minimum) and monthly air temperatures at 18 meteorological stations from the China Meteorological Administration (CMA)located over the Heihe River Basin and its adjacent area.There are 12 meteorological stations located over the Heihe River Basin (including Qilian,Yeniugou,Tuole,Shandan,Zhangye,Gaotai,Jiuquan,Jinta,Dingxin,Wutonggou,Jihede,and Ejinaqi) and 6 meteorological stations are located in the adjacent area of the basin (including Yongchang,Menyuan,Gangcha,Yumen,Mazongshan,and Guaizihu) (Figure 1).

Mean annual ground surface temperature (MAGST)is the mean daily ground surface temperatures in a year.Maximum annual ground surface temperature(MaxAGST) and minimum annual ground surface temperature (MinAGST) are the maximum and minimum daily ground surface temperature in a year,respectively.

Figure 1 The distribution map of the meteorological stations from CMA over the Heihe River Basin and its adjacent area.The black line is the river system of Heihe River

Generally,when performing the calculation based on daily data we compute the annual freezing/thawing index as the sum of surface temperature for all days with below/above 0 °C during the freezing/thawing period.Moreover,the freezing index is computed over a period from Jul.1 through to Jun.30 of the following calendar year,while the thawing index is calculated from Jan.1 through Dec.31 of the same calendar year(Zhanget al.,1997;Frauenfeldet al.,2007;Wuet al.,2011b).The freezing/thawing index (FI/TI) is calculated by the following formulae:

whereFIandTIare the surface freezing and thawing index,respectively;Tiis daily surface temperature;MFandMTrepresent to the ending date of freezing and thawing period,respectively.

MAGST and surface freezing/thawing index time series at meteorological stations all contained continuous observation data over the 1972/1959–2006/2005 period except Yeniugou,Ejinaqi,Wutonggou,Jinta and Jihede.For data comparability in 1960/1959–2006/2005,we recovered the missing data at these 5 stations through the linear regression method based on nearby station data at significant level of at leastp≤0.05.The missing data of air temperature at Wutonggou and Jihede was also recovered in a similar way.

4 Results

4.1 Air temperature warming

The linear tendency rate for spring,summer,autumn and winter air temperatures as well as MAAT estimated by the linear least squares regression method at meteorological stations in 1960–2008 over the Heihe River Basin is presented in table 1.The tendency rate for summer air temperature at Shandan station is not significant at significant level ofp≤0.05.Significant increases in summer air temperature are observed at 11 other meteorological stations,with the linear tendency rate ranging from 0.18 °C/decade to 0.41 °C/decade.Winter air temperature in 1960–2008 is significantly increased except at Wutonggou and Dingxin,with the linear tendency rate of 0.44–0.85 °C/decade.Spring and autumn air temperatures as well as MAAT in 1960–2008 at the 12 meteorological stations reveal significant trends of 0.19–0.45 °C/decade,0.20–0.55 °C/decade,and 0.26–0.52 °C/decade,respectively.

Table 1 The linear tendency rate for spring,summer,autumn and winter air temperatures as well as MAAT (°C/decade) estimated by the linear least squares regression method at the 12 meteorological stations in 1960–2008 over the Heihe River Basin

Based on monthly air temperature data of the 12 meteorological stations of CMA from 1960 through 2008,all the meteorological stations are composited in order to structure an integrated long-term time series over the Heihe River Basin.Estimating spring,summer,autumn and winter air temperatures from 1960 through 2008 reveals a statistically significant trend of 0.31 °C /decade,0.28 °C/decade,0.37 °C/decade and 0.50 °C /decade,and a net change of 1.5 °C,1.4 °C,1.8 °C,and 2.5 °C over the 49-year period,respectively(Figure 2).The linear tendency rate for winter air temperature is obviously bigger than others.The coefficients of determinationR2of the fitting trend line are 0.25,0.30,0.37 and 0.33,respectively,at significant level ofp<0.001.In a similar way,MAAT for 1960–2008 reveals a statistically significant trend of 0.37 °C /decade (R2=0.57,p<0.001) and a net change of 1.8 °C over the 49-year period (Figure 3).

The aforementioned conclusion is consistent with the following studies in Northwest China and on the Qinghai-Tibet Plateau (QTP).For example,the linear tendency rate of MAAT was from-0.34 to 0.88 °C/decade in 1951–2006 in Northwest China,and the mean value was 0.3 °C/decade (Zhai,2008).Analyzing data at 72 meteorological stations on the QTP,Weiet al.,(2003) thought that air temperature on the QTP was becoming warmer.The tendency rate of air temperature for most meteorological stations was between 0.2–0.3 °C/decade in winter and spring,and it was between 0.1–0.2 °C/decade from June through September.MAAT revealed a statistically significant trend of 0.28 °C/decade for 1961–2004 in Qinghai Province (Wanget al.,2007).The tendency rate for MAAT was 0.32 °C/decade for 1959–2008 in Northwest China (Zhaoet al.,2011).

4.2 Ground surface temperature trend

The MAGSTs for 1972–2006 at the 12 meteorological stations reveal significant trends of 0.44–0.90 °C/decade (shown in table 2).The tendency rate for MaxAGST at Qilian,Tuole,Wutonggou and Jihede is not significant at significant level ofp≤0.05.The MaxAGSTs for 1972–2006 at 8 other meteorological stations reveal significant trends of 0.59–2.05 °C/decade (Table 2).The averaged MAGST time series in the period between 1972 and 2006 indicates a statistically significant trend of 0.58 °C/decade (Figure 4).Estimating MaxAGSTs from 1972 through 2006 reveals a statistically significant trend of 1.27 °C/decade,and a net change of 5.2 °C over the 41-year period.The averaged MinAGST time series in the same period remains unchanged as a whole (Figure 5).

Figure 2 Averaged time series of seasonal air temperature in 1960–2008 over the Heihe River Basin.Grey and dotted lines in bold present three years moving lines and the linear least squares regression lines,respectively (same in figures 3,4,5,6)

Figure 3 Averaged time series of MAAT in 1960–2008 over the Heihe River Basin

Figure 4 Averaged time series of MAGST for 1972–2006 over the Heihe River Basin

4.3 Freezing/thawing index trend

The surface freezing index time series in 1959–2005 show significant decreasing trends except at Wutonggou,Gaotai and Jihede,with the linear tendency rate ranging from-74.9 °C/decade to-20.6 °C/decade.The surface thawing index in 1960–2006 show significant increasing trends of 33.6–172.7 °C/decade except at Tuole.Significant trends in the ratio of freezing index to thawing index(RFT) are observed at 9 other meteorological stations except at Qilian,Wutonggou and Gaotai,with the linear tendency rate ranging from-0.05/decade to 0.00/decade (Table 2).

In a similar way,the composite time series of freezing/thawing index and the RFT in 1959–2006 are structured.Estimating surface freezing/thawing index reveals a statistically significant trend of-42.5 °C-day/decade and 85.4 °C-day/decade in 1959–2006,respectively.The RFT in 1960–2005 over the basin exhibits a statistically significant trend of-0.018 per decade and a net change of-0.083 over the 46-year period (Figure 6).And it is determined by the decreasing freezing index and the increasing thawing index trends.

Similar results have been determined by other studies.Based on the analysis of ERA-40,CRU TS2.1 and CAI 1.02 datasets,Frauenfeldet al.,(2007) reveals a similar trend for air freezing/thawing index in 1958–2002 in the seasonally frozen ground regions between 20°N and 50°N on the Northern Hemisphere.The air freezing/thawing index at 7 meteorological stations along the Qinghai-Tibet Railway has decreased by 16.7–59.1 °C-day/decade and increased by 19.8–45.6 °C-day/decade in 1966–2004,respectively (Jianget al.,2007).The surface freezing/thawing index is also undergoing a negative/positive trend in 1961–2001 on the QTP,albeit of different magnitude (Wuet al.,2008).The surface freezing index displays decreasing trends (-120 to-20 °C-day/decade) at 6 meteorological stations over the period 1987–2005 in Mongolia,while exhibiting increasing trends (10 to 390 °C-day/decade) at 14 other stations.However,the surface thawing index all shows a statistically significant increase with a rate of 16–37 °C-day/decade at a significance level of 0.05 except at Baruunturuu station (Wuet al.,2011b).The freezing/thawing index at 16 meteorological stations,which are located in or adjacent to permafrost regions on the central QTP,has decreased by 111.2 °C-day/decade and increased 125.0 °C-day/decade in 1980–2007,respectively (Wuet al.,2013).

Table 2 The linear tendency rate for MAGST (°C/decade),MaxAGST (°C/decade),surface freezing/thawing index (°C-day/decade) and the RFT (/decade) estimated by the linear least squares regression method at the 12 meteorological stations in 1972/1959–2006/2005 over the Heihe River Basin

Figure 6 Averaged time series of (a) surface freezing index,(b) thawing index and (c) the RFT in 1959–2006 over the Heihe River Basin

5 Discussion

The land surface and atmosphere could adjust the ground surface thermal regimes and air temperature changes to make their changing tendency accordant,through various thermodynamic processes of the land-atmosphere interaction,such as,radiative flux transfer,and sensible heat flux exchange.

The MAGST,MaxAGST and RFT time series are significantly correlated with MAAT atR=0.96,R=0.61 andR=-0.80 at significant level ofp<0.01,indicating that a variance of 92% in MAGST,37% in MaxAGST and 64% in RFT can be accounted for by changes in MAAT,respectively.Freezing index time series is correlated with winter air temperature atR=-0.82 (p<0.01),indicating that 67% of freezing index variability can be accounted for by winter air temperature.The correlation between thawing index and summer air temperature is statistically significant and strongR=0.85 (p<0.01),indicating that 72% of thawing index variability can be accounted for by summer air temperature.

Undoubtedly,the increase in MAGST and the decline in freezing index will greatly weaken maximum seasonal freeze depth.It is calculated that maximum seasonal freeze depth in 1960–2007 exhibits a statistically significant trend of-4.0 cm/decade and a net change of-19.2 cm in the 48-year period over the Heihe River Basin (Wang,2013).

The increase in thawing index implies a profound influence on the southern/lower limit of permafrost,ALT,expansion of taliks,as well as permafrost temperature and depth.Since the 1980s,the lower limit of permafrost has risen about 50–80 m in the source area of the Yellow River on the QTP (Jinet al.,2009).ALT and ground temperatures at the bottom of the ALT for the 3 observation fields (China01,China02 and China04) have increased by 4 cm/a and 0.06 °C/a from 1999 through 2007/08 along the Qinghai-Tibet Highway (Zhaoet al.,2010).The increase in ALT has been observed at a rate of about 6.3 cm/a from 2006 through 2010 along the Qinghai-Tibet Railway.The mean rising rate of permafrost temperature at the depth of 6.0 m and the mean annual ground temperature(MAGT) at depth of zero amplitude is about 0.02 °C/a and 0.012 °C/a,respectively (Wuet al.,2011a).

It is estimated that the total area of permafrost region occupies approximately 11% of the Heihe River Basin (Zhanget al.,2012).Both the significant decline in freezing index and increase in thawing index might be expected to accelerate the permafrost degradation in the upper reaches of the Heihe River Basin.The extensive permafrost degradation will exert profound influence on the surface energy and water exchanges,carbon budgets,ecosystem dynamics,hydrological processes,and engineering infrastructures (Cheng and Zhao,2000;Nelsonet al.,2001;Zhanget al.,2005;Cheng and Wu,2007;Jinet al.,2009;Zhaoet al.,2010;Wuet al.,2013).

6 Conclusions

Significant increases in summer and winter air temperatures are observed at meteorological stations over the Heihe River Basin,with the linear tendency rate of 0.18–0.41 °C/decade except at Shandan and 0.44–0.85 °C/decade except at Wutonggou as well as Dingxin,respectively.Spring and autumn air temperatures as well as MAAT in 1960–2008 at the 12 stations reveal significant trends of 0.19–0.45 °C/decade,0.20–0.55 °C/decade,and 0.26–0.52 °C/decade,respectively.Estimating spring,summer,autumn and winter air temperatures and MAAT from 1960 through 2008 reveals a statistically significant trend of 0.31 °C/decade,0.28 °C/decade,0.37 °C/decade,0.50 °C/decade and 0.37 °C/decade,respectively.Due to the warming air temperature,MAGSTs,MaxAGSTs,and surface thawing index time series exhibit increasing trend for most or even all the meteorological stations over the basin,while the surface freezing index and the RFT time series reveal a decreasing trend.

The MAGSTs for 1972–2006 at the 12 meteorological stations reveal significant trends of 0.44–0.90 °C/decade.The MaxAGSTs for 1972–2006 at eight other meteorological stations except at Qilian,Tuole,Wutonggou and Jihede reveal significant trends of 0.59–2.05 °C/decade.The averaged MAGST time series in the period between 1972 and 2006 indicates a statistically significant trend of 0.58 °C/decade.Estimating MaxAGST from 1972 through 2006 reveals a statistically significant trend of 1.27 °C/decade.The MinAGST time series remains unchanged as a whole.

The surface freezing index time series in 1959–2005 show significant decreasing trends except at Wutonggou,Gaotai and Jihede,with the linear tendency rate ranging from-74.9 °C/decade to-20.6 °C/decade.The surface thawing index in 1960–2006 show significant increasing trend of 33.6–172.7 °C/decade except at Tuole.Significant trends in RFT are observed at 9 other meteorological stations except at Qilian,Wutonggou and Gaotai,with the linear tendency rate ranging from-0.05/decade to 0.00/decade.Estimating surface freezing/thawing index and RFT reveals a statistically significant trend of-42.5 °C-day/decade,85.4 °C-day/decade and-0.018/decade in 1959–2006,respectively.

This study is supported by the Chinese Academy of Sciences Key Research Program (No.KZZD-EW-13),the Natural Science Foundation of China (Nos.91025013,91325202),the State Key Laboratory of Frozen Soil Engineering (No.SKLFSE-ZY-06),CAS,and the Major Research Plan of the National Natural Science Foundation of China (No.2013CBA01802).

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