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Temperature variations at Lake Qinghai on decadal scales and the possible relation to solar activities
ARTICLE IN PRESS
Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 138–144 www.elsevier.com/locate/jastp
Temperature variations at Lake Qinghai on decadal scales and the possible relation to solar activities Hai Xu, Xiaoyan Liu, Zhaohua Hou Institute of Earth Environment, Chinese Academy of Sciences, Xi’an, China Received 30 May 2007; received in revised form 16 September 2007; accepted 17 September 2007 Available online 22 September 2007
Abstract Temperature variations at Lake Qinghai, northeastern Qinghai–Tibet plateau, were reconstructed based on four highresolution temperature indicators of the d18O and the d13C of the bulk carbonate, total carbonate content, and the detrended d15N of the organic matter. There are four obvious cold intervals during the past 600 years at Lake Qinghai, namely 1430–1470, 1650–1715, 1770–1820, and 1920–1940, synchronous with those recorded in tree rings at the northeast Qinghai–Tibet plateau. The intervals of 1430–1470, 1650–1715, and 1770–1820 are consistent with the three coldest intervals of the Little Ice Age. These obvious cold intervals are also synchronous with the minimums of the sunspot numbers during the past 600 years, suggesting that solar activities may dominate temperature variations on decadal scales at the northeastern Qinghai–Tibet plateau. r 2007 Elsevier Ltd. All rights reserved. Keywords: Lake Qinghai; Temperature; Solar activity; Qinghai–Tibet plateau
1. Introduction It is well known that the Earth’s temperature is influenced by variable factors, such as solar activity, atmospheric circulation, the complex topography, different land cover, and the greenhouse gases. The dominating factor that controls local temperature variation is different between different regions. Therefore, although the general trend of temperature variations over wide geographic areas has been figured out by numerous works, it is still urgent to make clear the details of regional temperature variations and the causes behind them. Corresponding author. Tel.: +86 29 88324172; fax: +86 29 88320456. E-mail address: [email protected] (H. Xu).
1364-6826/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2007.09.006
The northeastern (NE) Qinghai–Tibet plateau is very sensitive to global climatic changes, with four planetary scale atmospheric circulations prevailing over there, namely the East Asian summer monsoon, the Indian summer monsoon, the Westerly, and the Asian winter monsoon. Temperature variations at this region have aroused wide attention. Yao et al. (2006) studied the temperatures during the last millennium based on d18O in ice cores. Kang et al. (2000) reconstructed the temperature variations from tree ring width. Liu et al. (2004) carried out a study of dendrochronology and discussed temperature variations at this region. Although similarities exist between those various proxy indices, there are also some differences both in timing and in magnitudes, which seriously limits the understanding of the temperature mechanisms.
ARTICLE IN PRESS H. Xu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 138–144
Much more evidence is necessary to shed light on the details of temperature variations at the NE Qinghai–Tibet plateau. On the other hand, previous work has suggested that climates on different timescales at the Qinghai–Tibet plateau may be driven by different forces. For example, based on the comparisons between climates recorded in ice cores in the Tibet plateau and those in Greenland, Yao et al. (2001a) pointed out that the climatic variations at the Tibet plateau on orbital timescales are dominantly controlled by solar irradiance. According to a temperature indicator of d18O in peat cellulose, Xu et al. (2006a) suggested that the quasi-100-year solar activity may be responsible for temperature variations on centennial timescales at Hongyuan, NE Qinghai–Tibet plateau. However, as revealed from ice cores (Wang et al., 2003) and tree rings (Xu et al., unpublished data), temperature variations on annual scales are primarily influenced by atmospheric circulations, like the ‘‘El Nin˜o–South Oscillation’’ (ENSO). Our question is: what is the controlling factor of temperature variations on decadal scales at the NE Qinghai–Tibet plateau? In this study, we studied temperature variations on decadal scales during the past 600 years based on temperature indicators extracted from Lake Qinghai, NE Qinghai–Tibet plateau. We compared the temperature indicators at Lake Qinghai with the proxy indices from tree rings nearby, and with the
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reconstructed solar activities. The results show that temperature variations at Lake Qinghai are synchronous with those at the NE Qinghai–Tibet plateau, and the main temperature events are generally in-phase with solar activities during the last 600 years. 2. Data 2.1. Temperature indicators from Lake Qinghai Lake Qinghai (361320 –371150 N, 991360 –1001470 E; Fig. 1) is located at the NE Qinghai–Tibet plateau. The meteorological records of the Gangcha station from 1958 to 2000 show that the temperature around this region varies between 8.9 1C and 13.7 1C in July, and between 16.4 1C and 10.74 1C in January, with a mean annual value of about 0.4 1C. Lake water is relatively cold, with an average temperature of about 10 1C for surface water and about 4 1C for bottom water in summer. The lake is frozen between December and March. Since the lake is very large and the anthropogenetic impacts are relatively small, Lake Qinghai is an ideal site for the study of palaeoclimatic changes. Surface sediments (about 30–40 cm long) in Lake Qinghai were collected with a self-designed gravity corer. The 210Pb and 137Cs radioactivities were measured to determine the sedimentation rate (see Xu et al., 2006b for details). An approach of
Fig. 1. Location of Lake Qinghai and some sites mentioned in the text.
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high-resolution multi-proxy indices was applied to evaluate the climatic significances of the proxy indices. Carbonates in the sediment of Lake Qinghai are mainly authigenic. Therefore, total carbonate content would be largely influenced by the ion concentration, which is closely linked to the evaporation of the lake water. Variations in d13Ccarb are governed by d13C of the dissolved inorganic carbon (DIC) in the DIC pool of the lake water. It has been supposed that the variations in d13CDIC of Lake Qinghai are closely linked to the fast evaporation of the lake water. As a result, variations in d13Ccarb can be attributed to the evaporation (see Xu et al., 2006c for details). Comparison between d18O of the lake water, river water, rainfall, and springs shows that evaporation is the controlling factor that determines d18O of the lake water. Since d18O of the lake water dominates d18Ocarb, variations in d18Ocarb can also be attributed to evaporation. Therefore, variations in total carbonate content, d13Ccarb, and d18Ocarb are dominated by evaporation of the lake water. Since evaporation at Lake Qinghai is primarily controlled by temperature, variations in d13Ccarb, d18Ocarb, and total carbonate content can be attributed to temperature variations (see Xu et al., 2006c for details). d15Ndetrended is from d15N of the organic matter after removal of the stratigraphic trend. Variation in d15Ndetrended is closely related to changes of the primary productivity in Lake Qinghai. When the primary productivity increases, the discrimination effect of 14N/15N triggers an increased d15Norg in the sediments (see Xu et al., 2006c for details). Since the primary productivity is mainly controlled by temperature at Lake Qinghai, the d15Ndetrended can be used to reflect the regional temperature variations (see Xu et al., 2006c for details). In summary, variations in d18Ocarb, d13Ccarb, total carbonate content, and d15Ndetrended can be attributed to temperature variations. The significances of these four indices have been verified by comparisons with instrumental temperature records and by comparisons with the temperature indicators from nearby sites (see Fig. 6 in Xu et al., 2006c). These four indices were normalized, and then stacked to one to reflect the temperature trend at Lake Qinghai. Some other researchers also studied the recent climatic changes at Lake Qinghai. However, most of those previous works ignored the compaction of the subsurface sediments during the early diagenesis, and the dates of these studies are largely different from those of our work (see Xu et al., 2006b for
details). As a result, we did not collect the data by other studies at Lake Qinghai. 2.2. Temperature indicators in tree rings and solar activity indices Previous work indicated that the tree ring widths at both Dulan (Yao et al., 2001b) and Qilan Mountain (Liu et al., 2004), NE Qinghai–Tibet plateau (see locations in Fig. 1), are sensitive to temperature variations. The variations in d13C of tree rings at Hongyuan (see location in Fig. 1) are also attributed to temperature variations (Xu et al., unpublished data). These data from tree rings were collected and were 10-piont averaged for comparison with the indices from Lake Qinghai. The Sun is the energy source of the Earth and its activity is generally regarded as the dominating factor for Earth’s temperature variations. Solar activities during the past several centuries can be reconstructed from the observations (Lean et al., 1995; Hoyt and Schatten, 1998; Solanki and Fligge, 2000). Meanwhile the proxy indices, such as the concentrations of cosmogenic 14C (Solanki et al., 2004) and 10Be (Beer et al., 2000; Usoskin et al., 2003), revealed much longer variations of the Sun. For example, Bard et al. (2000) reconstructed the solar irradiance during the last 1200 years based on atmospheric 14C concentration from tree rings and on 10Be concentration from the South Pole ice core; the reconstructed solar irradiance correlated well with the observed long-term variation of sunspot numbers. To investigate the solar–temperature relationship, we collected the reconstructed solar activities by Lean et al. (1995) and the reconstructed solar irradiance by Bard et al. (2000). 3. Temperature variations during the last 600 years at Lake Qinghai As shown in Fig. 2, the long-term trend of temperature variations is increasing, inferred from tree ring widths and isotopic indices in tree rings. This long-term trend can also be detected in d18O of Dunde ice core, Guliya ice core, and Dasuopu ice core (refer to Fig. 2 in Yao et al. (2006)), suggesting an increasing temperature trend during the past 600 years at the NE Qinghai–Tibet plateau. The d18O of the bulk carbonate in Lake Qinghai also shows an increasing trend (See Fig. 3 in Xu et al. 2006c), which could be attributed to the increasing evaporation caused by increasing temperature. However,
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Fig. 2. Comparison of temperature variations at the northeast Qinghai–Tibet plateau during the past 600 years. (a) d13C of tree rings in Hongyuan, northwestern Sichuan plateau (Xu et al., unpublished data); (b) temperature-controlled indices from Lake Qinghai (see text for details); (c) tree ring widths in Dulan (redrawn from Yao et al., 2001b); (d) tree ring widths in Qilian Mountain (Liu et al., 2004). See locations of the sites in Fig. 1.
the long-term trends of other indices in Lake Qinghai are not so obvious, which may be because of the different processes during which the climatic signals were registered. Four obvious cold intervals were detected, namely 1400–1500, 1650–1715, 1790–1820, and 1920–1940. During 1400–1500, tree ring indices indicated a decrease of temperature variations; however, the indices in Lake Qinghai showed no obvious decrease, with only a small decrease at 1430–1470. During 1650–1715, both the indices from Lake Qinghai and the tree ring widths at Dulan and Qilian showed a striking decrease in temperature (Fig. 2). The records in ‘‘History of Qing Dynasty’’ (http://www.guoxue.com/shibu/24shi/qingshigao/qsg_ 040.htm, in Chinese) show that it was very cold from the end of the Ming Dynasty to the beginning of the Qing Dynasty, corresponding to this cold interval. There were nine strong decreases in temperature variations during the two decades of 1673–1692, with the most striking one during 1689–1692. The cold climate resulted in freezing of rivers widely. Numerous human beings, animals, and trees were frozen to death. As reviewed by Wang et al. (2003), the winter temperature index showed an obvious decrease during 1650–1699. At north China, cold winter was most frequent during 1650–1700, with a ratio of 5.4 cold years per decade, and the frost
years were most frequent during 1620–1700 (Wang et al., 2003). At the middle and lower reaches of the Yangtze River, the interval of 1650–1700 was significantly cold. Han River, Lake Taihu, Lake Dongting, and Lake Poyang were frozen continuously for four winters. The coldest winter was around 1651–1675; the south river-icing lines can reach Hengyang in Hunan province and Ji’an in Jiangxi province (Wang et al., 2003). Another cold interval occurred around 1790–1820. This cold interval can be detected in the variations in the indices from Lake Qinghai, especially in that of the d13Ccarb (Xu et al., 2006c). This cold interval can also be detected in tree ring width at Dulan (Fig. 2). The records in ‘‘History of Qing Dynasty’’ show that there were seven striking decreases during this period of time, namely 1792, 1794, 1796, 1798, 1805, 1814, and 1819, with the most striking cold years at 1792, 1794, 1796, and 1798. It was relatively cold during 1920–1940 (Fig. 2). This cold interval can be detected in variations in indices from Lake Qinghai, tree ring width at Dulan and Qilian, and the d13C in tree rings at Hongyuan. The cold intervals of 1400–1500, 1650–1715, and 1770–1820 are generally consistent with the three typical cold valleys of the Little Ice Age (LIA) in China. More and more evidence indicates that the LIA occurred worldwide. However, the dates of the
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detected, corresponded to the coldest period of the LIA. After that, a large amount of evidence has been supplied for the solar–Earth climate relationship. For example, the sea surface temperatures (SSTs) of the Atlantic, the Pacific, the Indian Ocean, and that of the global average correlate well with the sunspot numbers (see Fig. 1 in Reid, 2000). The concentration of 10Be in Dye3 correlated well with temperature variations of the north hemisphere. Solar irradiance reconstructed from the observed solar cycle length and the observed sunspot numbers correlated well with temperature variations of the north hemisphere (see Fig. 8 in Beer et al., 2000). Solar activity can explain 75% of the total variance of temperature variations on decadal scales at the Arctic area during the last 130 years (Soon, 2005). Solar activity inferred from the concentration of 10Be also synchronized with the D–O events inferred from the variations of d18O in GISP2 (van Geel, et al., 1999). The temperature variations inferred from d18O in peat cellulose at Jinchuan, NE China, show nearly a ‘‘one to one’’ relationship with the variation of solar activity (Hong et al., 2000). We compared the temperature variations at Lake Qinghai and the variation of solar activity during the last 600 years. As shown in Fig. 3, temperature
cold intervals of LIA in China are different at different regions. For instance, Zhu (1973) pointed out that the three cold intervals in East China occurred at AD 1470–1520 a, AD 1620–1720 a, and AD 1840–1890 a. The d18O in peat cellulose in Jinchuan showed three cold intervals around AD 1550 a, AD 1650 a, and AD 1750 a (Hong et al., 2000). The three cold intervals of LIA occurred around AD 1451–1550 a, AD 1601–1690 a, and AD 1790–1880 a recorded in Dunde ice core (Yao and Thompson, 1992), and around 1370–1400a AD, 1550–1610a AD, and 1780–1880a AD recorded in Hongyuan peat sediments (Xu et al., 2006a). These differences in timing may reflect the differences in temperature variations at different regions in China. They may also be attributed to different dating techniques for different archives, and/or different processes during which the climatic signals were registered. Therefore, both dating technique and the understanding of natural processes should be of special concern for future studies. 4. Solar activity and temperature variations on decadal scales at NE Qinghai–Tibet plateau
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Fig. 3. Temperature variations at Lake Qinghai and the solar activities during the past 600 years. (A) Stacked curve of the temperaturecontrolled indices in Lake Qinghai (see text for details); (B) total solar irradiance (TSI) reconstructed from sunspot numbers (Lean et al., 1995); (C) TSI reconstructed from atmospheric 14C recorded in tree rings and 10Be in South Pole ice core (Bard et al., 2000).
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variation at Lake Qinghai is obviously consistent with the solar activity on decadal scales. The cold period inferred from tree ring width during 1400–1500 correlated with the Spo¨rer minimum (1402–1516) (Hsu, 1998). The cold period of 1650–1715 corresponded with the Maunder minimum (1645–1715). Hsu (1998) pointed out that the advance of the glacier during the LIA was most significant around 1700. Climate during 1770–1820 6.00E+06
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was cold, corresponding to the Dalton minimum (Fig. 3). Another cold interval occurred at 1920–1940, which can also be supported by the decrease of solar irradiance at this period of time (Fig. 3). Fig. 4 shows the power spectrum of the temperature indicators in Lake Qinghai. The spectrum of the d13C series is very similar to that of the d18O series, suggesting common driving forces for these two indices. The 26-, 20-, 19-, and 17-year periodicities are close to the 22-year cycle of the sun, suggesting that the quasi-22-year solar activity may be responsible for the temperature variations on decadal scales at the NE Qinghai–Tibet plateau. However, the physics behind the solar–earth climate relationship is not fully understood. The observations during the last two decades indicate a perturbation of 0.1% of the total solar irradiance. Based on the solar cycle length, Zhang et al. (1994) supposed a perturbation of 0.2–0.6% of solar irradiance since the Maunder minimum. Some other studies also showed a perturbation of about 0.2–0.6% based on proxy indices (see Reid, 2000, and references therein). How should such small perturbations of the solar irradiance lead to the observed global warming, what is the mechanism behind it, etc., are still open questions. Much more evidence and discussions are necessary for future studies.
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Temperature variations are similar in trends at the NE Qinghai–Tibet plateau. The three strong decreases in temperature corresponded with the three coldest intervals of the LIA during the past 600 years. These three coldest intervals also correspond with the three solar minimum during the past 600 years, namely the Spo¨rer, the Maunder, and the Dalton minimums. Such a relationship suggests that solar activities are possibly the controlling factor of temperature variations at the NE Qinghai–Tibet plateau on decadal scales.
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Fig. 4. Power spectra of the d18O of bulk carbonate (A), the d13C of bulk carbonate (B), the d15N of the organic matters (C), and the total carbonate content (D). The solid curves and the dotted curves represent the 95% and 99% significant levels, respectively. Power spectrum analysis was performed using Redfit 35 (Schulz and Mudelsee, 2002).
Acknowledgements The authors are grateful to Prof. X.M. Shao for providing tree ring data. This work is supported by the National Basic Research Program of China (No. 2004CB720207), and the National Science Foundation of China (No. 40673071).
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References Bard, E., Raisbeck, G., Yiou, F., Jouzel, J., 2000. Solar irradiance during the last 1200 years based on cosmogenic nuclides. Tellus 52B, 985. Beer, J., Mende, W., Stellmacher, R., 2000. The role of the sun in climate forcing. Quaternary Science Reviews 19, 403. Eddy, J.A., 1977. The Maunder minimum. Science 192 (4245), 1189. Hong, Y.T., Jiang, H.B., Liu, T.S., Zhou, L.P., Beer, J., Li, H.D., Leng, X.T., Hong, B., Qin, X.G., 2000. Response of climate to solar forcing recorded in a 6000-year d18O time-series of Chinese peat cellulose. The Holocene 10 (1), 1. Hoyt, D.V., Schatten, K.H., 1998. Group sunspot numbers: a new solar activity reconstruction. Solar Physics 181, 491. Hsu, K.J., 1998. Sun, climate, hunger, and mass migration. Science in China (Series D) 41 (5), 449. Kang, X.C., Zhang, Q.H., Graumlich, L.J., Sheppard, P., 2000. Reconstruction and variation of climate in Dulan region, Qinghai during the last 2000a. Advance in Earth Science 15 (2), 215 (in Chinese with English abstract). Lean, J., Beer, J., Bradley, R., 1995. Reconstruction of solar irradiance since 1610: implications for climate change. Geophysical Research Letters 22, 3195. Liu, X.H., Qin, D.H., Shao, X.M., Chen, T., Ren, J.W., 2004. Temperature variations during the last millennium in MidQilan Mountain inferred from tree rings. Science in China (Series D) 34 (1), 89. Reid, G., 2000. Solar variability and the Earth’s climate: introduction and overview. Space Science Reviews 94, 1. Schulz, M., Mudelsee, M., 2002. REDFIT: estimating red-noise spectra directly from unevenly spaced paleoclimatic time series. Computers and Geosciences 28 (3), 421. Solanki, S.K., Fligge, M., 2000. Reconstruction of past solar irradiance. Space Science Reviews 94, 127. Solanki, S.K., Usoskin, I.G., Schuˇssler, M., Beer, J., 2004. Unusual activity of the sun during recent decades compared to the previous 11,000 years. Nature 43, 1084. Soon, W.W.H., 2005. Variable solar irradiance as a plausible agent for multidecadal variations in the Arctic-wide surface air temperature record of the past 130 years. Geophysical Research Letters 32, L16712. Usoskin, I.G., Solanki, S.K., Schuˇssler, M., 2003. MillenniumScale sunspot number reconstruction: evidence for an unusually active Sun since the 1940s. Physical Review Letters 91, 211101-1–211101-4.
Van Geel, B., Raspopov, O.M., Renssen, H., van der Plicht, J., 1999. The role of solar forcing upon climate change. Quaternary Science Reviews 18, 331. Wang, N.L., Thopson, L.G., Davis, M.E., Mosley-Thompson, E., Yao, T.D., Pu, J.C., 2003. Influence of variations in NAO and SO on air temperature over the northern Tibet Plateau as recorded by d18O in Malan Ice core. Geophysical Research Letters 30, 2167. Wang, S., Liu, J., Zhou, J., 2003. The climate of little ice age maximum in China. Journal of Lake Sciences 15 (4), 369 (in Chinese with English abstract). Xu, H., Hong, Y.T., Lin, Q.H., Zhu, Y.X., Hong, B., Jiang, H.B., 2006a. Temperature responses to quasi-100-yr solar variability during the past 6000 years based on d18O of peat cellulose in Hongyuan, eastern Qinghai–Tibet Plateau, China. Palaeogeography, Palaeoclimatology, Palaeoecology 230, 155. Xu, H., Ai, L., Tan, L.C., An, Z.S., 2006b. Geochronology of a surface core in the northern basin of Lake Qinghai: evidence from 210Pb and 137Cs radionuclides. Chinese Journal of Geochemistry 25 (4), 301. Xu, H., Ai, L., Tan, L.C., An, Z.S., 2006c. Stable isotopes in bulk carbonates and organic matter in recent sediments of Lake Qinghai and their climatic implications. Chemical Geology 235, 262. Yao, T.D., Thompson, L.G., 1992. Temperature variations in the past 5000 years recorded in Dunde ice core. Science Sinica (B) 10, 1089 (in Chinese). Yao, T.D., Xu, B.Q., Pu, J.C., 2001a. Climatic changes on orbital and sub-orbital time scale recorded by Guliya ice core in Tibet Plateau. Science in China (Series D) 44 (supp.), 360. Yao, T.D., Yang, M.X., Kang, X.C., 2001b. Comparative study of the climate changes in the past 2000 years by using ice core and tree ring records. Quaternary Sciences 21 (6), 514 (in Chinese with English abstract). Yao, T.D., Qin, D.H., Xu, B.Q., Yang, M.X., Duan, K.Q., Wang, N.L., Wang, Y.Q., Hou, S.G., 2006. Temperature change over the past millennium recorded in ice cores from the Tibetan plateau. Advances in Climate Research 2 (3), 99 (in Chinese). Zhang, Q., Soon, W.H., Baliunas, S.L., Lockwood, G.W., Skiff, B.A., Radick, R.R., 1994. A method of determining possible brightness variations of the Sun in past centuries from observations of solar-type stars. The Astrophysical Journal 427 (Part 2), L111. Zhu, K.Z., 1973. A preliminary study on the climatic fluctuations during the last 5000 years in China. Science Sinica 14, 226 (in Chinese).
Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 138–144 www.elsevier.com/locate/jastp
Temperature variations at Lake Qinghai on decadal scales and the possible relation to solar activities Hai Xu, Xiaoyan Liu, Zhaohua Hou Institute of Earth Environment, Chinese Academy of Sciences, Xi’an, China Received 30 May 2007; received in revised form 16 September 2007; accepted 17 September 2007 Available online 22 September 2007
Abstract Temperature variations at Lake Qinghai, northeastern Qinghai–Tibet plateau, were reconstructed based on four highresolution temperature indicators of the d18O and the d13C of the bulk carbonate, total carbonate content, and the detrended d15N of the organic matter. There are four obvious cold intervals during the past 600 years at Lake Qinghai, namely 1430–1470, 1650–1715, 1770–1820, and 1920–1940, synchronous with those recorded in tree rings at the northeast Qinghai–Tibet plateau. The intervals of 1430–1470, 1650–1715, and 1770–1820 are consistent with the three coldest intervals of the Little Ice Age. These obvious cold intervals are also synchronous with the minimums of the sunspot numbers during the past 600 years, suggesting that solar activities may dominate temperature variations on decadal scales at the northeastern Qinghai–Tibet plateau. r 2007 Elsevier Ltd. All rights reserved. Keywords: Lake Qinghai; Temperature; Solar activity; Qinghai–Tibet plateau
1. Introduction It is well known that the Earth’s temperature is influenced by variable factors, such as solar activity, atmospheric circulation, the complex topography, different land cover, and the greenhouse gases. The dominating factor that controls local temperature variation is different between different regions. Therefore, although the general trend of temperature variations over wide geographic areas has been figured out by numerous works, it is still urgent to make clear the details of regional temperature variations and the causes behind them. Corresponding author. Tel.: +86 29 88324172; fax: +86 29 88320456. E-mail address: [email protected] (H. Xu).
1364-6826/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2007.09.006
The northeastern (NE) Qinghai–Tibet plateau is very sensitive to global climatic changes, with four planetary scale atmospheric circulations prevailing over there, namely the East Asian summer monsoon, the Indian summer monsoon, the Westerly, and the Asian winter monsoon. Temperature variations at this region have aroused wide attention. Yao et al. (2006) studied the temperatures during the last millennium based on d18O in ice cores. Kang et al. (2000) reconstructed the temperature variations from tree ring width. Liu et al. (2004) carried out a study of dendrochronology and discussed temperature variations at this region. Although similarities exist between those various proxy indices, there are also some differences both in timing and in magnitudes, which seriously limits the understanding of the temperature mechanisms.
ARTICLE IN PRESS H. Xu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 138–144
Much more evidence is necessary to shed light on the details of temperature variations at the NE Qinghai–Tibet plateau. On the other hand, previous work has suggested that climates on different timescales at the Qinghai–Tibet plateau may be driven by different forces. For example, based on the comparisons between climates recorded in ice cores in the Tibet plateau and those in Greenland, Yao et al. (2001a) pointed out that the climatic variations at the Tibet plateau on orbital timescales are dominantly controlled by solar irradiance. According to a temperature indicator of d18O in peat cellulose, Xu et al. (2006a) suggested that the quasi-100-year solar activity may be responsible for temperature variations on centennial timescales at Hongyuan, NE Qinghai–Tibet plateau. However, as revealed from ice cores (Wang et al., 2003) and tree rings (Xu et al., unpublished data), temperature variations on annual scales are primarily influenced by atmospheric circulations, like the ‘‘El Nin˜o–South Oscillation’’ (ENSO). Our question is: what is the controlling factor of temperature variations on decadal scales at the NE Qinghai–Tibet plateau? In this study, we studied temperature variations on decadal scales during the past 600 years based on temperature indicators extracted from Lake Qinghai, NE Qinghai–Tibet plateau. We compared the temperature indicators at Lake Qinghai with the proxy indices from tree rings nearby, and with the
139
reconstructed solar activities. The results show that temperature variations at Lake Qinghai are synchronous with those at the NE Qinghai–Tibet plateau, and the main temperature events are generally in-phase with solar activities during the last 600 years. 2. Data 2.1. Temperature indicators from Lake Qinghai Lake Qinghai (361320 –371150 N, 991360 –1001470 E; Fig. 1) is located at the NE Qinghai–Tibet plateau. The meteorological records of the Gangcha station from 1958 to 2000 show that the temperature around this region varies between 8.9 1C and 13.7 1C in July, and between 16.4 1C and 10.74 1C in January, with a mean annual value of about 0.4 1C. Lake water is relatively cold, with an average temperature of about 10 1C for surface water and about 4 1C for bottom water in summer. The lake is frozen between December and March. Since the lake is very large and the anthropogenetic impacts are relatively small, Lake Qinghai is an ideal site for the study of palaeoclimatic changes. Surface sediments (about 30–40 cm long) in Lake Qinghai were collected with a self-designed gravity corer. The 210Pb and 137Cs radioactivities were measured to determine the sedimentation rate (see Xu et al., 2006b for details). An approach of
Fig. 1. Location of Lake Qinghai and some sites mentioned in the text.
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high-resolution multi-proxy indices was applied to evaluate the climatic significances of the proxy indices. Carbonates in the sediment of Lake Qinghai are mainly authigenic. Therefore, total carbonate content would be largely influenced by the ion concentration, which is closely linked to the evaporation of the lake water. Variations in d13Ccarb are governed by d13C of the dissolved inorganic carbon (DIC) in the DIC pool of the lake water. It has been supposed that the variations in d13CDIC of Lake Qinghai are closely linked to the fast evaporation of the lake water. As a result, variations in d13Ccarb can be attributed to the evaporation (see Xu et al., 2006c for details). Comparison between d18O of the lake water, river water, rainfall, and springs shows that evaporation is the controlling factor that determines d18O of the lake water. Since d18O of the lake water dominates d18Ocarb, variations in d18Ocarb can also be attributed to evaporation. Therefore, variations in total carbonate content, d13Ccarb, and d18Ocarb are dominated by evaporation of the lake water. Since evaporation at Lake Qinghai is primarily controlled by temperature, variations in d13Ccarb, d18Ocarb, and total carbonate content can be attributed to temperature variations (see Xu et al., 2006c for details). d15Ndetrended is from d15N of the organic matter after removal of the stratigraphic trend. Variation in d15Ndetrended is closely related to changes of the primary productivity in Lake Qinghai. When the primary productivity increases, the discrimination effect of 14N/15N triggers an increased d15Norg in the sediments (see Xu et al., 2006c for details). Since the primary productivity is mainly controlled by temperature at Lake Qinghai, the d15Ndetrended can be used to reflect the regional temperature variations (see Xu et al., 2006c for details). In summary, variations in d18Ocarb, d13Ccarb, total carbonate content, and d15Ndetrended can be attributed to temperature variations. The significances of these four indices have been verified by comparisons with instrumental temperature records and by comparisons with the temperature indicators from nearby sites (see Fig. 6 in Xu et al., 2006c). These four indices were normalized, and then stacked to one to reflect the temperature trend at Lake Qinghai. Some other researchers also studied the recent climatic changes at Lake Qinghai. However, most of those previous works ignored the compaction of the subsurface sediments during the early diagenesis, and the dates of these studies are largely different from those of our work (see Xu et al., 2006b for
details). As a result, we did not collect the data by other studies at Lake Qinghai. 2.2. Temperature indicators in tree rings and solar activity indices Previous work indicated that the tree ring widths at both Dulan (Yao et al., 2001b) and Qilan Mountain (Liu et al., 2004), NE Qinghai–Tibet plateau (see locations in Fig. 1), are sensitive to temperature variations. The variations in d13C of tree rings at Hongyuan (see location in Fig. 1) are also attributed to temperature variations (Xu et al., unpublished data). These data from tree rings were collected and were 10-piont averaged for comparison with the indices from Lake Qinghai. The Sun is the energy source of the Earth and its activity is generally regarded as the dominating factor for Earth’s temperature variations. Solar activities during the past several centuries can be reconstructed from the observations (Lean et al., 1995; Hoyt and Schatten, 1998; Solanki and Fligge, 2000). Meanwhile the proxy indices, such as the concentrations of cosmogenic 14C (Solanki et al., 2004) and 10Be (Beer et al., 2000; Usoskin et al., 2003), revealed much longer variations of the Sun. For example, Bard et al. (2000) reconstructed the solar irradiance during the last 1200 years based on atmospheric 14C concentration from tree rings and on 10Be concentration from the South Pole ice core; the reconstructed solar irradiance correlated well with the observed long-term variation of sunspot numbers. To investigate the solar–temperature relationship, we collected the reconstructed solar activities by Lean et al. (1995) and the reconstructed solar irradiance by Bard et al. (2000). 3. Temperature variations during the last 600 years at Lake Qinghai As shown in Fig. 2, the long-term trend of temperature variations is increasing, inferred from tree ring widths and isotopic indices in tree rings. This long-term trend can also be detected in d18O of Dunde ice core, Guliya ice core, and Dasuopu ice core (refer to Fig. 2 in Yao et al. (2006)), suggesting an increasing temperature trend during the past 600 years at the NE Qinghai–Tibet plateau. The d18O of the bulk carbonate in Lake Qinghai also shows an increasing trend (See Fig. 3 in Xu et al. 2006c), which could be attributed to the increasing evaporation caused by increasing temperature. However,
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Fig. 2. Comparison of temperature variations at the northeast Qinghai–Tibet plateau during the past 600 years. (a) d13C of tree rings in Hongyuan, northwestern Sichuan plateau (Xu et al., unpublished data); (b) temperature-controlled indices from Lake Qinghai (see text for details); (c) tree ring widths in Dulan (redrawn from Yao et al., 2001b); (d) tree ring widths in Qilian Mountain (Liu et al., 2004). See locations of the sites in Fig. 1.
the long-term trends of other indices in Lake Qinghai are not so obvious, which may be because of the different processes during which the climatic signals were registered. Four obvious cold intervals were detected, namely 1400–1500, 1650–1715, 1790–1820, and 1920–1940. During 1400–1500, tree ring indices indicated a decrease of temperature variations; however, the indices in Lake Qinghai showed no obvious decrease, with only a small decrease at 1430–1470. During 1650–1715, both the indices from Lake Qinghai and the tree ring widths at Dulan and Qilian showed a striking decrease in temperature (Fig. 2). The records in ‘‘History of Qing Dynasty’’ (http://www.guoxue.com/shibu/24shi/qingshigao/qsg_ 040.htm, in Chinese) show that it was very cold from the end of the Ming Dynasty to the beginning of the Qing Dynasty, corresponding to this cold interval. There were nine strong decreases in temperature variations during the two decades of 1673–1692, with the most striking one during 1689–1692. The cold climate resulted in freezing of rivers widely. Numerous human beings, animals, and trees were frozen to death. As reviewed by Wang et al. (2003), the winter temperature index showed an obvious decrease during 1650–1699. At north China, cold winter was most frequent during 1650–1700, with a ratio of 5.4 cold years per decade, and the frost
years were most frequent during 1620–1700 (Wang et al., 2003). At the middle and lower reaches of the Yangtze River, the interval of 1650–1700 was significantly cold. Han River, Lake Taihu, Lake Dongting, and Lake Poyang were frozen continuously for four winters. The coldest winter was around 1651–1675; the south river-icing lines can reach Hengyang in Hunan province and Ji’an in Jiangxi province (Wang et al., 2003). Another cold interval occurred around 1790–1820. This cold interval can be detected in the variations in the indices from Lake Qinghai, especially in that of the d13Ccarb (Xu et al., 2006c). This cold interval can also be detected in tree ring width at Dulan (Fig. 2). The records in ‘‘History of Qing Dynasty’’ show that there were seven striking decreases during this period of time, namely 1792, 1794, 1796, 1798, 1805, 1814, and 1819, with the most striking cold years at 1792, 1794, 1796, and 1798. It was relatively cold during 1920–1940 (Fig. 2). This cold interval can be detected in variations in indices from Lake Qinghai, tree ring width at Dulan and Qilian, and the d13C in tree rings at Hongyuan. The cold intervals of 1400–1500, 1650–1715, and 1770–1820 are generally consistent with the three typical cold valleys of the Little Ice Age (LIA) in China. More and more evidence indicates that the LIA occurred worldwide. However, the dates of the
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detected, corresponded to the coldest period of the LIA. After that, a large amount of evidence has been supplied for the solar–Earth climate relationship. For example, the sea surface temperatures (SSTs) of the Atlantic, the Pacific, the Indian Ocean, and that of the global average correlate well with the sunspot numbers (see Fig. 1 in Reid, 2000). The concentration of 10Be in Dye3 correlated well with temperature variations of the north hemisphere. Solar irradiance reconstructed from the observed solar cycle length and the observed sunspot numbers correlated well with temperature variations of the north hemisphere (see Fig. 8 in Beer et al., 2000). Solar activity can explain 75% of the total variance of temperature variations on decadal scales at the Arctic area during the last 130 years (Soon, 2005). Solar activity inferred from the concentration of 10Be also synchronized with the D–O events inferred from the variations of d18O in GISP2 (van Geel, et al., 1999). The temperature variations inferred from d18O in peat cellulose at Jinchuan, NE China, show nearly a ‘‘one to one’’ relationship with the variation of solar activity (Hong et al., 2000). We compared the temperature variations at Lake Qinghai and the variation of solar activity during the last 600 years. As shown in Fig. 3, temperature
cold intervals of LIA in China are different at different regions. For instance, Zhu (1973) pointed out that the three cold intervals in East China occurred at AD 1470–1520 a, AD 1620–1720 a, and AD 1840–1890 a. The d18O in peat cellulose in Jinchuan showed three cold intervals around AD 1550 a, AD 1650 a, and AD 1750 a (Hong et al., 2000). The three cold intervals of LIA occurred around AD 1451–1550 a, AD 1601–1690 a, and AD 1790–1880 a recorded in Dunde ice core (Yao and Thompson, 1992), and around 1370–1400a AD, 1550–1610a AD, and 1780–1880a AD recorded in Hongyuan peat sediments (Xu et al., 2006a). These differences in timing may reflect the differences in temperature variations at different regions in China. They may also be attributed to different dating techniques for different archives, and/or different processes during which the climatic signals were registered. Therefore, both dating technique and the understanding of natural processes should be of special concern for future studies. 4. Solar activity and temperature variations on decadal scales at NE Qinghai–Tibet plateau
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Fig. 3. Temperature variations at Lake Qinghai and the solar activities during the past 600 years. (A) Stacked curve of the temperaturecontrolled indices in Lake Qinghai (see text for details); (B) total solar irradiance (TSI) reconstructed from sunspot numbers (Lean et al., 1995); (C) TSI reconstructed from atmospheric 14C recorded in tree rings and 10Be in South Pole ice core (Bard et al., 2000).
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variation at Lake Qinghai is obviously consistent with the solar activity on decadal scales. The cold period inferred from tree ring width during 1400–1500 correlated with the Spo¨rer minimum (1402–1516) (Hsu, 1998). The cold period of 1650–1715 corresponded with the Maunder minimum (1645–1715). Hsu (1998) pointed out that the advance of the glacier during the LIA was most significant around 1700. Climate during 1770–1820 6.00E+06
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was cold, corresponding to the Dalton minimum (Fig. 3). Another cold interval occurred at 1920–1940, which can also be supported by the decrease of solar irradiance at this period of time (Fig. 3). Fig. 4 shows the power spectrum of the temperature indicators in Lake Qinghai. The spectrum of the d13C series is very similar to that of the d18O series, suggesting common driving forces for these two indices. The 26-, 20-, 19-, and 17-year periodicities are close to the 22-year cycle of the sun, suggesting that the quasi-22-year solar activity may be responsible for the temperature variations on decadal scales at the NE Qinghai–Tibet plateau. However, the physics behind the solar–earth climate relationship is not fully understood. The observations during the last two decades indicate a perturbation of 0.1% of the total solar irradiance. Based on the solar cycle length, Zhang et al. (1994) supposed a perturbation of 0.2–0.6% of solar irradiance since the Maunder minimum. Some other studies also showed a perturbation of about 0.2–0.6% based on proxy indices (see Reid, 2000, and references therein). How should such small perturbations of the solar irradiance lead to the observed global warming, what is the mechanism behind it, etc., are still open questions. Much more evidence and discussions are necessary for future studies.
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Temperature variations are similar in trends at the NE Qinghai–Tibet plateau. The three strong decreases in temperature corresponded with the three coldest intervals of the LIA during the past 600 years. These three coldest intervals also correspond with the three solar minimum during the past 600 years, namely the Spo¨rer, the Maunder, and the Dalton minimums. Such a relationship suggests that solar activities are possibly the controlling factor of temperature variations at the NE Qinghai–Tibet plateau on decadal scales.
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Fig. 4. Power spectra of the d18O of bulk carbonate (A), the d13C of bulk carbonate (B), the d15N of the organic matters (C), and the total carbonate content (D). The solid curves and the dotted curves represent the 95% and 99% significant levels, respectively. Power spectrum analysis was performed using Redfit 35 (Schulz and Mudelsee, 2002).
Acknowledgements The authors are grateful to Prof. X.M. Shao for providing tree ring data. This work is supported by the National Basic Research Program of China (No. 2004CB720207), and the National Science Foundation of China (No. 40673071).
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