Journal of Geographical Sciences

© 2007 Science in China Press Springer-Verlag

Received: 2007-06-16 Accepted: 2007-08-16 Foundation: National Natural Science Foundation of China, No.40401054; National 973 Program of China, No.

2005CB422004; CAS Hundred Talents Program; Knowledge Innovation Program of CAS, No. KZCX3-SW-339/334

Author: Zhang Yongjun (1978–), Ph.D Candidate, specialized in paleoclimate. E-mail: atlaszhang@itpcas.ac.cn

www.scichina.com www.springerlink.com

DOI: 10.1007/s11442-007-0431-0

Seasonal air temperature variations retrieved from a Geladaindong ice core, Tibetan Plateau

ZHANG Yongjun1,2, KANG Shichang1,3, QIN Dahe3, REN Jiawen3, ZHANG Yulan1,2, Bjorn GRIGHOLM4, Paul MAYEWSKI4

1. Institute of Tibetan Plateau Research, CAS, Beijing 100085, China; 2. Graduate University of the Chinese Academy of Sciences, Beijing 100049, China; 3. Key Laboratory of Cryosphere and Environment, CAREEI, CAS, Lanzhou 730000, China; 4. Climate Change Institute, University of Maine, Orono, ME 04469, USA

Abstract: A 70-year history of precipitation δ18O record has been retrieved using an ice core drilled from a plat portion of the firn area in the Guoqu Glacier (33º34′37.8″ N, 91º10′35.3″ E, 5720 m a.s.l.) on Mt. Geladaindong (the source region of Yangtze River) during October and November, 2005. Based on the seasonality of δ18O records and the significant positive rela-tionships between monsoon/non-monsoon δ18O values and summer/spring air temperature from the nearby meteorological stations, the history of summer and spring air temperature have been reconstructed for the last 70 years. The results show that both summer and spring air temperature variations present similar trends during the last 70 years. Regression analysis indicates that the slope of the temperature-δ18O relationship is 1.3℃/‰ for non-monsoon δ18O values and spring air temperature, and 0.4℃/‰ for monsoon δ18O values and summer air temperature. Variation of air temperature recorded in the ice core is consistent with that in the Northern Hemisphere (NH), however, the warming trend in the Geladaindong region is more intense than that in the NH, reflecting a higher sensitivity to global warming in the high elevation regions. In addition, warming trend is greater in spring than in summer.

Keywords: ice core δ18O; summer; spring; air temperature variations; Mt. Geladaindong; Tibetan Plateau

1 Introduction

The global mean land surface air temperature has risen by 0.3–0.6℃ in the past 100 years since the 1900s (Folland et al., 2001). The global warming would influence the world environment, and a lot of Chinese scientists applied themselves to this research (Chen et al., 1998; Chu et al., 2005; Du et al., 2001; Li et al., 1993; Lin et al., 1993; Liu et al., 2000; Ren et al., 2005a, 2005b; Tang et al., 2005; Yao et al., 1994, 2000; Zhang et al., 2006; Zhai et al., 1997; Zhu et al., 2001). The sensitivity to the global warming is different in different places

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because of the latitude difference, altitude difference, season switch and “urban heat island effect”. Based on the analysis of observational data of Chinese meteorological stations, since the 1950s the land surface mean air temperature amplitude in northern China has been hig-ger than in the southern (Chen et al., 1998; Ren et al., 2005a, 2005b; Zhang et al., 2006; Zhai et al., 1997), and high altitude region higher than low altitude region, and winter higher than summer, and because of the intensification of urbanization process, the “urban heat island effect” has also become more and more remarkable (Chu et al., 2005; Du et al., 2001; Li et al., 1993; Lin et al., 1993; Liu et al., 2000; Tang et al., 2005; Yao et al., 1994, 2000; Zhu et al., 2001). The basic fact of Chinese climate change was confirmed, and the probable reasons were also discussed. These researches made us understand the impact of global warming on our living environment more deeply. However, almost all these researches have been concentrated in the densely populated areas but rarely in the high altitude and frigid regions.

Everyone knows the Yangtze River and Yellow River which foster millions of people in China were originated in the high altitude and frigid regions. Since the 1970s, the floods of the Yangtze River and flow-breaking of the Yellow River have taken place frequently, seri-ously influencing the harmonious and sustainable development among the people, resources and environment in these drainage areas. Therefore, it is important for deeply studying the climate change of the source areas of the rivers (SARs). Lack of meteorological observa-tional data not only in the SARs but also in the Tibetan Plateau restricted further studies on the climate change. Some scientists studied the air temperature change using the ice core δ18O record as a proxy and obtained good results (Tian et al., 2006; Yao et al., 1991a, 1996, 1998, 2006). Wang et al. (2003) thought the air temperature in the northern Tibetan Plateau had a clearly descent trend since the 1970s based on the Malan ice core δ18O record. This result was opposite to the conclusion of Mustage ice core (Tian et al., 2006), Guliya ice core (Yao et al., 2006), and Dasuopu ice core (Yao et al., 2006) in the southern Tibetan Plateau. This indicated that the climate change had regional characteristics in the Tibetan Plateau, and the research on air temperature of Geladaindong region was necessary.

Mt. Geladaindong, located in the center of the Tibetan Plateau, is the summit of Tanggula Mountains, and also the headstream of the Yangtze River. Due to the lack of the long time meteorological observational data, in-depth research of climate change is restricted in this area. Therefore, climate change would lead to glacier retreat, permafrost degradation, and environmental deterioration of the SARs. This would not only destroy the farming/pasturing of the source area but also restrict the economic and social developments in the downstream region of the Yangtze River. It is thus significant for studying the air temperature change of the source area, specially the seasonal air temperature change. Based on the Geladaindong ice core δ18O record from firn basin of the Guoqu glacier, the spring and summer air tem-perature time series were reconstructed, and the annual and decadal variational characteris-tics of spring and summer air temperature since 1935 were also discussed in this paper.

2 Samples collection and analysis

In the surroundings of Mt. Geladaindong with an elevation of 6621 m a.s.l., 105 glaciers covering an area of 333 km2 are developed (Zhang et al., 1981). In November, 2005, the

ZHANG Yongjun et al.: Seasonal air temperature variations retrieved from a Geladaindong ice core 433

Sino-US Cooperation Expedition Team recovered two deep ice cores (74 m and 147 m) and a shallow ice core (26 m). At the same time three snowpits (SP1, SP2 and SP3) were also collected (Figure 1). This paper analyzed the 74 m ice core (33o34′37.8″N, 91o10′35.3″E, 5720 m a.s.l.) and SP2 (33º34′37.8″ N, 91º10′ 35.3″E, 5720 m a.s.l.).

These ice cores were returned frozen to the State Key Laboratory of Cryospheric Science, Chinese Academy of Sciences. Using the continuous ice core melter system (Osterberg et al., 2006), the 74 m core was cut at intervals between 2 and 5 cm and 2336 samples were

Figure 1 Location of ice cores and snowpits on the Guoqu Glacier of Mt. Geladaindong, Tibetan Plateau in 2005

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available. The anion and ion of all samples and Beta activity were analyzed in Climate Change Institute, University of Maine, USA. Anion was analyzed by ICS-2500, and the testing accuracy was 1 ppb. Ion was analyzed by ICS-2000, and the testing accuracy was 1 ppb. The oxygen isotopic ratio (δ18O) was analyzed by the Gas Stable Isotope Ratio Mass Spectrometry (MAT-253) in Institute of Tibetan Plateau Research, Chinese Academy of Sciences, and the testing accuracy is ±0.2‰.

3 Dating

Previous studies showed that δ18O of precipitation was the function of air temperature at the height of water vapor condensate processing, and it had a high positive correlation with the land surface air temperature (Dansgaard et al., 1964). Previous studies on δ18O in the pre-cipitation and ice core over the Tibetan Plateau also indicated in the northern Tibetan Plateau the high positive correlation between the δ18O and air temperature (Tian et al., 2001; Yao et al., 1996; Zhang et al., 1995). Otherwise, in the southern Tibetan Plateau, though the af-fected by the “amount effect” (Wake et al., 1995), δ18O still had a high positive correlation with the air temperature at the long time scale (Tian et al., 2001; Thompson et al., 1995; Yao et al., 2006). Hence, using the ice core δ18O as the proxy of air temperature, the air tem-perature variation over the Tibetan Plateau could be reconstructed. The climate boundary between the northern and southern Tibetan Plateau was along Mt. Tanggula (Tian et al., 2001; Wei et al., 2003). We can prove whether Geladaindong ice core δ18O from Mt. Tang-gula was affected by “amount effect” through the snowpit record. Figure 2 showed the δ18O and K+ concentration of Geladaindong snowpit versus depth and compared with the

Figure 2 The δ18O and K+ concentration of Geladaindong snowpit versus depth and comparsion with the monthly δ18O of precipitation at Lhasa

ZHANG Yongjun et al.: Seasonal air temperature variations retrieved from a Geladaindong ice core 435

monthly δ18O of precipitation at Lhasa (Tian et al., 1997). In monsoon season, δ18O of pre-cipitation in Geladaindong, like Lhasa in the southern Tibetan Plateau, was influenced by “amount effect”, and its value was less than its in non-monsoon. This indicated that in sum-mer the India monsoon controlled Geladaindong and in non-monsoon (spring and fall) the regional convectional precipitation was dominating. In other words, the δ18O value of pre-cipitation would descend sharply from spring to summer, and this accorded with Yao et al. observational results in Mt. Tanggula very much (Yao et al., 1991b).

The study on particles from Geladaindong snowpit (Wake et al., 1994) showed the ion concentration in non-monsoon season is much higher than it in monsoon season, and this was in accordance with K+ concentration variation of Geladaindong snowpit in this paper. Hence, based on the seasonal variation of ice core δ18O and dominating ions, we dated this ice core. The accuracy of the dating is verified with known nuclear bomb horizons (1963) identified in the well-preserved total β-activity from the ice core. We estimate the date of the core back to 1935 at the depth of the 47 m. This combination of dating techniques yields an annually resolved chronology that is as accurate as to within ±1 year.

4 δ18O record of Geladaindong ice core revealing the spring and summer air temperature variation Figure 3 showed that δ18O of Geladaindong ice core had a clearly seasonal switch. Based on this conclusion, we could divide the δ18O into two parts, and then counted the mean annual δ18O values of monsoon season and non-monsoon season. We compared non-monsoon δ18O with monsoon δ18O of Geladaindong ice core during 1935–2004, and got that the variation of non-monsoon δ18O had a similar trend to monsoon δ18O, but a little difference existed.

Figure 3 Variations of δ18O and K+ concentration in the upper 47 m of the Geladaindong ice core and dating of the core (Dashed lines represent annual boundaries.)

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From the 1930s to the end of the 1960s, the fluctuation of δ18O values was relatively little not only in non-monsoon season but also in monsoon season. Since the 1970s, values of δ18O in non-monsoon season had a continuous but slow descent, and presented minimum at the end of the 1980s. At the same time, values of monsoon δ18O had a dramatic decrease and then reached the minimum in the mid-1970s, followed by an increasing trend from the end of 1970s to the early 1980s. It had other descent trend in the 1980s, and then reached the second minimum at the end of the 1980s and the early 1990s. Since the 1990s, values of δ18O between non-monsoon season and monsoon season ascended dramatically. The δ18O variations not only in the monsoon season but also in the non-monsoon season have the same trend, and the three phases were partitioned: calm period during 1935–1970 with very little fluctuation in this period, minimum period during 1971–1990, and dramatically as-cending period during 1991–2004. Otherwise, the significant characteristic of Geladaindong ice core δ18O variation was ascending dramatically since the 1990s. In this period, the linear trend in non-monsoon season was 1.3‰·(10a)−1, and 1.7‰·(10a)−1 for monsoon season.

Previous studies indicated that monsoon precipitation was dominating and more than 80% of the annual precipitation arrived during June–September over the Tibetan Plateau, and most precipitation in the non-monsoon period concentrated in spring and fall (Wang et al., 2003; Qiao et al., 1994). After analyzing correlation between precipitation δ18O of non-monsoon season and monsoon season and mean monthly air temperature1 from Amdo meteorological station during 1966–2003, which is the nearest station to the core site, we found that the correlation coefficient between the 5-year running means of δ18O in

Figure 4 Variations of annual non-monsoon (a) and monsoon (b) δ18O in the Geladaindong ice core during 1935–2004 (Coarse lines for 5-year running mean, dashed lines for linear trend.)

1 Monthly mean air temperature data in Amdo meteorological station during 1966–2003 come from China Climate Center, China Meteorological Administration.

ZHANG Yongjun et al.: Seasonal air temperature variations retrieved from a Geladaindong ice core 437

non-monsoon season and mean April air temperature in Amdo was 0.46 (P<0.01), and the correlation coefficient between annual δ18O in monsoon season and mean air temperature from July to September was 0.38 (P<0.05), 0.76 (P<0.001) for the 5-year running means. This suggested that the non-monsoon δ18O record reflected the spring air temperature varia-tion, and monsoon δ18O record reflected the summer air temperature variation. In contrast with the non-monsoon season, the correlation during monsoon season was better. This could be explained through (1) in Geladaindong most precipitation concentrated in monsoon sea-son and precipitation was very little in non-monsoon season, and (2) comparisons between ice core δ18O and instrument records, we could not date the ice core at the month scale ac-curately. Especially for the non-monsoon season with little precipitation, this would lead to low correlation between δ18O and air temperature.

Figure 5 Comparisons between Geladaindong ice core δ18O (fine lines) and air temperature (coarse lines) at Amdo station (a) Non-monsoon δ18O and April air temperature; (b) monsoon δ18O and July–September air temperature

Figure 5 illustrated the comparisons between Geladaindong ice core δ18O and air tem-perature at Amdo station. δ18O and air temperature not only in non-monsoon season but also in monsoon season had varied similarly. In monsoon season, they had a same ascending trend since the 1990s. In non-monsoon season, both fluctuations were little. Based on their linear correlation between the 5-year running means of δ18O in non-monsoon season and in monsoon season and mean April, July–September at Amdo meteorological station respec-tively, the regressive equations were established:

δ18O(non-monsoon)=0.8T–9.4 (r=0.46) (1) δ18O(monsoon)=2.3T–28.0 (r=0.76) (2)

Equation (1) shows that 1‰ change in δ18O during non-monsoon season is equal to 1.3℃, and equation (2) shows that 1‰ change in δ18O during monsoon season is equal to 0.4℃.

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Figure 6 Comparison between non-monsoon δ18O in Geladaindong ice core and spring air temperature in the Northern Hemisphere during 1935–2004 (Fine lines for annual values and coarse lines for 5-year running mean values.)

Figure 7 Comparison between monsoon δ18O in Geladaindong ice core and summer air temperature in the Northern Hemisphere during 1935–2004 (Fine lines for annual values and coarse lines for 5-year running mean values.)

This is consistent with Yao’s results (Yao et al., 1996, 2006). Figures 6 and 7 give comparisons between non-monsoon/monsoon δ18O in Geladaindong

ice core and spring/summer air temperature in the Northern Hemisphere (NH) during

ZHANG Yongjun et al.: Seasonal air temperature variations retrieved from a Geladaindong ice core 439

1935–2004. Figure 6 shows that in the past decades δ18O and air temperature have a similar ascending trend, but there is a little difference in their variational details. For example, from the 1940s to the mid-1970s the NH air temperature (Jones et al., 2006, http://cdiac.esd.ornl.gov/trends/temp/jonescru/ jones.html) has a slow descending trend, and it reaches the minimum in the mid-1970s. At the same time, for δ18O in Geladaindong ice core this slow descending lasts to the end of the 1980s, and lags behind the NH air tempera-ture variation. This indicates that there are differences between regional and global air tem-perature variations. Otherwise, based on the instrument records around Geladaindong, cold period in the 1980s is found. The correlation coefficient between δ18O in non-monsoon sea-son and spring air temperature in NH is 0.29 (P<0.02). Figure 7 indicates the annual fluctua-tions of monsoon δ18O in the ice core record are highly correlated with the NH air tempera-ture. The correlation coefficient between them is 0.32 (P<0.01), and 0.41 (P<0.001) for 5-year running mean. The lowest annual δ18O and the NH air temperature occur in the end of the 1970s, and they have an ascending trend, while in the last 20 years this trend is very acute.

In the past 20 years, the δ18O record not only has a remarkable ascending trend in non-monsoon season but also in monsoon season. Both spring and summer air temperatures in Geladaindong have an increasing trend under the global warming background. Based on the regression analysis results, since the 1990s the NH spring air temperature has risen 0.42℃, and 0.39℃ for summer. From the δ18O record of Geladaindong ice core, the Geladain-dong spring air temperature has risen 2.4℃, and 1.0℃ for summer. Hence, the warming magnitude, no matter in spring or in summer derived from the Geladaindong ice core record, is much larger than that of the NH. This indicates that the high elevation region is more sen-sitive to global warming.

5 Conclusions

The spring and summer air temperature changes during 1935–2004 are reconstructed by Geladaindong ice core.

(1) δ18O record in Geladaindong ice core could be divided into three phases: calm period during 1935–1970, minimum period during 1971–1990, and dramatically ascending period during 1991–2004. Otherwise, the significant characteristic of Geladaindong ice core δ18O variation was ascending dramatically since the 1990s. During this period, the linear trend in non-monsoon season was 1.3‰·(10a)−1, and 1.7‰·(10a)−1 for monsoon season.

(2) The detailed annual δ18O in ice core record allowed us to compare it with the local meteorological station air temperature data. These results show that the variation of non-monsoon δ18O is consistent with the local spring air temperature record, and the varia-tion of monsoon δ18O reflects the local summer air temperature change. Regression analysis shows that 1‰ change in δ18O during non-monsoon season is equal to 1.3℃, and 1‰ change in δ18O during monsoon season is equal to 0.4℃.

(3) In the past 20 years, the δ18O record no matter in non-monsoon season or in monsoon season has a remarkable ascending trend. Both spring and summer air temperatures in Ge-ladaindong have an increasing trend under the global warming background. The warming magnitude no matter in spring or in summer derived from the Geladaindong ice core record,

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is much larger than that of the NH. This indicates that the high elevation region is more sen-sitive to global warming.

Acknowledgements: The authors would like to thank all members of the Sino-US Cooperative Expedi-tion Team, 2005. We also thank Qu Dongmei for sample analysis in the laboratory.

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