The Mongol empire’s expansion and rethinking research trends in Chinese historical climatology

With the rise of interdisciplinary studies and novel approaches to historical research, scholars—including historians and scientists—have embarked on establishing the new subfield of historical climatology (Ingram, 1978; Brázdil et al., 2005; White et al., 2022). Meticulous reconstruction of reliable historical climate data is central to the aims of historical climatology. Through the application of various rigorous and advanced scientific methodologies, current scientists are able to provide highly reliable data on past climates. It is important to recognize that climate change in the past unfolded as a long-term process, spanning centuries or even millennia. Consequently, scientists typically generate and reconstruct long-term climate data to accurately capture these historical fluctuations. This wealth of data allows for a thorough re-examination of significant historical events and facilitates the identification of overarching historical patterns.

However, it is worth exploring whether new scientific data can lead to a reliable understanding of history. Historical climatology research methods are employed to examine whether scientific data align with historical interpretations. In the field of Chinese historical climatology, the Granger Causality Test is a statistical hypothesis method adopted by many scholars to research the relationships between climate and war or conflict.

This article aims to explore the relationship between the Mongol westward expansion and climate, specifically examining scholars’ use of the Granger Causality Test to link scientific data with historical interpretations. The westward expansion of the Mongol Empire (thirteenth to fourteenth centuries) is one of the most important events in world history, captivating scholars’ interest in its causes and consequences, with climate as a focal factor. The first section of this study introduces recent relevant publications on climate and the Mongol Empire. Based on this section, the focus will then be on reviewing the limitations of using the Granger Causality Test as a research method in historical climatology, along with an examination of other research methods in historical climatology.

The Mongol Empire, founded by Genghis Khan (1162–1227) in 1206, became the largest contiguous land empire in world history, stretching from the Pacific Ocean in the east to the Danube River (modern-day Hungary) and the Persian Gulf in the west by the late thirteenth century. Before 1206, Genghis Khan was merely one of many tribal leaders vying for dominance in the steppe regions south and southeast of Lake Baikal. However, his decisive victories over the Kereit and Naiman Turks secured his undisputed authority over present-day Mongolia, setting the stage for a series of expansive campaigns.

The first major campaign, from 1205 to 1209, targeted the Tangut kingdom of Xi Xia, a northwestern border state of China. Ultimately, the Xi Xia king submitted to Genghis Khan. In 1219, the assassination of Genghis Khan’s Muslim envoys by the Khwārezmians in Otrar triggered a war with the Khwārezmian Sultanate in West Turkistan. Mongol forces went on to capture and plunder key cities such as Bukhara, Samarkand, and the capital Urgench in 1220 and 1221. By 1227, the Mongol dominions spanned vast territories from Central Asia to the central plains of China (Juvaini, 1997; Rossabi, 2011; May, 2012). This vast empire encompassed a multitude of peoples, religions, and civilizations.

The Mongol Empire not only facilitated the expansion of territory but also promoted artistry, technological innovations, and trade across its vast ___domain (Nyamdaa, 2023). Under Mongol rule, trade along the Silk Road flourished, with safer and more efficient travel routes. The Mongols played a significant role in the spread of knowledge and technology, leaving an enduring impact on world history.

Understanding the driving forces behind this unprecedented expansion is crucial. The historical conflict between nomadic steppe-dwellers and settled agricultural civilizations provides important context. While raids by steppe nomads on settled populations were common, they had never before escalated into a quest for global domination, as seen in Genghis Khan’s invasions. Scholars have long sought to explore why the Mongols embarked on their westward expansion. Among the many factors considered, the influence of climate has garnered significant attention from researchers.

Ellsworth Huntington published Civilization and Climate in 1915, in which he proposed a series of correlations between climate and human civilization. He addressed topics such as food supply, natural resources, parasites and diseases, human livelihood, and habits (Huntington, 1915). Huntington presented climate as the most powerful force influencing migration, racial mixing, and natural selection. As a social Darwinist and environmental determinist, he believed that human civilization and culture are the result of a gradual and complex evolutionary process driven by the fundamental forces of nature (Fleming, 1998; Coscioni, 2020). In 1907, Huntington proposed that the westward expansion of the Mongol Empire was spurred by the onset of a global dry period in the thirteenth century, prompting the Mongols to engage in outward military aggression for survival (Huntington, 1907). This argument was endorsed by subsequent renowned scholars (Lamb, 1954; Jenkins, 1974; Toynbee, 1987; Brown, 2005). They, respectively, discuss periods of drought, of cooling, and of both simultaneously, and conclude that the deterioration of living conditions was a factor that encouraged the Mongolian invasions. In addition to drought, they suggest that sudden temperature decreases and the simultaneous occurrence of cold and humidity played roles in driving Mongol outward aggression.

Environmental determinism is the belief that physical geographic factors, such as climate and terrain, have a significant and direct impact on human activities and outcomes. It is inevitable that human societies and activities are influenced by weather and climate, but the way people respond is not necessarily uniform. In their latest research, Dagomar Degroot et al. summarize how different societies have developed varied strategies to cope with climate change, emphasizing that these responses were not uniform but rather influenced by local contexts, environmental conditions, and social structures (Degroot et al., 2021). Specifically, while some societies may have faced significant challenges or even collapsed due to climatic pressures, others were able to adapt and thrive through various means, such as exploiting new opportunities, developing resilient energy systems, expanding trade networks, and implementing political or institutional adaptations.

Environmental determinism is a form of reductionism that simplifies data and outcomes into direct relationships. Meanwhile, the role of humans in history becomes entirely passive. The argument that the Mongols sought outward expansion due to deteriorating climate raises questions: If the Mongols were experiencing climate deterioration, how did they find the courage and determination to wage wars? And why would they establish a vast Eurasian empire if they were only trying to solve survival problems? The biggest flaw of environmental determinism is that it places humans in a passive and reactive position in history. As a result, environmental determinism no longer holds a significant place in the academic study of the relationships between human societies and climate change.

However, such linear cause-and-effect reasoning, as a form of reductionism, is still applied in some statistical research methods, particularly in studies that use the Granger Causality Test to explore the relationships between climate, wars, and the decline of dynasties in Chinese history.

As new knowledge and technologies emerge, more accurate historical climate data become available (introduced below). While modern advancements in methods and technologies have realized more accurate historical climate data, historians recognize that interpreting these data and assessing the impact on human history requires contextualization within historical frameworks.

The Granger Causality Test emerged as a tool in this interdisciplinary pursuit. According to Anil Seth’s summary, “if a signal X₁ ‘Granger-causes’ a signal X₂, then past values of X₁ should contain information that helps predict X₂ above and beyond the information contained in past values of X₂ alone. Its mathematical formulation is based on linear regression modeling of stochastic processes” (Seth, 2007). Geographer David Zhang and his team have emerged as leading scholars proficient in leveraging the Granger Causality Test within the realm of Chinese historical climatology. Across three seminal articles, the extensive research of Zhang et al. explores the correlation between temperature fluctuations and the frequency of conflicts, as well as the rise and fall of dynasties (Zhang et al., 2005, 2006, 2007). Focusing on ancient China, Zhang posits that colder climatic phases could counteract progress achieved during warmer periods, leading to social unrest and dynastic collapse. Employing the Granger Causality Test, Zhang conducted comparative analyses of climate changes over the past millennium alongside major conflict outbreaks, suggesting that colder periods correlate with heightened societal instability and conflicts between pastoralists and agriculturalists, ultimately shaping dynastic declines. Criticisms have been leveled at Zhang’s conclusions (Fan, 2010, 2015), but parallel studies by researchers such as Zhang et al. (2010) have yielded congruent conclusions.

David Zhang relies on the assumption that changes in climate influence historical events. His analysis hinges on the premise that reductions in thermal energy inputs diminish the carrying capacity of the land, leading to population reductions within traditional agrarian societies. By formulating hypotheses linking climate fluctuations to historical events, Zhang seeks to establish causal relationships between environmental factors and human societies over time.

Similar studies by Qing Pei in collaboration with David Zhang explored the relationship between climate change and nomadic migration in historical China (Pei and Zhang, 2014). Using the Granger Causality Test and other methodologies, the authors explored the causal pathway linking climate change to nomadic migration and conflicts between pastoralists and agriculturalists. Their research suggests that periods of low rainfall and temperature coincide with peaks in nomadic migration, corroborating the significant role of climate change in driving migration over the past two millennia. Pei and his team further elucidated the dynamics of conflicts between nomads and agriculturalists under the influence of climate change, with nomadic migrations often initiated in response to subsistence pressures induced by climatic fluctuations. Pei references the work of Pederson et al. and concludes, “Mongols and other nomads almost conquered the entire Eurasian continent in the 12^th and 13^th centuries during a period of drought” (Pei et al., 2019). Pei suggests that the Mongols, as nomads, achieved this conquest in response to climate-induced subsistence pressures. However, it is challenging to be persuaded that the article of Pederson et al. article can substantiate this conclusion.

Statistical approaches, especially the Granger Causality Test, focus on long time periods. While there are indeed trends in China’s climate changes over 1000 years, the problem remains that humans in history are not entirely passive, and their responses to climate change are not uniform or singular. The use of the Granger Causality Test in historical climatology can be seen as an evolved form of environmental determinism, grounded in statistical methods. However, it still fundamentally operates on the premise that, predominantly, natural forces dictate human societal responses.

Raphael Neukom et al. (2019) discovered no indication of preindustrial universally consistent cold and warm periods over the last 2000 years. Throughout history, China has been a vast land with highly diverse climatic conditions from south to north. Such complexity cannot be understood through simple statistics, let alone through the lens of environmental determinism.

Extensive analysis reveals that, in the realm of historical China, precipitation exerted a statistically more significant influence on nomadic migration than did temperature. This finding is supported by various scholars’ investigations, implicitly relying on the Granger Causality Test (Ying and Kung, 2011; Hsiang and Burke, 2014; Wischnath and Buhaug, 2014). Collectively, these studies converge on the idea that climate change was closely linked to war, conflict, and social unrest in China’s history. However, it is important to note that the Granger Causality Test, designed to assess long-term relationships between time series, may have limited applicability in historical climatology due to the transient and sporadic nature of social unrest and natural disasters.

It is implausible to assume that imperial administrators remained inert during periods of temperature-induced disaster and famine, or that empires lacked mechanisms to buffer social upheaval. In such scenarios, causal inference becomes unavoidable. Consequently, the simplistic assumption that “Climate change leads to natural disasters and famine, which in turn results in wars, conflicts, and social unrest, ultimately culminating in the fall of a dynasty,” warrants scrutiny. For example, David Morgan, taking the Mongol Empire as example, proposes that the “decline and fall” paradigm does not provide an adequate explanatory framework (Morgan, 2009). Nicola Di Cosmo (2023), based on his literature review of climate and the Mongol conquests, advocates for a nuanced analysis that takes into account the timing, routes, and tactical decisions involved.

The limitation of the Granger Causality Test lies in its failure to adequately consider causal inference. The intricate interplay between humans and climate throughout history further complicates matters. Su et al. uncovered that warmer periods saw increased warfare between nomadic and farming groups in ancient China, contrasting with David Zhang’s findings spanning the period from the Western Han Dynasty to the Tang Dynasty (Su et al., 2016, 2021). When employing the Granger Causality Test, meticulous verification of historical facts is imperative to avoid misleading conclusions. While some studies support the aforementioned sequence (Fang and Liu, 1992; Ying and Kung, 2011), others may diverge (Su et al., 2016), underscoring the necessity of a nuanced approach in historical analysis.

Examining Chinese history as an illustrative case reveals that long-term climate trends may not always directly correlate with specific historical events. Climate operates in cycles, with variations occurring within individual periods. Moreover, the proposed sequence of climate change leading to disasters, followed by conflicts and unrest, often results in localized rather than widespread turmoil. Declines in population due to warfare or crisis management efforts can mitigate conflicts. For example, warfare between northern nomadic groups and central plains dynasties often shifted southward into Chinese territory, dampening unrest elsewhere (Fan, 2015).

However, during periods of prolonged prosperity and population growth, large-scale and sustained warfare may arise due to commercial interests, as seen in ancient Athens, the Roman Empire, and modern Britain (Jackson, 2013; Rawlings, 2013; Mahony and Endfield, 2018; Margaritelli, 2020). The Crusades, spanning 200 years, exemplify this trend, occurring independently of climate fluctuations (Phillips, 2010). Considering the combination of recent research on the Mongol Western Expedition and climate, David Zhang and his team’s conclusions appear incongruous. Climate warming facilitated the mobility of Mongol cavalry, while deteriorating conditions led to their retreat from Hungary (related research introduced below). John Masson Smith Jr. conducted a comprehensive analysis of multiple unsuccessful Mongol campaigns in Syria, highlighting logistical challenges such as limited access to pasture and water, which significantly hampered Mongol military operations (Smith, 1984).

It becomes apparent that David Zhang and his team overlooked historical circumstances in Chinese dynasties. During periods of economic prosperity and national power, external expansion was unnecessary under China’s tributary system (Fairbank, 1968; Su et al., 2021). Strong national power during warm and humid periods often fueled expansionist ambitions and led to external conflicts. The Mongol Empire’s conflicts with Japan stemmed from the refusal of the latter to acknowledge Kublai Khan’s authority, rather than climate-induced aggression (Turnbull, 2010; Anderson, 2019). Economic and commercial activities under China’s tributary system fostered harmonious relations among China, neighboring countries, and nomadic groups (Jagchid and Symons, 1989).

Ensuring the accuracy of historical facts is paramount when employing the Granger Causality Test, to prevent erroneous conclusions. Long-term trends may not always align with individual historical events, and localized conflicts may diminish as populations decrease or crisis-alleviating measures are implemented. Large-scale wars or rebellions, such as the An Lushan Rebellion (755), the Mongol Western Expedition, the Ming–Qing transition (1644), and the Taiping Rebellion (1851–1864), involved multifaceted factors beyond climate and conflict. The Granger Causality Test faces a significant challenge in overlooking human agency in history, and accurate interpretation of climate data necessitates meticulous historical research.

Study of the historical relationships between climate and human societies involves interdisciplinary approaches. Climate science and history alone are no longer sufficient to meet the research needs of scholars, nor can they capture the complete picture. Researchers in historical climatology advocate for interdisciplinary participation, where geography, climatology, and archeology play crucial roles in the collection of climate data, and history, sociology, and economics provide interpretive frameworks. However, the establishment of accurate climatic data remains indispensable. Most importantly, any interpretation must not contradict historical facts.

It is understandable that early twentieth-century research, lacking sufficient scientific data, was unable to fully grasp the climate conditions of the thirteenth century. However, advancements in scientific and technological understanding of historical climate change and data collected in the Mongol region have allowed new viewpoints, as evidenced by the work of two groups of scholars.

A team led by Rosanne D’Arrigo studied 1738 years (AD 262–1999) of temperature variation based on tree-ring widths of Siberian pine at Solongotyn Davaa in northern Mongolia. Although D’Arrigo et al.'s (2001) work focused on climatology, her article suggests that the early-mid-thirteenth century was an extended warm period, which may have facilitated the travels of Genghis Khan and his followers. However, D’Arrigo et al. jump to conclusions without providing a detailed explanation of how the Warm Period facilitated the expansion of the Mongol Empire. Nevertheless, undoubtedly, their discovery challenges the assertions made by scholars over the past century and represents a significant shift in explanatory frameworks by integrating historical facts with climate data.

Meanwhile, a research group led by Wu Wenxiang from the Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, undertook the task of reconstructing climatic data for the Mongol Western Expedition period. Drawing from a diverse array of sources—including ice cores, tree rings, marine cores, and stalagmite records—they combined their findings with studies on ancient climates in other Eurasian regions. This comprehensive approach aimed to address the significant lack of climate data from China, particularly within the Mongolian region, which had led to substantial inaccuracies in earlier studies. Unlike previous scholars such as Huntington, who primarily relied on regional climate data from Europe and the United States, Wu Wenxiang’s team uncovered a more nuanced picture. They found that, during the same period as the global dry and cold phase, there was no consistent trend either of temperature rise or fall, or of drought or humidity, across various regions of China. Wu Wenxiang’s research showed that, contrary to longstanding beliefs, the time of the Mongol Western Expedition coincided with the middle-to-late stages of the Medieval Warm Period, during which the Mongolian region experienced relative humidity (Wu et al., 2009). Notably, the theory of a dry and cold period as a catalyst for Mongol expansion faces its greatest challenge in the fact that the Mongols chose to expand westward instead of southward, where regions would have been expected to be warmer and more humid.

The work of Wu Wenxiang’s team opens the door to further historical study, particularly concerning the lack of significant adversaries during the Mongol Western Expedition. The paramount factor lies in the Mongols’ core advantage—their formidable cavalry, heavily reliant on horses (Rossabi, 1994). Had Mongolia faced scarcity of water and grasslands due to drought or adverse environmental conditions in Central Asia, this would undoubtedly have hindered the effectiveness of the Mongol cavalry (Nicolle, 1990; May, 2017, 2023). Wu et al. further discuss how the humid and warm period facilitated the advance of the Mongol army and enhanced the effectiveness of the Mongol cavalry. It is clear that the study of Wu et al. offers more historical knowledge related to the climate to explore the factors in the Mongols’ success. Of course, the success of the Mongol westward expansion also relied on advanced military technology and gunpowder that they had learned about from the Song and Jin dynasties (Haw, 2013; May, 2015; Andrade, 2016).

Neil Pederson, one of D’Arrigo’s team members, has been at the forefront of tree-ring and climate research in the Mongolian region. In 2012, Pederson and his team’s project was first reported in Science (Hvistendahl, 2012). In 2014, an interdisciplinary team consisting of Pederson, Hessl, Baatarbileg, Anchukaitis, and Di Cosmo—experts in climate science, history, and related fields—published research findings based on ancient trees growing on barren rock in the Khangai Mountains of central Mongolia, which provided compelling evidence. Their growth patterns showed a period of sustained warmth and rainfall, a climatic anomaly not seen in over a millennium across Central Asia. This unique climatic phenomenon facilitated a remarkable chapter in history: Genghis Khan, the legendary leader, took advantage of these favorable conditions. With grasslands flourishing, the region experienced an unprecedented surge in grass production, nurturing vast numbers of war horses, and other livestock critical to the power of the Mongol Empire (Pederson et al., 2014). While Neil Pederson’s tree-ring data may vary from those of Wu Wenxiang’s team, the two studies converge on a singular conclusion: the critical role of climatic conditions in shaping the trajectory of Mongol power. Historian Di Cosmo (2023), a team member of Pederson, integrates primary sources and historical studies on the Mongol army, its organization, and customs, to explain how the Mongol Empire was assisted by the climate.

Aaron Putnam et al’s article provides additional support for Pederson’s conclusions, offering a distinct geological perspective. Putnam’s analysis of geological data indicates that the core of the inner Asian desert belt experienced wetter conditions compared with the preceding period, prompting a southward expansion of grasslands during the period of the Mongol rise. These conditions facilitated the expansion of steppe pastoralists from the Mongol Empire across the drylands of Asia. Grass served as a crucial energy source, driving the military conquests of the Mongol Empire powered by horse-based warfare (Putnam et al., 2016). Che Ping and Lan Jianghu offer a similar explanation in their article published in 2021. During the Mongol Empire, horses were the primary means of transportation for military conquests. Each cavalry soldier typically took three or more remounts, meaning the number of horses was at least three times that of the soldiers, which put enormous pressure on local forage. The expansion of the Mongol Empire coincided with increased wetness in Central Asia during the early Little Ice Age, while northern China experienced significant aridification. This climate shift likely facilitated the movement of the Mongol cavalry across Central Asia toward Eastern Europe, because the greening of mid-latitude Asian deserts eased their passage (Che and Lan, 2021).

Beginning from historical facts, it becomes evident that the initial objective of the Mongol Western Expedition was directed toward Khwarazm in Central Asia, with commercial disputes serving as its root cause. Genghis Khan had dispatched trading caravans along the Silk Road, and, upon their passage through Khwarazm, they were unlawfully detained. Despite Mongolia’s diplomatic efforts to resolve the situation peacefully, tensions escalated into war (Juvaini, 1997; Farrokh, 2012; McLynn, 2016). Therefore, Genghis Khan’s decision to engage in warfare stemmed not from deteriorating living conditions on the grasslands due to adverse climate effects, but rather from the ambition to bolster trade with various Central and Western Asian nations. This suggests a relatively prosperous economic environment at the time. The argument that adverse climate conditions damaged Mongolian agriculture, prompting outward expansion, lacks substantial evidence. Secondly, the war for the Mongol conquest of Khwarazm unfolded from 1219 to 1221, during which time the Mongols besieged the city for 2 years. Had Mongolia itself been affected by adverse weather conditions, sustaining a 2-year war and ultimately conquering Khwarazm would be implausible (Farrokh, 2012). Throughout this extended conflict, the Mongol army received a consistent supply of provisions, indicating a robust logistical network that would not have been feasible in adverse environmental circumstances. Lastly, it is crucial to recognize the Mongols’ inclination toward resolving issues through warfare (Fletcher, 1986). Even commercial disputes often escalated into armed conflicts, showing a predisposition toward military solutions within Mongol culture and governance.

Genghis Khan initially campaigned southward against the Jin Dynasty rather than embarking on a western expedition. After defeating the Jin Dynasty, his forces continued their conquests further south. By around 1214, the Mongol army had already initiated a comprehensive siege against the Jin Dynasty (Wright, 2022). Therefore, movement southward by the Mongol forces preceded the Western Expedition, which was later triggered by commercial trade disputes. Consequently, in terms of argumentation, the fact that the Mongols initially moved southward contradicts the notion of adverse climate conditions playing a role. Historically speaking, the cause of the Mongol invasion to the West had no relation to adverse climate conditions.

Historians have long been intrigued by why the Mongol army did not seek to control Hungary (Rogers, 1996). In a 2016 study by Ulf Büntgen and Nicola Di Cosmo, the Mongol invasion of Eastern Europe—particularly their sudden withdrawal from Hungary in AD 1242—was examined using both documentary sources and tree-ring chronologies. Their research identifies a period of warm and dry summers from 1238 to 1241, followed by a shift to cold and wet conditions in early 1242. This weather change likely led to reduced pastureland, weakening the Mongol army’s power and contributing to their decision to abandon Hungary and return to Russia (Büntgen and Di Cosmo, 2016). However, there is ongoing debate among scholars. For instance, Pinke and Pow do not fully agree with Büntgen and Di Cosmo’s conclusions (Pinke et al., 2017; Pow, 2019). Büntgen and Di Cosmo’s study suggests that adverse weather conditions played a role in the Mongols’ withdrawal and thus that climatic deterioration brought peace to Hungary because the Mongols retreated (Büntgen and Di Cosmo, 2017). This challenges the assumption made by the Granger Causality Test that climate deterioration necessarily leads to war or conflict.

The Mongol Western Expedition was not driven by climate deterioration but rather reflected strategic decisions, notably (contrary to common belief) in the decision to withdraw from Hungary (Sinor, 1999). However, it is essential to note that the Mongols’ demand for foreign commercial trade may have been driven by a period of prosperity brought about by a warm and humid climate. In instances where they could not establish commercial networks, warfare ensued.

Kublai Khan’s preparations for war against Japan in 1268 and the subsequent dispatch of a vanguard force to Korea in 1273 illustrate this pattern (Turnbull, 2010; Anderson, 2019). However, the Mongol forces encountered challenges in sustaining themselves on Korean soil because there were limited grazing grounds for their cavalry, highlighting the importance of favorable weather conditions for Mongol military operations. The latest research not only challenges conventional understanding but also underscores the need for a refined research methodology concerning the relationships between short-term weather, longer-term climate change and historical events. Long-term climatic trends alone cannot fully explain specific historical occurrences.

The study of historical climatology in China has become increasingly interdisciplinary. In particular, research on the relationships between the Mongol westward expansion and climate involves collaboration between historians and geographers, resulting in research findings that are more thoroughly validated.

David Zhang’s work primarily focuses on temperature changes, but this approach is clearly insufficient. The present paper references broader research by scholars, shifting the focus on the Mongol expansion to examine the relationship between moisture and vegetation productivity. While temperature often reflects long-term trends, moisture changes are linked to shorter cycles and more extreme events. Therefore, such research cannot be simply understood through the lens of a single factor like long-term trends.

Cold and dry climates have alternated with warm and humid climates throughout China’s history. Zhu Kezhen (1890–1974) was a prominent figure in meteorology, renowned for his pioneering work in tracing climate changes throughout China’s historical epochs. Although many scholars have challenged Zhu’s periodization, the framework he established has remained an important foundation for subsequent research. In his seminal 1973 article, Zhu identified four warm and four cold periods, laying a framework for understanding China’s climatic history (Zhu, 1973). At the same time, the rise and fall of Chinese dynasties also alternated. For example, during the Tang Dynasty, there were natural disasters and epidemics even during periods of prosperity (Fan, 2010). During the Qing Dynasty, the volcanic eruption of Mount Tambora in present-day Indonesia in 1815 triggered a global economic downturn (Cao et al., 2012). The most important factor in facing natural crises is social vulnerability—if the state is strong, it can address challenges in many ways (Pfister, 2010). However, if analyzing historical events from a statistical perspective, especially using the Granger Causality Test, focusing solely on trends without understanding how human societies respond to nature fails to provide a true understanding of the relationships between climate change and history. Moreover, climate change is only a broad trend; even during the Little Ice Age, there were warm periods (Neukom et al., 2019). China is vast, and there are climatic differences between regions, particularly in the south, which is more affected by monsoons and is located in a subtropical zone (Fan, 2015). As a result, colder climates have less impact on agriculture in these areas. Therefore, statistical research methods have significant limitations.

The four-phase economic cycle—prosperity, recession, depression, and recovery (Schumpeter, 1964)—is similar to the alternation between periods of cold and warm climates. When an empire encounters a cold period, it may experience a series of negative environmental impacts, such as crop failures, economic depression, population decline, external invasions, and migrations. However, as population decreases and migrations occur, the empire’s bureaucratic structures may reorganize, which can help generate positive effects when the climate eventually warms. Furthermore, based on Chinese history, environmental destruction by humans has continued regardless of the climate turning warm or cold (Rawski, 1975; Osbrone, 1998; Elvin, 2004; Marks, 2012).

In contrast, during a warm period, a society may show a series of indicators of prosperity, such as a strong national power leading to external conquests, overexploitation of natural resources, and a shift to cultivating high-value crops. However, as the population grows and becomes more concentrated, and as bureaucratic corruption increases, the society’s vulnerability significantly escalates. This heightened vulnerability makes it difficult to cope with the deteriorating natural environment when the climate eventually cools again. Therefore, causality is complex and cannot be fully understood through linear thinking.

This type of reasoning is also supported by historical evidence. Paul Kennedy pointed out that any empire tends to grow increasingly large, causing maintenance costs to rise continually, eventually leading to collapse as the nation’s resources are exhausted (Kennedy, 1987). This supports the argument that empires are inevitably destined to decline. McNeill, viewing China’s environment from a global perspective, found that China historically had a diverse range of ecosystems with highly complementary resources, which gave the Chinese Empire a great deal of resilience. China’s farmlands, dams, and canals required significant labor and capital; if the government failed to invest, the situation would deteriorate, but once reinvestment occurred, production could quickly recover (McNeill, 1998).

Using the Granger Causality Test as a research method is just an extension of environmental determinism and a form of linear thinking. It implies that human societies passively accept the control of the natural environment. For example, in Chinese history, during the Western Han dynasty (BC 202–AD 9) with the Xiongnu and the Song dynasty (960–1279) with the Liao and Jin states, trade was used as a means to quell conflicts, demonstrating that human agency played a significant role in shaping historical outcomes (Jagchid and Symons, 1989). In addition, intermarriage for pacification also played an important role in peacemaking during warm periods in Chinese history (Su et al., 2021).

The Mongol westward expansion had absolutely no relation to climate deterioration, and more importantly, it cannot be understood through linear thinking. Genghis Khan did not embark on external invasions due to crop failures; rather, his large-scale westward campaigns were driven by the desire to secure and ensure the smooth flow of trade routes between East and West.

Exploration of the expansion of the Mongol Empire and its relationship with climate change reveals a complex interplay of factors shaping historical events. The traditional narrative linking adverse climate conditions to Mongol aggression is challenged by recent multidisciplinary research, which emphasizes strategic decisions and economic motivations as primary drivers of expansion. While the Granger Causality Test offers insights into correlations between climate fluctuations and historical events, its application in historical climatology requires careful consideration of contextual factors and meticulous verification of historical facts. Criticisms regarding the oversimplification of causal relationships and the overlooking of human agency in historical events warrant further scrutiny and refinement of research methodologies. Moving forward, interdisciplinary collaboration and a nuanced approach to historical analysis are essential in advancing our understanding of the intricate relationships between climate change and human history. By integrating diverse perspectives and methodologies, researchers can better navigate the complexities of historical climatology and avoid misleading conclusions. Ultimately, the quest to unravel the mysteries of the past requires a holistic approach that considers the multifaceted interactions between climate, society, and human agency.