Demographic impact of climate change on northwestern China in the late imperial era

Quaternary International xxx (2016) 1e11

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Demographic impact of climate change on northwestern China in the late imperial era Harry F. Lee a, *, David D. Zhang a, Qing Pei b, c, Xin Jia d, Ricci P.H. Yue a a

Department of Geography and International Centre for China Development Study, The University of Hong Kong, Pokfulam Road, Hong Kong Department of Social Sciences, The Education University of Hong Kong, 10 Lo Ping Road, Tai Po, New Territories, Hong Kong c School of Environment and Natural Resources, Renmin University of China, Beijing, 100872, China d School of Geographic and Oceanographic Sciences, Nanjing University, Nanjing, Jiangsu, 210023, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Available online xxx

A growing body of quantitative (large-N) studies examines the climateeman nexus in historical agrarian China. Their dominant approach is to take the whole of China as a spatial aggregate, without taking into account the role of regional context. Furthermore, quantitative investigations of the climateeman nexus in northwestern (NW) China remain scarce. This study seeks to address the above issues. We focused on the extent to which periodic fluctuation of precipitation and temperature affected positive checks (famine, epidemics, nomadic invasion, and rebellion) and subsequently population growth dynamics in Sha'anxi, Gansu, and Ningxia, NW China in AD 1500e1911. Multiple Wavelet Coherence and Partial Wavelet Coherence analyses were applied to handle the non-linear and non-stationary nature of the climateeman nexus. Our results show that drought was the common stressor of various positive checks in NW China at the multi-decadal (32e64 year) time-scale. In addition, there was a coherence break of the relationship in AD 1700e1820, owing to the tremendous increase of subsistence brought about by land reclamation policy and the introduction of foreign food crops. Yet, the coherence resumed afterwards, resulting in more disastrous demographic consequences. We highlighted that the climateeman nexus is not entirely deterministic in nature, even in environmentally-fragile NW China. The relationship is mediated by social factors, particularly government policies. Still, if those measures are made at the expense of the environment, although humans might be able to win over nature in the short-term, the final outcome could be catastrophic in the long-term. This study substantiates the above notion with empirical quantitative evidence. © 2016 Elsevier Ltd and INQUA. All rights reserved.

Keywords: Climate change Drought Temperature Positive checks Climateeman nexus Northwestern China

1. Introduction The societal impact brought by global climate change has received international attention. In recent years, a growing body of quantitative studies (i.e., large-N study) has examined the impact of climate change on historical agrarian China. Their basic premise is that “in the pre-industrial period, the main source of livelihood was agriculture; traditional agriculture was very much dictated by the whims of climate and weather conditions; any deterioration of climate would trim agricultural production; yield reduction would trigger famine, tax revolt, and a weakening of state power; the deficit in livelihood resources was aggravated by the population expansion

* Corresponding author. E-mail address: [email protected] (H.F. Lee).

accumulated in the previous favorable climate. Thus, human crises were likely to erupt during the period of deteriorating climate” (hereafter climateeman nexus) (Lee and Zhang, 2015, p.237). Along with this rubric of research, several aspects of Chinese societies have been correlated with climate change, such as armed conflicts (Zhang et al., 2005, 2006, 2007b, 2010; Zhang and Lee, 2010; Jia, 2013), epidemics (Pei et al., 2015b), migrations (Pei and Zhang, 2014; Pei et al., 2016a), agricultural production (Yin et al., 2015), economic fluctuations (Wei et al., 2014, 2015b, 2015a; Pei et al., 2016b), geo-politics (Zhang et al., 2015), and population growth dynamics (Lee et al., 2008, 2009; Lee and Zhang, 2010a, 2013; Lee, 2014). The above studies demonstrate the adverse socio-economic and demographic effects brought by climate deterioration (cooling in particular) via statistical means, shedding light on the role of global climate change in shaping Chinese history in a macrohistorical perspective.

http://dx.doi.org/10.1016/j.quaint.2016.06.029 1040-6182/© 2016 Elsevier Ltd and INQUA. All rights reserved.

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Nevertheless, the findings of the above studies are also subject to debate. One of the key issues is that the dominant approach of the abovementioned studies is to take the whole of China as a spatial aggregate, assuming the vast territory of China to be uniform. However, the impact of climatic forcing may be buffered by various political, socio-economic, and technological institutions, at least in some places and in some instances (Fan, 2010). Crisis occurs only if the forcing exceeds the buffering capacity of human society (in terms of migration, economic change, innovation, trade, peaceful resource redistribution, and so on) for a significantly long period. Also, in regions where population pressure and agricultural dependence on climate were less important, the climateeman nexus was weaker and less apparent (Zhang et al., 2007a, 2011a). As the socio-economic context, such as population pressure and agricultural dependency that characterizes institutional capacity to adapt to social pressure, varies across places, the societal impact brought by global climate change may be regionally diversified. As highlighted by Catto and Catto (2004), in some regions, climate change has been a driving force of social crises; in other regions, the climate has played an important supporting role. In addition, there are also regions where the effect of climate change is secondary. Owing to the above concern, a scholarly attempt has been made to scale down the quantitative analysis of the historical climateesociety nexus to smaller geographic regions in China. The roles of regional geographic context and cultural factors in mediating the nexus have also been highlighted (Pei et al., 2016a). Yet, the attempt is characterized by strong regional bias. Even though the historical climateeman nexus in eastern China (Zhang et al., 2007b; Bai and Kung, 2011) and northern China (Kung and Ma, 2014; Chen, 2015) has been repeatedly examined, the one in northwestern (NW) China still remains poorly investigated. So far, only the frequency of natural disasters (Lee and Zhang, 2010b, 2011, 2012) in relation to climate change has been quantitatively explored. A related line of research is about using archaeological records to trace the changes of subsistence strategy, human settlement pattern, and culture in different climatic episodes (Dong et al., 2012a, 2012b, 2013a, 2013b; Jia et al., 2012, 2016a, 2016b; Guan et al., 2014; Jia et al., 2016c, 2016d; Zhou et al., 2016) as well as the human impact on the biophysical environment (Zhou et al., 2012; Zhao et al., 2013; Ren et al., 2015). Given that some important historical events with disastrous social and demographic consequences, such as late Ming peasant rebellions (c. early seventeenth century) and the Dungan Revolt (c. 1862e1877 and 1895e1896), originated in NW China, and that they may have been triggered by deteriorating climate (Zheng et al., 2014), the historical climateeman nexus in NW China is worth exploring, particularly in a quantitative manner. The present study seeks to fill the abovementioned research gap. As various aspects of human societies (e.g., armed conflicts, epidemics, migration, and so on), which are covered in the previous climateeman studies, are eventually linked to population growth dynamics (Lee and Zhang, 2010a, 2013; Lee, 2014), we focus on the extent to which periodic fluctuation of temperature and precipitation affected population checks, and subsequently population growth dynamics, in NW China in recent human history. 2. Materials and methods 2.1. Study area and study period NW China includes the current provinces/autonomous regions of Sha'anxi, Gansu, Ningxia, Qinghai, and Xinjiang, comprising about one-third of China's land area. It is also the region where the Hexi Corridor is located and was of significant military importance to ancient China (Li, 1998). Its total area is 3.09 million km2, and it overlaps with the fringe of the monsoon limit, where the

precipitation regime is simultaneously influenced by multiple atmospheric circulation systems, such as Asian Summer Monsoon, Winter Monsoon, and Westerlies (Zhao, 1986). Most of the regions in NW China are arid, with the mean annual rainfall less than 250 mm. In the western plains, annual precipitation is in the range of 50e150 mm; in the Taklimakan Desert, precipitation is even less than 25 mm. On the other hand, annual evaporation in NW China is more than 1400 mm on average and about 2000e3000 mm in the desert areas (Zhao, 1986; Deng et al., 2005). Subject to the above climatic background, NW China is the transition zone from farming to animal husbandry, and is highly susceptible to climate and environmental change. Even minor natural or man-made disturbances could cause permanent ecological damage in the region (Zhao, 2006). In this study, subject to the availability of fine-grained historical population checks and written records of population size, our study area was delineated as Sha'anxi, Gansu, and Ningxia as per current administration boundary (Fig. 1). As our study area covers more than 50% of the inhabited area in NW China, it could be a reasonable representation of NW China. In addition, we delimited our study period to AD 1500e1911 [the late imperial era, spanned over Ming (c. AD 1368e1644) and Qing (c. AD 1644e1911) dynasties], which is commonly covered in all of our datasets (cf. Section 2.2). 2.2. Data sources 2.2.1. Paleo-climate data Temperature and precipitation are two major climatic elements. The geographic coverage of two recently published regional paleoclimate reconstructions overlapped with our study area, and they were employed in this study. The first one is the annual mean temperature variability in the West Qinling Mountains in AD 1500e1995, which is derived from 73 proxy records based primarily on treeering chronologies (Yang et al., 2013) (Fig. 2A). The second one is the annual geographic extent of drought anomalies (a proxy indicator of precipitation) in NW China in AD 1470e2008, which is derived from the dryness/wetness grade series of 19 sites in NW China based primarily on historical documents (Lee et al., 2015c) (Fig. 2B). To date, these are the paleo-climatic records with the highest temporal resolution in our study area. We employed them to examine the combined and the individual effect of temperature and precipitation fluctuations on population checks in NW China. 2.2.2. Positive checks data Famines, epidemics, and wars are coined as positive (or Malthusian) checks in Thomas Malthus' (1798) classic treatise An Essay on Population. They play a significant role in balancing food subsistence and population size (Malthus, 1798). We correlated our climate data with those historical positive check incidents that happened within our study area. The sources of our positive checks data are stated below: As regards famine, it is always difficult to define the term objectively. As a remedial measure, in reference to Xiao et al. (2015), we took cannibalism as an indicator of famine. Our cannibalism data were obtained from a multi-volume compendium Collection of Meteorological Records in China over the Past Three Thousands Years (Zhang, 2004). Famine was counted according to the number of counties with cannibalism in a year (Fig. 2C). As regards epidemics, our data were obtained and aggregated from three independent datasets, namely Collection of Meteorological Records in China over the Past Three Thousands Years (Zhang, 2004), Historical Records of Infectious Diseases in China (Li, 2004), and Epidemic records in historical China (Zhang, 2007).

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Fig. 1. Map of China. Our study area is delineated as Sha'anxi, Gansu, and Ningxia in NW China. Arrows represent Asian Summer Monsoon (including East Asian Summer Monsoon and Indian Summer Monsoon), Westerlies, and Winter Monsoon. The shaded belt is the region between the 200e400 mm isohyets, which is the approximate present-day northern fringe of Asian Summer Monsoon.

Epidemics were counted according to the number of counties affected by epidemics in a year (Fig. 2D). As regards wars, our data came from a multi-volume compendium Tabulation of Wars in Ancient China, which exhaustively records information on the wars that took place in China from 800 BC to AD 1911 (Editorial Committee of Chinese Military History, 1985). War was counted in terms of the number of battles. Based on the types of participants, particularly leaders of the two sides in the armed conflicts, we further put war into two categories, namely nomadic invasion (Fig. 2E) and rebellion (Fig. 2F). 2.2.3. Population size data We retrieved historical population size data within the current political boundaries of Sha'anxi, Gansu, and Ningxia from Lu and Teng's (2006) Chinese provincial population dataset published in Examination of Historical Chinese Population in Various Provinces and Districts (Fig. 2G). As Lu and Teng (2006) give estimates of Chinese population size at irregular time intervals, and the common logarithm of the data points was taken, linearly interpolated and then anti-logged back to create an annual time-series. This method avoids distortions of the population growth rate in data interpolation, which has been applied in our previous studies (Lee et al., 2008, 2009). 2.3. Statistical methods 2.3.1. Wavelet analysis Climateeman nexus is highly complex and contextual in nature (Lee et al., 2015b; Pei et al., 2016a), implying that the association between climate change and human societies could be non-linear and non-stationary. Hence, it may be inappropriate simply to apply traditional techniques (which make the assumption that the statistical properties of the time-series do not vary with time) to analyze the nexus. Wavelet analysis is a powerful tool that is already in use throughout science and engineering. By performing a

local time-scale decomposition of the signal in time-series, wavelet analysis is germane in analyzing non-stationary systems (Torrence and Compo, 1998; Cazelles et al., 2007, 2008). The continuous wavelet transform decomposes the time-series into both time and frequency components, the calculation of the wavelet power spectrum quantifies in the time-frequency domain the distribution of the variance of the time-series (Grinsted et al., 2004; Cazelles et al., 2008). In this study, we employ wavelet coherency analysis to present coherencies between time-series, in order to identify any significant associations between time-series of specific frequencyetime domain (Grinsted et al., 2004; Cazelles et al., 2008). Multiple Wavelet Coherence (MWC) and Partial Wavelet Coherence (PWC) analyses (Ng and Chan, 2012) were applied to explore the combined and individual (stand-alone) effect of drought and temperature anomalies on positive checks. MWC is designated for seeking the wavelet coherence of multiple independents on a dependent, while PWC is used to seek the wavelet coherence between two time-series after eliminating the influence of their common dependence (Ng and Chan, 2012). Morlet wavelet is employed to decompose signals, which is generally regarded as an efficient means of detecting variations in the periodicities of geophysical signals along time-series in a continuous manner (Rigozo et al., 2008). 2.3.2. One-way ANOVA One-way ANOVA is used to test for significant differences between group means (De Vaus, 2002). In this study, we applied this method to supplement our wavelet analysis. When any “unusual period” was revealed by the wavelet analysis, we applied One-way ANOVA to compare the values of various time-series between the unusual and the rest of our study periods. 2.3.3. Moving correlation We employed moving correlation analysis to inspect the possible non-stationary association between temperature and

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Fig. 2. Climate change, positive checks, and population growth dynamics in NW China, AD 1500e1911. (A) Drought anomalies; (B) temperature anomalies; (C) famine; (D) epidemics; (E) nomadic invasion; (F) rebellion; and (G) total population size.

drought anomalies at multi-decadal time-scale, in which climate variability is usually discussed (Lee et al., 2015b). Moving correlation, r(nw), is the correlation coefficient calculated between two time-series for all points within the correlation window. The window is moved along the time-series and then r(nw) is calculated at every point along the time-series. The moving correlation window was set at a 51-point (year) width and moved to every point along the temporal dataset.

2.3.4. Control variables The link from climate to positive checks can be summarized as: deteriorating climate causes poor harvest, followed by economic hardship, and eventually positive checks. However, when the climateeman nexus was examined in this study, we did not control the effect of agricultural production and food price on positive checks. This is because those factors themselves are affected by climate variation (Pei et al., 2013, 2014, 2015a). As emphasized by

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Hsiang et al. (2013), if they are included as control variables, they may incorrectly absorb the signal contained in our concerned climate variable. Furthermore, their inclusion may result in a biased estimate because populations differ in unobserved ways and become artificially correlated with climate. 3. Results 3.1. Overall picture of the climateeman nexus in NW China In terms of the counts of positive checks, two periods are revealed to be chaotic periods in NW China in our study time span: AD 1600e1700 and AD 1850e1911. NW China was also characterized by frequent natural disasters and social instability in the above periods (Jiang, 1993; Yuan, 1994). Subject to frequent positive checks, total population size was stagnated and only increased by 3.85% in AD 1578e1661, from 10.8 to 11.2 million, while total population size dropped by 43.71% in AD 1820e1912, from 26.5 to 14.9 million (Lu and Teng, 2006). We applied MWC to investigate the combined effect of drought and temperature anomalies on various positive checks in NW China. Results are presented in Fig. 3. Significant regions can be seen on almost the entire spectra in every panel. Despite discontinuous coherence in 2e4 and 4e8 year periodicities prevalent in various panels in Fig. 3, some coherence bands are relatively consistent. For famine, there are two dominant coherence bands, one of 32e64 year periodicities, and one of 64e128 year periodicities (Fig. 3A). For epidemics, there are two dominant coherence bands, one of 16e32 year periodicities, and one of 32e64 year periodicities (Fig. 3B). For nomadic invasion, there are two dominant coherence bands, one of 16e32 year periodicities, and one of 32e64 year periodicities (Fig. 3C). For rebellion, there are two dominant coherence bands, one of 32e64 year periodicities, and one of 64e128 year periodicities (Fig. 3D). As shown in Fig. 3, the 32e64 year coherence band is common in all of the four population checks. The band also covers the chaotic periods in AD 1600e1700 and 1850e1911. In addition, the 16e32 year periodicities are also apparent in famine, epidemics, and nomadic invasion in those chaotic periods. For rebellion, the 16e32 periodicities are significant in AD 1850e1911 and nearly significant in AD 1600e1700. The abovementioned common periodicities in coherence imply that those crisis periods are associated with the synthesis of precipitation and temperature fluctuation at the decadal to multi-decadal (16e64 year) time-scales. Yet, regarding the commonly shared 32e64 year periodicities, there is a break of the coherence around AD 1700e1820 in all of the panels in Fig. 3, which will be further examined in Section 3.3. 3.2. Individual effect of drought and temperature anomalies on positive checks We employed PWC to check the individual effect of drought and temperature anomalies on various positive checks in NW China. When the influence of temperature anomalies was controlled, continuous coherence band in 32e64 year periodicities between drought anomalies and positive checks is observed, though there is a coherence break in AD 1700e1820 (Fig. 4). The 64e128 year coherence between drought anomalies and famine remains continuous. Alternatively, when the influence of drought anomalies was controlled, no continuous coherence band between temperature anomalies and positive checks could be observed (Fig. 5), which suggests that the temperature anomalies are not strongly related to positive checks. Also, the areal extent and the pattern of significant regions of various panels in Fig. 5 are not as continuous/regular as those in Fig. 4, implying that

Fig. 3. Multiple wavelet coherence (MWC) analysis, which presents the combined effect of drought and temperature anomalies on various positive checks in NW China. (A) MWC of famine, drought anomalies, and temperature anomalies; (B) MWC of epidemics, drought anomalies, and temperature anomalies; (C) MWC of nomadic invasion, drought anomalies, and temperature anomalies; and (D) MWC of rebellion, drought anomalies, and temperature anomalies. The black contour indicates significant periodicities (p < 0.05) against red noise. The legend indicates coherence values, which vary from dark blue (low values) to dark red (high values). The region outside the cone of influence, where edge effects might distort the picture, is shaded. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

drought anomalies were more important than temperature anomalies in driving various positive checks in NW China during our study period, particularly at the multi-decadal and centennial time-scales.

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Fig. 4. Partial wavelet coherence (PWC) analysis, which presents the stand-alone effect of drought anomalies on various positive checks in NW China, with the influence of temperature anomalies controlled. (A) PWC of famine, drought anomalies, and temperature anomalies; (B) PWC of epidemics, drought anomalies, and temperature anomalies; (C) PWC of nomadic invasion, drought anomalies, and temperature anomalies; and (D) PWC of rebellion, drought anomalies, and temperature anomalies. The black contour indicates significant periodicities (p < 0.05) against red noise. The legend indicates coherence values, which vary from dark blue (low values) to dark red (high values). The region outside the cone of influence, where edge effects might distort the picture, is shaded. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 5. Partial wavelet coherence (PWC) analysis, which presents the stand-alone effect of temperature anomalies on various positive checks in NW China, with the influence of drought anomalies controlled. (A) PWC of famine, temperature anomalies, and drought anomalies; (B) PWC of epidemics, temperature anomalies, and drought anomalies; (C) PWC of nomadic invasion, temperature anomalies, and drought anomalies; and (D) PWC of rebellion, temperature anomalies, and drought anomalies. The black contour indicates significant periodicities (p < 0.05) against red noise. The legend indicates coherence values, which vary from dark blue (low values) to dark red (high values). The region outside the cone of influence, where edge effects might distort the picture, is shaded. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Another point is that when compared with the figure of MWC (Fig. 3), there is substantial reduction of high-frequency (2e16 year) coherence bands in the figures of PWC (Figs. 4 and 5), which reveals that the high-frequency cycles of various positive checks may be

attributable to the synergistic effect of drought and temperature anomalies. Nevertheless, this makes it hard to assess the standalone contribution of drought and temperature anomalies to positive checks at the inter-annual to decadal time-scales.

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3.3. Break of the 32e64 year coherence in AD 1700e1820 In the above wavelet coherency analysis, neither the combined nor the individual effect of drought and temperature anomalies could explain the 32e64 year (multi-decadal) cycles of positive checks during AD 1700e1820. To see how this period of coherence break is different from the other periods, we applied One-way ANOVA analysis to compare the annual means of famines, epidemics, nomadic invasion, and rebellion between the coherency break period (AD 1700e1820) and the rest of our study period (AD 1500e1699 and 1821e1911). Our results show that the period of coherence break is characterized by significantly fewer positive checks (p < 0.1) (Table 1). Table 1 One-way ANOVA of positive checks in NW China, AD 1500e1911.

*

Variable

F

dfa

dfb

Annual means 1700e1820

Other periods

Famine Epidemics Nomadic invasion Rebellion

4.870** 5.828** 3.047* 7.417***

1 1 1 1

410 410 410 410

0.01 0.32 0.01 0.06

0.43 0.78 0.14 0.62

Significant at 0.1 level. Significant at 0.05 level. Significant at 0.01 level. a Degree of freedom between groups. b Degree of freedom within groups.

**

***

We proceeded to seek explanation for the coherence break from both physical and social factors. From the side of physical factors, we examined whether the coherence break is attributable to the unusual association between drought and temperature anomalies in AD 1700e1820. We applied 51-point (year) moving correlation analysis to compare the drought and temperature anomalies. Results are presented in Fig. 6. There exists a positively significant correlation between the drought and temperature anomalies in the coherence break period, which means that the combination of climatic elements was either cool and humid or warm and dry during the time. The positive correlation is also revealed by highresolution lake sediment records in NW China (He et al., 2015; Song et al., 2015). Indeed, such combination is not ideal for agricultural production (Su et al., 2014), especially in arid regions (Hsu,

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1998; Zhang et al., 2011b). According to the climateeman nexus on which previous studies are grounded (cf. Section 1), there should be more frequent positive checks in the period. But, the reality shows the opposite. This suggests that explanation for the coherence break should be sought from non-climatic factors. We turned to the side of social factors, and examined the important policies that were enacted by Qing government during the time. We found that the period is coincident with the active implementation of land reclamation policy in NW China. Chinese population was less than 100 million when Qing dynasty was established in AD 1644, at which time the population began to increase incessantly, reaching 150 million in AD 1724 (Lu and Teng, 2006). As food supply was unable to catch up with the rapid population growth in the major agricultural regions in China, there was exigency to cater for the subsistence needs of those surplus population (Zhao, 2006; Jia et al., 2016a). The land reclamation policy in Qing dynasty was mostly to promote a garrison system and to encourage immigration to border regions to relieve population pressure. Related policies provided various incentives to civilians or soldiers, including farming cattle and tillage implements, seeds, silver (as currency), and levies or tax exemptions. This substantially encouraged people to open up previously unwanted land such as forest, grassland, and hilly wasteland for cultivation (Zhao, 2006). Large-scale immigration of people into NW China occurred in Qianlong Times (AD 1736e1795). The period is also marked by the rapid expansion of population in our study area, as total population size increased by 135.9% in AD 1661e1820, from 12.2 million to 26.5 million, which was in proportion to the population growth in the whole of China (Lu and Teng, 2006). The population increase is also envisaged by the expansion of human settlements in Gansu and Qinghai in terms of their number and geographic coverage (Dong et al., 2012b; Jia et al., 2016a). Commercial sectors in NW China were not well-developed during the time (Dai, 1988), which is unlikely to account for such a drastic increase of population. In unison, foreign (New World) food crops such as maize and sweet potatoes were also introduced in NW China around the eighteenth century. Those crops did not compete with existing Asian staples, as they could be planted in places where nothing else could grow, such as poorer soils in loess or hilly regions (Guo, 1995; Lv, 2007; Penuel and Statler, 2011). Maize and sweet potatoes were companion crops. The former one was grown on sunny hills, while the latter one on shady hills. In mountainous regions, maize could

Fig. 6. Moving correlation curve between the temperature and drought anomalies in NW China (black line). Correlation window, nw, is equal to 51-point (year). The width of the window is indicated in the lower right-hand corner. The thin dashed horizontal line indicates the 95% confidence level. The corresponding values are plotted at the centers of the window periods.

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give farmers provisions for six months (Embree and Gluck, 1999). On one hand, those foreign crops increased the total farming area and consequently the supply of food, animal feed, and fuel in NW China. On the other hand, owing to their environmental stress tolerance, they also served as famine crops and helped people to sustain themselves during the periods of harvest failure (Penuel and Statler, 2011). Peasants in northern China were ordered by imperial edicts to plant sweet potatoes to prevent famine in the eighteenth century (Embree and Gluck, 1999). The synergistic effect of land reclamation policy and the introduction of foreign crops engendered a tremendous increase of subsistence in NW China in the early Qing dynasty (Cao, 2002; Yao, 2003b; Zhao, 2006). Hence, despite the abovementioned unfavorable combination of climatic elements, the multi-decadal link between climate change and positive checks could be detached. 4. Discussion 4.1. Drought anomalies and the climateeman nexus Population growth is constrained by food subsistence. When population growth overshoots the available subsistence, poverty and consequently positive checks occur (Malthus, 1798). In the preindustrial era, subject to the low level of technology, agricultural production and subsistence strategies were largely contingent upon climatic conditions (Galloway, 1986; Jia et al., 2016a, 2016b, 2016c, 2016d). Such climate dependence also shaped human settlement pattern and even cultural evolution (Jia et al., 2012; Dong et al., 2012a, 2013a). Even with improved farming technology in recent decades, when annual mean temperature (precipitation) drops by 1  C (100 mm), crop yield per mu will be decreased by 10% in China (Zhang, 1982). On the other hand, the vast majority of households were rural and derived most of their income from agricultural activity (Pei et al., 2013, 2014, 2015a). Therefore, climate-induced agricultural shrinkage and its subsequent economic shocks could result in social turmoil. This is especially true at the multi-decadal to centennial time-scales. The above phenomenon has been empirically demonstrated in pre-industrial China (Lee and Zhang, 2010a, 2013; Lee, 2014; Pei et al., 2015b), Europe (Zhang et al., 2011a; Lee et al., 2013, 2015b, 2016), the Northern Hemisphere (Zhang et al., 2011b), as well as the entire globe (Zhang et al., 2007a). When taking China as a spatial aggregate, multi-decadal to centennial cooling appears as the common stressor in disrupting socio-political order and population growth dynamics throughout Chinese history (Lee et al., 2008, 2009; Lee and Zhang, 2010a, 2013; Lee, 2014). The same conclusion is also applicable at the regional level, such as eastern China (Zhang et al., 2007b) and northern China (Kung and Ma, 2014; Chen, 2015). However, in this study, we find that drought, rather than cooling, is the stressor in NW China. The above disparity may be attributable to the influence of regional geography. Most of the regions in China are covered by Asian Summer Monsoon (Fig. 1), which ensures the steady supply of moisture (Zhao, 1986). Hence, fluctuation of temperature is relatively more imperative in modulating the change of human carrying capacity (agricultural production) in those regions. But, in NW China (i.e., our study area) where the climate is arid/semi-arid and a huge part of the region is not covered by Asian Summer Monsoon (Zhang et al., 2006), precipitation availability is revealed to be the most critical variable in determining human carrying capacity and, subsequently, the economy and society there (Yuan, 1994; Pei and Zhang, 2014; Pei et al., 2016a). Reduced rainfall could decrease grassland productivity in semi-arid lands by 40e90% and increase desertification (Huq et al., 2004). In unison, NW China is also the agro-pastoral ecotone (cf. Introduction). The

people there are sustained by both farming and pastoral activities. A pastoral system is a system that directly interacts with the ecosystem (Pratt et al., 1997), which is highly vulnerable to drought (Fang and Liu, 1992; Shi and Ha, 2002; Pei and Zhang, 2014). Pastoral society was even more vulnerable to drought in the past, owing to its lower level of buffering and technological capacity compared to agricultural society (Pei and Zhang, 2014; Pei et al., 2016a). This may have further reinforced the demographic impact of precipitation fluctuation in NW China during our study period. 4.2. Break and resumption of multi-decadal climateeman nexus The question arises why the multi-decadal connection between climate change and positive checks, which was once detached in AD 1700e1820, was resumed afterwards. To resolve this query, it is imperative to thoroughly consider the historical and regional context of NW China. Although the synthesis of land reclamation policy and the introduction of foreign crops engendered a tremendous increase in the subsistence level in NW China, it was contingent upon the availability of farmland. It was once sustained by the transformation of pastoral land into farmland (Guo, 1995; Lv, 2007). Nevertheless, with the increase of population, there was no idle, deserted cultivated land left in the second half of the eighteenth century. To meet the land demand, the government directed people to move to mountains, hills, and wastelands, despite the poor ecological conditions in those areas (Zhao, 2006). Those areas were marginal or even unsuitable for any farming activities. For instance, in newly opened-up farmland in the Hexi region, crop yield per mu only ranged between 86.2 and 129.2 market catties despite huge labor input, in comparison to 160e170 market catties in ordinary farmland (Li, 1992). Even though the multi-decadal climateeman nexus in NW China could be detached in the short-term, it was made at the expense of the environment. There are some scholarly estimates about such maneland conflict. In the middle of the Qing dynasty, in the Hexi Corridor, the magnitude of human impact on the environment was nearly four times heavier than that in Ming dynasty and earlier periods (Cheng et al., 2011). Furthermore, the density of population had exceeded the critical index of population pressure in arid land and the rate of water resource utilization had exceeded 40% (Wang et al., 2003). What followed was the reduction of tree cover and the desiccation of lakes (Wang et al., 2002; Dong et al., 2012b), and eventually severe desertification and soil erosion, particularly in those ecologically-marginal areas that had been opened up for farming (Zhao, 2006). The Mu Us Desert, which locates at the transitional belt from temperature grassland to desert grassland, was once vegetated. Subject to the land reclamation policy in Qing dynasty, desertification process was accelerated there, which eventually became a typical man-made desert (Zhao, 2006). In Hehuang region, over 50,000 mu of farmland was abandoned during the period AD 1799e1847 because of soil erosion (Yao, 2003a). Even worse, because of those unsustainable measures, the disastrous impact of climate deterioration is magnified in the long-term (Coombes and Barber, 2005; Fraser, 2011). As shown in Fig. 3, the multi-decadal climateesociety link resumed since the AD 1820s. In the first chaotic period in AD 1600e1700, frequent positive checks induced by deteriorating climate only resulted in the stagnation of population growth. But, in the second chaotic period in AD 1850e1911, frequent positive checks induced by deteriorating climate resulted in the drastic reduction of population (Fig. 2G). The demographic impact in the second chaotic period was much more disastrous and more longlasting than that in the first chaotic period. In AD 1949, total population size in our study area was still 3.2% lower than that in AD 1820 (Lu and Teng, 2006).

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The population collapse in the second chaotic period is mainly brought by the Dungan Revolt, which is often attributed to religious difference between Muslim and Han (Cao, 2002). However, religious difference is a time-unvarying factor, which cannot really explain the timing of the revolt. Our results suggest that the carrying capacity of NW China was shrunk by climate deterioration, and its effect was further magnified by the severe environmental degradation during the time. This might have raised tensions over dwindling subsistence resources, and then fueled existing conflict into socio-economic and population collapse in the second half of the nineteenth century. 4.3. Knowledge contribution In this study, we supplemented the existing literature by verifying quantitatively the historical impact of climate change on population growth dynamics in NW China, a topic that is overlooked in previous research. In addition, the non-linear and nonstationary association between climate change and human societies was also handled in our methodology. We found that the demographic impact of climate (drought in particular) is especially apparent at the multi-decadal time-scale. Even though we highlighted the importance of climatic factors in shaping human societies, the coherence break of the multi-decadal influence of climate change on positive checks in AD 1700e1820 suggests that the climateeman nexus is not entirely deterministic in nature, even in environmentally-fragile regions like NW China. In fact, the relationship is mediated by social factors (government policies in particular). Therefore, when examining the climateeman nexus, we should not settle for an environmentally-deterministic conclusion, as institutional measures often played a part. But, if those measures are made at the expense of the environment, though humans could win over nature in the short-term, the final outcome could be catastrophic in the long-term. This study substantiates the above notion with empirical quantitative evidence. 5. Conclusions Drought anomalies, which are a proxy of precipitation change, were imperative in driving various positive checks in Sha'anxi, Gansu, and Ningxia, NW China in the late imperial era at the multidecadal time-scale, while their connection was, in part, mediated by government policies. Even though drought anomalies were the important stressor, drought itself is often modulated by temperature change in NW China (Lee and Zhang, 2011; Lee et al., 2015a). Therefore, the influence of temperature should not be neglected. Our findings may have some implications for the present day's marginal societies within the context of global warming. For the countries locating in the Sahel, their environmental setting as well as long-term climatic variability is very similar to that in NW China, as both of them locate at the fringe of the monsoon limit (Shi and Ha, 2002). The genocide in Darfur since AD 2003 has been attributed to environmental causes (i.e., the synthesis of warm and drought) (United Nations Environment Programme, 2007). Based on our findings in this study, perhaps the genocide could have been prevented if appropriate institutional measures had been taken, provided that those measures were not made at the expense of Mother Nature. Acknowledgements This research was supported by the Hui Oi-Chow Trust Fund (201502172003), Research Grants Council of The Government of the Hong Kong Special Administrative Region of the People's Republic of China (HKU758712H, HKU745113H, and 17610715), China

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