Letters Article ID: 1673-1719 (2007) Suppl.-0026-05 Amplitude and Rhythm of Winter Half-year Temperature Change in Eastern China for the Past 2000 Years Ge Quansheng 1, Zheng Jingyun 1, Liu Jian 2 1 Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China; 2 Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China Abstract: The amplitude and rhythm of temperature changes at inter-decadal and inter-centennial timescale were studied, based on the winter-half year temperature change series reconstructed from historical phenological events in eastern China for the past 2000 years, together with the temperature change simulation from ECHO-G model for the past 1000 years, and the quasi-periods of temperature fluctuation were discussed by using wavelet analysis. The results indicate: 1) the maximal amplitude of winter half-year temperature change in eastern China at decadal and centennial scale, was above 2=and =respectively. The reconstructed result indicates that the amplitude of warming during the 20th century was identical with the maximum amplitude before the 20th century in eastern China, but the simulated result suggests that the amplitude of warming has exceeded the maximum amplitude. 2) The rhythms of temperature change at centennial to millennial scale in eastern China were about 100-year, 250-year, 400-year, 600-year and 1000-year. The 20th century, the 1stJ3rd century and the 9thJ13th century were warm peaks at inter-centennial scale as well as at millennial scale. It is implicated that the warming during the 20th century should be attributed to not only anthropogenic effect, but also natural climate variation. Key words: winter half-year; temperature change; amplitude; rhythm; the past 2000 years; eastern China Introduction As the context of global climate warming in the 20th century, the climate variability for the past millennia has been a focus of global change study. Although the global or the Northern Hemispheric temperature variation for the past millennia has been reconstructed in some studies, arguments about the temperature reconstruction still exist [1]. Besides, the complexity of climate change and the uncertainty of its reconstruction technology, the regional difference of climate change are very important factors resulting in those debates [2]. This paper analyzes the amplitude and rhythm of temperature variation in eastern China for the past millennia, based on the winter half-year temperature change series reconstructed from historical phenological events for the past 2000 years [3], together with the temperature change simulation from ECHO-G model for the past 1000 years in eastern China [4]. Since the instrumental record shows that the temperature variations are very similar in China and in the Northern Hemisphere, this study is not only important for deeply understanding the regional difference of climate change during the historical period, but also valuable for comprehending the cold/warm change in the Northern Hemisphere, even in the global wide area. 1 Data The data of this study include two series. One is winter half-year temperature anomaly series in eastern China for the past 2000 years (Fig.1a), which was reconstructed from the phenological cold/warm events recorded in Chinese historical documents and results derived from some related studies in different periods and different sites in eastern Received: November 3, 2006; revised: March 8, 2007 Corresponding to Zheng Jingyun (E-mail: zhengjy@igsnrr.ac.cn) 2007 National Climate Commission of China. All right reserved 26
Temperature anomaly/ 1.0 J J J1.5 (a) (b) 1 200 400 600 800 1000 1200 1400 1600 1800 2000 Year/AD Fig. 1 Winter half-year temperature change in eastern China over the past 2000 years (grey bar: decadal mean; solid line: 30-year mean; dashed line: mean of the last millennium) (a) reconstructed series based on phenological events from historical documents (from ref. x3z = ), (b) simulation series based on ECHO-G model (from ref. x4z = ) China (particular in North China and in the middle and lower reaches of Yangtze River) with a 30-year resolution, but the two periods of the 960sJ1100s and the 1500sJ1990s have a higher resolution of 10-year [3]. Another is the simulated winter half-year temperature change from the ECHO-G model for the past 1000 years in eastern China with a 10-year resolution (Fig.1b). The model consists of the spectral atmospheric model ECHAM4 and the global ocean circulation model HOPE-G, which were implemented and developed at the Max Planck Institute of Meteorology (MPI) in Hamburg, Germang. The external driving forcings for the simulation include solar activity, greenhouse gas concentrations in the atmosphere (including CO 2 and CH 4 ) and an estimation of radiative effects of stratospheric volcanic aerosols for the period of 10001990 [4]. In order to facilitate the comparison between the reconstructed and simulated results, the reference period for those two series in this study is set as 1951J1980. 2 Trends and amplitudes of temperature changes Figure 1 (a) indicates that there were 7 cold/warm phases with a duration more than 100 years during the past 2000 years (Table 1). Differences in average winter halfyear temperature between cold and warm phases range from 0.6 to 0.7, which indicates that the amplitude of winter half-year temperature at centennial scale in the eastern China is 0.6J0.7. The 20th century is a warm phase within the past 2000 years in the eastern China and the average winter half-year temperature for this phase is 0.6 higher than that for the last cold phase (the 1320sJ1910s). Although it is a little higher than that for the Medieval Warm Period (the 930sJ1310s), yet it is still lower than those for two warm periods (the 930sJ1100s and 1200sJ1310s) respectively, and also lower than the one of warm phase in Sui and Tang Dynasty (the 570sJ770s). For winter halfyear temperature anomalies at 30-year interval, in the past 2000 years there were 9 identical intervals, 23 warmer intervals, and 34 colder intervals, relative to the mean winter half-year temperature over 1951J1980. Among the 23 warmer intervals there were 5 significant warmer ( higher) intervals, two warmest (0.9and 0.7=higher) intervals during 1230sJ1250s and 1260sJ1280s in the late Medieval Warm Period, respectively. While the interval of 1981J2000 is just identical with the intervals of the 690sJ 710s and 1080sJ1100s, their means of winter half-year temperature anomaly are all. Meanwhile, there were 21 evident colder ( lower) intervals, and three coldest (1.1, 1.0 and 0.8 lower) intervals in the 1650sJ 1670s, 480sJ500s, and 1860sJ1880s, respectively. Besides, there were 5 centennial-scale periods of warming or cooling with the change rate of 1.0=per 100 years in the past 2000 years, particularly in the cold/warm transition periods. The 20th century warming is a rapid warming course from cold phase (Little Ice Age) to warm phase, and its change rate of temperature reaches 1.1 per 100 years, which is not unique one when compared with other rapid warming courses in the past 2000 years,. However, for the winter half-year temperature at a 10-year resolution, the warming-rate (the 1870sJ1960s) from the Little Ice Age to the 20th century warming phase is 1.71 per 100 years, higher than the maximum cen- 27
Ge Quansheng et al.: Amplitude and Rhythm of Winter Half-year Temperature Change in Eastern China for the Past 2000 Years Table 1 The cold/warm phase at more than centennial scale in eastern China during the past 2000 years (All temperatures in this Table are averaged winter half-year temperature anomalies from the normal of 1951J1980 [6] ) Cold / Warm Phase Mean temperature of phase / Maximum temperature range for each cold / warm phase Warmest 30-year and its anomaly / Coldest 30-year and its anomaly / 0sJ200s 4 90sJ110s 0.4 30sJ50s J0.2 570sJ770s 0.23 690sJ710s 600sJ620s 750sJ770s Warm Phase 930sJ1310s 930sJ1100s 8 0.27 1230sJ1250s 1080sJ1100s 0.9 1140sJ1160s 930sJ950s J 1110sJ1190s J0.33 1170sJ1190s J0.2 1140sJ1160s J 1200sJ1310s 0.43 1230sJ1250s 0.9 1290sJ1310s 1920sJ1990s 0.20 1980sJ1990s 1950sJ1970s 210sJ560s J0.47 240sJ260s 480sJ500s 360sJ380s Cold Phase 780sJ920s 1320sJ1910s J0 J0.39 840sJ860s 1380sJ1400s 1500sJ1520s J 810sJ830s 1650sJ1670s J0.7 J1.1 1740sJ1760s tennial warming rate in the course from the cold valley to the warm peak (the 1650sJ1740s with the change rate of 1.64 per 100 years) within the Little Ice Age. Furthermore, the warming-rate during the 20th century warming is 0.88/100 a (the 1900sJ1990s), also higher than the maximum warming-rate (0.62/100 a) in the early Medieval Warm Period (the 1000sJ1090s). Because there are only two periods (the 960sJ1100s and 1500sJ1990s) with the data at 10-year resolution, it is hard to compare the warming-rate on decadal time-scale in the 20th century warm phase with those in other warm phases or in the cold to warm transition periods in the past 2000 years, and to identify whether the warming-rate during the 20th century is the largest or not. However, it is evident that the warming during the 20th century is very rapid, which probably resulted from the enhanced greenhouse effect induced by human activities at least. Comparing Fig.1 (a) with Fig.1 (b), we can see that the inter-centennial variations of cold/warm phase during the past 1000 years in eastern China, derived from the ECHO-G model simulation and the reconstruction from the historical documents, are similar. The correlation coefficient between the two series is 0.35, and significant at the α= 5 significance level. The promineat climate fluctuations are consistent between the model simulations and reconstructed results, for the warming in 1000J1250 AD, the changing from warm to cold after 1250 AD, the persistent cooling in 1380sJ1870 AD, and the persistent warming since 1870 AD. Meanwhile, both the simulated and reconstructed results indicate that the period of 1650sJ1670s was the coldest 30 years during the last 1000 years. However, during the period of 1000J1500 AD and after 1850 AD, there are phase differences of 30J50 years between the simulated and reconstructed results, that is, the peak and valley of reconstructed series lag behind those of simulated one. The reason for the phase difference might be that the model did not consider the long-term variation of land surface vegetation and changes of aerosol resulted from modern industrialization. Viewed from the temperature amplitudes, the simulated results for the past 1000 years indicate that the average temperature anomaly from the normal of 1951J 28
1980 of centennial cold/warm phase are Jfor the 1000sJ1250s, for the 1260sJ1880s, and for the 1890sJ1980s, respectively, which shows that the simulated temperature amplitude is per 100 years. For 30-year resolution, the maximum simulated variation amplitude during the past millennium is 2.1 (J 1.5 to 0.6), almost identical with that (J1.1=to 0.9) from the reconstruction, and for 10-year resolution, it is 2.3 (J1.7 to 0.6), very close to the reconstructed one 2.4 (J1.4 to 1.0). However, the simulation indicates that the warming amplitude during the 20th, particularly during the late 20th century has exceeded the maximum amplitude before the 20th century. It is diagnosed in the simulation that the rapid warming during the 20th century is mainly resulted from the increase of greenhouse gases (CO 2 and CH 4 ) and the enhancement of greenhouse effect [4]. 3 Rhythm of temperature variation To understand the rhythm of temperature variation for the past millennium, the Morlet wavelet transform [7] of both simulated and reconstructed series are performed and the results presented in Fig. 2, wherefrom it can be seen that the two series are consistent with each other. Fig. 2 (a) shows that the significant rhythms of temperature oscillation are 250-year, 600-year and 1000-year in the eastern China for the past 2000 years. Meanwhile, the weak rhythm signal is also shown on 100-year scale. The rhythm signal of 400- year becomes significant since 1200 AD. Figure 2 (b) also exhibits that there are significant rhythm signals with 100-year, 200-year, 300-year, 400-year, 600-year and 1000-year in eastern China for the past millennium. Compared with Fig.2 (a), the phase of temperature oscillation from the simulation is completely identical with that from reconstructed result on 1000-year scale. The consistent phase signal of temperature oscillation between simulation and reconstruction is also shown on 400-year scale since 1100 AD, while they are similar between simulation and reconstruction on 600-year and 250-year scales after 1500 AD. However, the simulation presents a strong rhythm signal on 100-year scale, versus a weak signal in reconstructed series, which might be related 1000 800 period/year 600 400 (a) 200 1000 800 period/year 600 400 (b) 200 1 200 400 600 800 1000 1200 1400 1600 1800 2000 Time/AD Fig. 2 Real part of Morlet wavelet spectrum for the winter half-year temperature change series in eastern China during the past 2000 years (a) reconstructed series, (b) simulation series (bold dot line: major oscillations in the time-scale of 200J600 years, bold dash line: comparison of phase in the time-scale of 400 years) 29
Ge Quansheng et al.: Amplitude and Rhythm of Winter Half-year Temperature Change in Eastern China for the Past 2000 Years with the higher time resolution of the model simulation. Furthermore, Fig. 2 clearly presents that the 1stJ3rd century, the 9thJ13th century (Medieval Warm Period) and the 20th century are 3 warm peaks of 1000-year scale oscillation, while the period of the 570sJ770s is only a warm peak of 200-year to 600-year scale. It is interesting to note that the 20th century is also a warm peak for other centennial scales. It seems to suggest that the warming in the 20th century should be attributed not only to the enhanced greenhouse effect induced by human activities, but also to the natural oscillation of climate. 4 Concluding remarks Based on the above analysis, the main conclusions are summarized here: 1) For both results of the reconstructed and simulated series, the maximum amplitude of winter half-year temperature change is more than 2.0 in the eastern China on decadal scale. The amplitude is 0.6J0.7 on centennial scale for the reconstructed result and for the model simulation. The reconstructed result indicates that the amplitude of warming during the 20th century is identical with the maximum amplitude of warming before the 20th century in eastern China on decadal to centennial scale. Compared with the reconstructed, the simulated result indicates that the amplitude of warming during the 20th century, particularly during the late 20th century has exceeded the maximum amplitude of warming before the 20th century. 2) The rhythms of temperature change on 100- to 1000-year scale in eastern China are 100-year, 250-year, 400-year, 600-year and 1000-year. The 20th century, the 1stJ3rd century, and the 9thJ13th century are warm peaks on 1000-year scale, but the latter also a warm peak on 100- year scale. It is implicated that the warming in 20th century should be attributed not only to the enhanced greenhouse effect induced by human activities, but also to the natural variation of climate. Acknowledgements This work was supported by the project from National Natural Science Foundation of China under grants No. 40331013 and 40571017. References [1] Wang Shaowu, Luo Yong, Zhao Zongci, et al. Debate still continues about temperature changes during the last millennium [J]., 2005, 1 (2): 76J79 (in Chinese) [2] Mann M E, Bradley R S, Hughes M K, et al. On past temperatures and anomalous late-20th century warmth [J]. EOS (Transactions, American Geophysical Union), 2003, 84 (27): 256J258 [3] Ge Quansheng, Zheng Jingyun, Fang Xiuqi et al. Winter half-year temperature reconstruction for the middle and lower reaches of the Yellow River and Yangtze River, China, during the past 2000 years [J]. The Holocene, 2003, 13 (6): 933J940 [4] Liu Jian, von Storch H, Chen Xing, et al. Simulated and reconstructed winter temperature in the eastern China during the last millennium [J]. Chinese Science Bulletin, 2005, 50 (24): 2872J2877 [5] Bradley R S. Instrumental records of past global change: lessons for the analysis of noninstrumental data [C]// Bradley R S. Global Change of the Past. Boulder, Colorado: UCAR/Office for Interdisciplinary Earth Studies, 1989: 103J116 [6] Zheng Jingyun, Ge Quansheng, Fang Xiuqi. Seeing the 20th century warming from temperature changes of winter-halfyear in eastern China for the last 2000 years [J]. Acta Geographica Sinica, 2002, 57 (6): 631J638 (in Chinese) [7] Torrence C, Compo G P. A practical guide to wavelet analysis [J]. Bulletin of the American Meteorological Society, 1998, 79 (1): 61J78 30