Abstract

Newly available paleoclimate data and a re-evaluation of the historical and archaeological evidence regarding the Uyghur Empire (744–840)—one of several nomadic empires to emerge on the Inner Asian steppe—suggests that the assumption of a direct causal link between drought and the stability of nomadic societies is not always justified. The fact that a severe drought lasting nearly seven decades did not cause the Uyghur Empire to collapse, to wage war, or to disintegrate gives rise to speculations about which of its characteristics enabled it to withstand unfavorable climatic conditions and environmental change. More broadly, it raises questions about the complex suite of strategies and responses that may have been available to steppe societies in the face of environmental stress.

Severe, prolonged droughts have been identified as a contributing factor in the decline of complex agricultural polities and civilizations, such as those of the Khmer city of Ankhor, the ancestral Puebloans, and the ancient Maya. In the case of pastoral societies, however, droughts have been understood as drivers of migrations, invasions, conquests, internal warfare, and other forms of political instability, although such inferences typically garnered limited support because of the dearth of both high-resolution paleoclimate data proximal to the steppe, and significant historical records emanating from the societies themselves. Historical analysis is inherently at odds with simplistic cause–effect models based on decontextualized correlations, and theories based on “push” and “pull” forces to explain the movements and conquests of nomadic peoples are necessarily reductionist. A different and more productive approach consists of integrating several types of evidence within the confines of case studies that can embrace the complexity of a historical analysis. This approach provides new insights into the relationship between climatic and environmental variability, on the one hand, and political and social change in steppe empires, on the other. Recently developed paleoclimatic data present an excellent opportunity not just to increase our knowledge of a still relatively obscure period in Asian history but especially to explore new questions and new hypotheses concerning the relationship between nomads and environmental factors, some of which have been already examined in recent studies, albeit in different historical contexts.1

Since the late first millennium b.c.e. and until the early modern period, several nomadic empires emerged on the Inner Asian steppes, exerting control over surrounding regions and transforming the Eurasian economic and political order from China to Central Asia and even to Europe. The imperial form of nomadic polities—militarily powerful, politically centralized, territorially extended, and inclusive of multiple ethnic and social groups—was relatively rare but historically far more relevant than other political formations. Notwithstanding similarities in their productive basis, culture, government, and social structures, steppe empires differed considerably in their overall economic development, duration, expansion, and interaction with other polities. We cannot account for these differences by constructing a general model of nomadic empire. We have to take a closer look at the historical circumstances in which each of them arose and the particular challenges that each of them confronted. On the historical palette of these empires, the Uyghur Empire (or qaghanate) presents special characteristics that, coupled with high-resolution paleoclimatic data, provide an ideal case for exploring the relationship between severe climatic downturn and its possible role in the evolution of a nomadic polity.2

The Uyghurs were a Turkic-speaking people of Mongolia who replaced the Türks as the rulers of the Inner Asian steppes in 744, establishing their political center in the Orkhon valley of central Mongolia. Their empire, which preceded that of the Mongols (established in 1206), played a critical role in fostering communication and trade between China and Central Asia. Several of its features were either unique or original to the Inner Asian empires. However, the reasons for the different choices that the Uyghurs made, as well as the reasons for their empire’s collapse, are still unclear, and given the dearth of historical sources, historical analysis by itself is insufficient to grasp fully the dynamics that occurred during the arc of its existence.

Their empire comprised parts of Central Asia and south Siberia, the Altai and the Tianshan Mountains, and the whole of Mongolia (see Figure 1). They soon became a major power on the Silk Road in the contest for domination over the trade routes and production centers that connected the Eurasian regions between China and the Mediterranean. The Uyghurs were key to international commerce; the spread of religions across Eurasia; and political relations between China, Tibet, and Central Asian powers. Their rule came to a rapid end in 840. Previous explanations for their decline include the combined action of internal political struggles, weather-related economic crises, and foreign invasions. However, the weakening and fall of the empire occurred after a long stretch of peaceful and productive relations with China and other neighbors. Although the immediate causes of its final and fatal crisis are known, the deeper reasons that accelerated or assisted it remain unclear. Paleoclimatic data add a new dimension to the puzzle, raising questions that can generate new thinking, if not new solutions.3

Fig. 1

Locations of the Uyghur Empire and Paleo-Climate Sites

Fig. 1

Locations of the Uyghur Empire and Paleo-Climate Sites

The paleoclimatic evidence shows that the center of the empire was exposed to a drought the duration of which was exceeded only once in the last 1700 years. This drought is assumed to have depleted the environment and, by inference, to have reduced the productive capacity of the local economy. However, the written sources, originating either with the Chinese or the Uyghurs, have not registered any of the phenomena usually associated with these adverse conditions, such as a drastic increase in animal mortality, depopulation, migrations, or aggressive military pressure. Our conclusions about the features of the Uyghur economic and political system are not meant to present a new climatic theory about its rise and fall but to suggest how climate might have affected them and, more broadly, how the case of the Uyghur Empire might lend insights about climatic vulnerability in relation to other societies, both settled and nomadic.

Climate Variability and the Uyghur Empire

To date, only limited paleoclimatic records of sufficient resolution for historical analysis were available to explore the climatic context of Central Asian nomads prior to the Mongols. Hence, until now, no studies have considered the capacity of nomadic polities to develop strategies that could shield them from the consequences of climatic downturns and allow them to maintain stability under conditions of environmental stress. The drought reconstruction from Mongolia presented below shows that north-central Mongolia was affected by a drought of sixty-eight years (783–850) covering the second half of the Uyghur Empire.4

Tree-Ring Chronology

The 1,773-year (240–2012) dendroclimatic reconstruction of past moisture variability is based on Siberian pine (Pinus sibirica) increment cores and cross-sections from a Holocene lava at Uurgat (ulp) in north central Mongolia. Samples were collected from living and dead trees on thin or absent soils surrounded by dark basalt. Trees growing on the lava today are widely spaced, stunted, and appear primarily moisture-limited. We measured total ring width to +/− 0.001 mm and crossdated samples using standard procedures (Figure 2).

Fig. 2

Tree-Ring Reconstruction of Moisture (240–2012) in Orkhon Valley

Fig. 2

Tree-Ring Reconstruction of Moisture (240–2012) in Orkhon Valley

To maintain low-frequency information without including age-related departures in growth unrelated to climate, we used conservative techniques in the program arstan to detrend and standardize the raw ring-width series. We used a hierarchical approach, including a negative exponential curve or straight line with slope ≤0 (typically samples containing, or close to, pith), straight line with slope=0 (typically non-strip bark samples without early growth), or straight line with slope 0 (typically strip-bark samples). Two samples with unusual growth trends were standardized using the Friedman SuperSmoother (alpha=9). We combined the individual series of tree-ring indices into a single chronology using a bi-weight robust mean. The chronology extends to 416 b.c.e. (Figure 3) but was truncated to the period when the average expressed population signal (eps)—a metric that quantifies how well a chronology based on a finite number of trees represents a hypothetical perfect or “true” chronology—is 0.85 (240–2012). The average correlation between tree-ring series (rbar) during the period of analysis is 0.59, indicating that tree growth is responding to a common environmental signal.5

Fig. 3

Reconstructed Drought with Uyghur Period Noted in Grey (A) and Number of Tree-Ring Samples (B)

Fig. 3

Reconstructed Drought with Uyghur Period Noted in Grey (A) and Number of Tree-Ring Samples (B)

Mongolia’s climate stations are sparse and have missing data, particularly since 1980. To calibrate the tree-ring reconstruction of moisture, we instead chose to use the gridded self-calibrating Palmer Drought Severity Index (scpdsi) derived from cru 3.20 precipitation and potential evapotranspiration fields. scpdsi uses the physically based Penman-Monteith equation for potential evapotranspiration rather than the Thornthwaite function used in pdsi. The Penman-Monteith equation incorporates daily temperature, radiation, wind speed, and humidity; the Thornthwaite uses only temperature and day length. Further, scpdsi applies actual vegetation cover rather than a reference crop, as in the original pdsi, and it is further corrected for seasonal snowpack dynamics. It also has a similar range of variability in diverse climates allowing different regions to be compared.6

We reconstructed average June–September scpdsi (target) using a linear regression model that relates average June–September scpdsi from a grid box (46–49N, 99–109E) to the mean of our detrended ring-width chronologies throughout the period from 1959 to 2012. The definition of the grid box was based on spatial-field correlations with scpdsi over Mongolia (Fig. 4A). We then validated our model and estimated prediction error using a split-period cross-validation approach by partitioning our time series into two periods (1959–1986 and 1987–2012) (Figure 4B). To evaluate the model fit, we used calibration and validation r2, re (reduction of error), and ce (coefficient of efficiency). We developed bivariate plots of runs of years below scpdsi=0 to compare the duration and severity of droughts over time (Figure 5).

Fig. 4

Relationship between Uurgat Tree-Ring Chronology and Instrumental Climate Record

Fig. 4

Relationship between Uurgat Tree-Ring Chronology and Instrumental Climate Record

Fig. 5

Cumulative Severity (Reconstructed June–September scpdsi) and Duration of Drought Years (scpdsi0) from 240 c.e. to 2012 c.e. with Marginal Histograms

Fig. 5

Cumulative Severity (Reconstructed June–September scpdsi) and Duration of Drought Years (scpdsi0) from 240 c.e. to 2012 c.e. with Marginal Histograms

Derived from living and dead trees within ∼100 km and located in the same watershed as the Uyghur capital of Kara Balghasun, the Uurgat reconstruction has the highest fidelity to the instrumental record near the Orkhon Valley, the seat of several steppe empires, including the Uyghur. The last 1,773 years witnessed three periods of severe drought—the 400s, 800s, and 1000s—followed by more frequent pluvial (wet) events—the 1300s, 1400s, and late 1900s (Figure 2). The Uyghur period (744–840) evinces an abrupt shift in moisture, consisting of moderate to wet conditions during the mid- to late 700s, followed by the extreme and protracted droughts of the late 700s to early 800s (Figure 2).

Beginning in 743, the reconstruction documents moderate to wet conditions that persisted through 782; in only fifteen of forty years was scpdsi 0. These four decades of relative moisture were followed by several persistent droughts centered on one of the most severe of the last 1,773 years. Dry conditions began in 783 and persisted nearly continuously for sixty-eight years. The most severe event began in 805 and continued for thirteen years with a cumulative scpdsi of −18. In the entirety of the tree-ring record presented herein, this extreme event was exceeded in duration and severity only by a seventeen-year drought that started in 1005 (Figure 5). The drought of 805 was followed by a less severe, ten-year drought beginning in 827 with cumulative scpdsi=−8, and a third, less sustained five-year drought beginning in 840 with cumulative scpdsi=−5. In general, dry conditions began in 783 and continued nearly unabated through 850, lasting for more than half of the total lifespan of the Uyghur Empire—fifty-seven out of ninety-six years.

Comparison with Other Proxy Records

Low-resolution paleoclimatic records for arid Central Asia support a warm, dry Medieval Climate Anomaly (mca) between approximately 800 and 1200, though the timing is not consistent across sites. Sediment records from the Mu Us Desert in Northern China and northern Mongolian lakes indicate that widespread drought affected the region around 800, though due to the resolution of these sediment records, the timing and duration of this dry period is uncertain. Historical sources about Tongwan city, located along a major Uyghur trade route, describe desertification in the early 800s, consistent with the calendar-dated droughts described herein, but the geographical extent and severity of this drought cannot be determined from the historical sources alone.7

We compared our time series of moisture from Uurgat tree rings to two existing paleoclimatic proxies from arid Central Asia with sufficient detail to resolve the Uyghur period (Figure 6). A well-dated, high-resolution record of ∂D of C26 and C28n-alkanoic acids from high elevation Lake Karakuli in the eastern Pamir Mountains documents that one of the driest periods in the last 2,000 years occurred during the early 800s (Figure 6A). Like the tree-ring reconstruction presented in this article, the leaf-wax record suggests that the severity of the drought during the 800s is as severe as recent aridity. We also compared our tree-ring reconstruction with the previously published Khorgo lava-pine tree-ring reconstruction of moisture (Figure 6B) from ∼200 km northwest of Uurgat (Figure 1). The Khorgo reconstruction, updated herein with an additional sixty samples to improve replication in the first millennium, is correlated with Uurgat at the annual scale (r=0.48, 240–2012), displaying similar decadal trends during the Uyghur period—neutral to moist conditions during the 700s followed by intense and protracted drought during the early 800s. The cessation of drought at Khorgo is earlier (836), but the arid excursions are more severe than those observed at Uurgat.8

Fig. 6

Paleoclimate Comparison

Fig. 6

Paleoclimate Comparison

To explore the spatial extent of moisture dynamics during the Uyghur Empire, we used the Monsoon Asia Drought Atlas (mada), an updated 0.5° gridded tree-ring reconstruction of scpdsi (which includes Uurgat and Khorgo tree-ring records as well as additional new chronologies from the Tibetan plateau and southern Siberia). For the early phase of the Uyghur Empire, mada reconstructs moist to neutral conditions across northern Asia (Figure 7). In contrast, from 783 to 844, mada clearly documents widespread drought extending from western Tibet to northern China. The wetter conditions in higher-latitude regions near Lake Baikal and northern China could be indicative of warm conditions at high-latitude tree-ring sites where tree growth may have been limited by temperature. Regardless, mada suggests that the 800s event was a mega-drought across northern Asia, including most of the Uyghur Empire and the Silk Road.9

Fig. 7

Spatial Pattern of Moisture (scpdsi) and Drought across Asia during the Early Expansion Phase of the Uyghur Empire (744–782) and during the Contraction and Collapse (783–844)

Fig. 7

Spatial Pattern of Moisture (scpdsi) and Drought across Asia during the Early Expansion Phase of the Uyghur Empire (744–782) and during the Contraction and Collapse (783–844)

The climate records show that the Uyghur Empire began under neutral to favorable moisture conditions that continued for approximately four decades. Beginning in 783, the climate of central Mongolia and much of arid Central Asia abruptly shifted to a drought, ranging from moderate to severe, that engulfed the region for nearly six decades. What impact (if any) such dire climate conditions had on the Uygher Empire is, however, unclear. The Empire did not abruptly collapse; the capital did not move to a more favorable location; and the Empire did not increase military activity to prey on surrounding people. Under such incontrovertibly arid conditions, the Uyghur Empire continued to survive and even prosper, raising the question of whether special conditions, accidental or purposeful, might have mitigated the effect of the drought. Could the Uyghurs have developed the capacity to withstand persistent drought?

The Uyghur Empire in History: General Features

A proper understanding of the key political, social, and economic aspects of Uyghur history is crucial to an analysis of how climatic conditions may have contributed to the challenges that the Uyghur Empire faced. Certain specific features of the Empire, generally regarded as cultural elements typical of steppe nomads, require close attention to identify the points at which natural constraints may have exerted an effect on Uyghur political and economic life. The Uyghur qaghanate replaced the second Türk Empire (682–744) in 744 after a few years of political turbulence. A peculiar feature of the Uyghur Empire, distinguishing it from its predecessors, was its relationship with China, then under the rule of the Tang dynasty. Earlier steppe empires typically entertained relations with China that were often hostile, in a pattern that alternated conflict with a peaceful co-existence governed by unstable treaties. The Uyghur leadership, however, immediately established a symbiotic political relationship with the Tang dynasty, allowing its military power to be used against China’s enemies, both internal and external. As a reward, the Uyghur political and commercial elite received from China massive amounts of silk—the most profitable commodity and main currency on the Silk Road—in exchange for horses, both at border markets and through commercial missions ostensibly intended to pay tribute to the Chinese emperor.10

The official conversion of the Uyghurs to Manichaeism under Mouyu qaghan brought the Sogdians into the fold of the Uyghur government, but a backlash against the Sogdians in 779 caused their expulsion under Tun Bagha, who assassinated the qaghan and usurped the throne. At his death in 789, however, the Sodgians were re-instated and their influence increased. From 789 to 795 the Uyghur Empire went through a period of political instability, marked by wars and violent successions. These events rattled the leadership, but they did not cause the Empire to collapse. In the conflict between Tibetans and Uyghurs from 789 to 792 for the control of the Tianshan region—and the trade routes on the Silk Road—the Uyghurs suffered military drawbacks but managed to hold onto the eastern part of the Tianshan region. They also retained control over such important towns and areas as Beiting (Beš-Balïq), Gaochang (Qočo), Qarashahr, and the Turfan depression where a number of Uyghur dignitaries and functionaries—many of them Manichaean, and possibly Sogdians—lived during the reign of the qaghan Bao-yi (808–821). The Uyghurs continued to play an active role in this region, although their expansion came to a halt. The internal politics of the Uighur Empire stabilized in 795 with the establishment of a new dynasty under minister Qutlug. From 795 to 805, the lucrative horses-for-silk trade with China was interrupted, but the sources are silent about whether lower productivity, a military need for more horses, or other factors were to blame.11

The resumption of commercial relations with China in the early ninth century coincided with the onset of the most severe drought period. The horse trade with China was probably made more difficult after 800 by the aridization of the Mu Us desert in the Ordos region below the great bend of the Yellow River, which could have affected the transport of horses from Mongolia to the Tang capital Chang’an. According to contemporary observers, such as the poet Bai Juyi (772–846), the Uyghur horses did not leave a single blade of grass on the route of the Yinshan mountains (skirting the Mu Us desert); and 60 or 70 percent of them died or were injured while being transported to China. The number of horses carried to China increased over time, and the Tang treasury continued to pay with quality silk, which proved to be a heavy financial burden.12

Meanwhile, the economy of the Silk Road, in which the Uyghurs were prime participants, had undergone a process of monetization whereby silk served as high-end currency in large commercial transactions, and coins sufficed for smaller purchases. A good portion of the silk certainly went to the Uyghur elites and circulated as gifts or rewards throughout the empire. However, given the size of the trade, much of it probably flowed into the commercial channels that connected China, Tibet, and Mongolia with Central and Western Asia. The prominent merchant families of Sogdian extraction within the Uyghur elite who controlled the Central Asian trade surely determined the bulk of its distribution. Yet, we do not know how silk was traded either within the Uyghur Empire or on the international markets of the Silk Road. What did the Uyghurs receive in exchange for the silk that they sold as a commodity or employed as currency?13

A reasonable conjecture is that silk supported the import of foodstuffs, drinks, and other supplies needed to provision the Uyghur capital of Kara Balghasun, the court, and the army in the Orkhon Valley, traveling from farming centers into the commercial networks managed by Sogdian traders. This notion, based only on circumstantial evidence, does not rise to the level of proof, but it provides an economic mechanism through which the negative effects of an environmental downturn might have been mitigated. The first element in its support is that agricultural production likely developed in the early ninth century with the construction of fixed settlements and the first phase in the adoption of a predominantly sedentary lifestyle by at least a portion of the population under Uyghur rule. Reports by the Arab envoy Tamim al-Bahr of 821 include the observation of agricultural fields on the route to the capital. Some of the archaeological evidence appears to show walled enclosures and thus the possible existence of agriculture in the suburban area of the Uyghur capital. These sparse clues indicate that the Uyghurs became more reliant than previous nomadic empires on agriculture, while their pastoral economy continued to supply horses for both military and commercial purposes. Archaeological research to date is not decisive about whether internal agricultural production relied on irrigation or was rain-fed, but the arid conditions prevailing in the region point toward irrigation.14

Finally, agricultural production on a greater scale was available in the oases and commercial towns between the Gansu corridor and the Tarim basin, some of which would have become accessible to the Uyghurs approximately from 755 to 790 (Figure 1). The Chinese censuses of the first half of the eighth century indicated flourishing communities at Gaochang, Xizhou, Beiting, Dunhuang, and Turfan, where the population numbered in the thousands or even tens of thousands. The increasingly prominent position of Sogdian elites within the Empire, and the above-mentioned presence of Manichean Uyghur functionaries in these cities, may indicate a greater dependency of the state on commercial networks and the people who controlled them.

The idea that the Uyghur capital Kara Balghasun and its surrounding areas received large quantities of foodstuffs and other necessities from other areas finds support in a parallel historical case. According to thirteenth-century sources, Karakorum, the capital of the Mongol Empire—a short distance from the Uyghur capital in the Orkhon valley—was supplied daily with 500 wagons carrying food and drink in 1230. Notwithstanding differences in the size, and presumably the population, of the two cities, the fact that transportation by cart and pack animal remained the same from the Uyghur to the Mongol period raises the theoretical possibility that the Uyghurs enjoyed a supply system resembling that of the Mongols. Such an analogy cannot constitute evidence per se, but it illustrates what could well have happened in an organized nomadic state.15

After the accession of the Chinese emperor Muzong (820–824), Uyghur–Tang relations improved to the point that a diplomatic marriage was arranged between the Tang Princess Taihe and the Uyghur qaghan. The peace treaty between Tibetans, Uyghurs, and the Tang of 822/3 marked the end of several decades of hostilities in the oasis region along the northwestern frontier of China, which, because of its agricultural produce and its location at the bottleneck of trade routes, had been fiercely contested. But the 822 treaty also meant that the Uyghur lost direct control of, and possibly access to, much of the region (Figure 1).16

The largest volume of trade between Tang China and the Uyghurs occurred in the 820s; the last recorded transactions were the Uyghurs’ exchange of 5,750 horses in 829 and 10,000 horses in 830 with China for a total of 230,000 pieces of silk. Although we do not know where the horses brought to China were raised, archaeological evidence suggests that some areas in northern Mongolia underwent a conversion from agricultural to pastoral use—by all appearances a rare occurrence for the premodern period. Uyghur efforts to increase revenue and minimize conflict during the 820s appear to signal a deepening of economic difficulty, or a shortage of horses, which were no longer sufficient to satisfy both military needs and commercial exchange.17

The Empire’s decline in the 830s has been attributed to internal rivalries and the rebellion of subject peoples, particularly the Kirghiz located in southern Siberia. Another contributing cause for the collapse of the Uyghur Empire was a catastrophic weather event in the winter of 839/40, in which either low temperatures or heavy snowfall were responsible for a high mortality of animals. Reconstructed climate data cannot confirm these events because they typically capture only summer conditions. The historical records, however, testify to the Uyghurs suffering heavy losses of livestock, followed by epidemics, and widespread famine during that winter. The Kirghiz took advantage of the crisis to invade the Uyghur Empire with a cavalry numbering 100,000, destroying the capital and killing the qaghan. Uyghur refugees pressed against the Chinese frontier in the fall of 840 to escape famine and their enemies, some of them initiating a prolonged period of hostilities with China. The Uyghurs who subsequently migrated to the Tarim Basin region (today’s Xinjiang) eventually created new kingdoms at Qočo and Beš-Balïq. Significantly, the Kirghiz did not establish themselves in the Orkhon Valley, as the second Türk Empire and the Uyghurs had before them, but returned north. The economic devastation caused by prolonged droughts and catastrophic frosts may well have made the Orkhon Valley unsuitable for them to establish the political and economic center of a new empire.18

The events marking the end of the Uyghur Empire appear to have arrived suddenly, but they came on the heels of prolonged droughts that persisted for nearly six decades, from the early 780s through the end of the Empire. The question is whether environmental stress was a contributing factor to the cessation of the horse-for-silk trade with China, and whether persisting climatic adversity intensified the political instability of the 830s.

The Integration of Climate Data Into the Study of the Uyghur Empire

Contrary to deterministic assumptions of fixed cause–effect relationships between climatic change and historical events, the case of the Uyghur Empire suggests several ways in which Uyghur society might have been able to maintain stability in the face of climatic stress.19

The peculiar features of the Uyghur Empire obviously cannot be explained solely in terms of adaptation to adverse climate conditions and environmental depletion, but such adaptations could have been instrumental in shielding the Uyghurs from the drought’s direst and most catastrophic consequences. At the macroeconomic level, the Uyghur Empire appears to have been far more diversified and sophisticated than any purely nomadic economy, given its agricultural activities, commercial interests, and traditional pastoral production. Its economic diversification parallels the cultural, religious, and ethnic composition of the Uyghur elites. The Uyghur qaghan’s conversion to Manichaeism during a sojourn in the Chinese capital opened the way to a close relationship with the Sogdian merchant families who controlled trade along the Silk Road involving several important cities in today’s northwest China—Turfan, Kucha, and Dunhuang.

The Uyghur urban centers, settlements, and palace compounds—such as the capital Kara Balghasun, Bay Baliq, and Por-Bazhyn—were most likely established during the second half of the eighth century, coinciding with the expansion and consolidation of the Empire. These settlements had different sizes and functions: The capital, a fortified city (with suburban areas), served mainly as a political and religious center, though it also contained some productive activities. Other settlements were either administrative centers, military outposts, market towns, or production centers. This analysis is hampered by an insufficient archaeological knowledge about the Uyghur period and by a lack of even basic estimates regarding its population and commercial networks (hopefully, ongoing excavations and archeological surveys will fill many of the gaps in the near future).20

Trade occupied a position of primary importance in their economy. If the Uyghur elite opted to become the most important commercial partner of Tang China in the silk trade, the position of dominance acquired by their Sogdian partners in international trade must not be underestimated. Uyghur military power and horse production provided a perfect combination to extract silk from China through a double system of service and trade. This relationship was the basis for a sophisticated mechanism that linked, through silk, the Uyghur nomadic aristocracy and traditional steppe economy of Mongolia with the trade routes, markets, and agricultural areas of Central Asia in which the Sogdian mercantile elites operated. Such a mechanism would have assisted in mitigating the effects of the drought by relying on exchange networks fueled by the influx of silk.21

The Empire may have been weakened by a long-term decline in grassland productivity that depressed the availability of horses for both military and commercial purposes. After 829/30, while the drought continued unabated, no silk seems to have arrived from China. Although it is impossible to determine the extent to which this dearth in grass affected the Uyghur economy and disrupted the horse-driven supply system, we can hardly ignore the internal rebellions and political conflicts that developed from c. 832. These events, compounded with the catastrophic winter of 839/40, made the Uyghurs too weak and divided to resist the Kirgiz onslaught.22

This study argues that the early decades of the Uyghur Empire—when the Uyghurs enjoyed amicable relations with China, formed a symbiotic relationship with the Sogdian elites, and expanded its territorial holdings into the Silk Road—established the foundation for a system of government and economy that, intentionally or not, partially sheltered it from severe environmental downturns. Counterfactually, we might ask whether the Uyghurs had other options at their disposal to adapt, at least gradually, to a multi-decadal drought. mada suggests that the drought was not only prolonged but also widespread, leaving little choice but to concentrate production in more suitable localities. One adaptive strategy practiced by previous nomadic empires might have been to increase revenues through plunder or tribute exacted by military force. However, the military capabilities of the Uyghurs seem to have ebbed already around the end of eighth century, and both the diplomatic marriage with the Tang in 821 and the peace concluded in 822/3 with the Tibetans hint that military solutions were not an option. Conceivably, the Uyghurs might have tried to innovate by increasing agricultural production, which would have required irrigation, and by investing in trade and transportation networks, but no unambiguous evidence for either strategy exists.

The heart of the matter is whether the apparent capacity to withstand adverse climatic conditions was due to a set of cultural and/or political choices, unrelated to climate, or to a modification of their nomadic ways toward a greater reliance on agriculture and trade. In support of the first alternative, the Uyghur Empire’s initial military rise and expansion (c. 744 to 780) may have been aided by conditions of high moisture that would have made the grasslands especially productive. The alliance with China, the conversion to Manichaeism, and the growing incorporation of Sogdian elites in their empire that took place during this period may have aided the Uyghurs, serendipitously, to withstand the climatic crisis. In the long run, however, some areas of society most likely suffered adverse effects, which might explain the internal splits and the final defeat at the hands of the Kirgiz. These developments owe as much to the sedentary habits of the Uyghur elites and their ineffective leadership as to the erosion of the pastoral resources that constituted the main support of a nomadic army.

The evidence from tree-ring data points to environmental conditions that alter the historical narrative of the rise and fall of the Uyghur Empire. Although several studies have focused on how climatic extremes affected agricultural societies, the fate of complex nomadic societies did not necessarily follow the same path. Steppe empires, at least from the first Türk Empire (established in the sixth century c.e.), and possibly earlier, were based on expansive geographical networks, diversified economies, and a degree of co-dependency with merchant classes. Even though these elements varied greatly between empires, they probably reacted to climatic variability in ways radically different from the ways in which agricultural and urban polities did. Once we account for socio-economic complexity, the case of the Uyghur Empire belies the general assumption that nomadic peoples are more vulnerable to climatic stress. The severe and protracted drought did not trigger migration, pillaging, or conquest. Sudden shocks, however, could have had a devastating effect on an economy already weakened. The catastrophic event of the winter of 839/40, which appears to be a dzud (a weather condition of extreme cold and heavy snowfall that causes high animal mortality) was likely the coup de grâce to an already compromised pastoral production. Contemporary studies of the dzud emphasize that the degree of calamity of the winter disaster is closely related to the drought conditions that preceded it. Thus, the collapse of the Uyghur Empire, generally understood as a rapidly evolving political crisis compounded by a catastrophic weather event, is also connected to multidecadal climatic change, dependency mechanisms, and geopolitical constraints.23

This article further offers answers to a few lingering questions about the Uyghur Empire. First, silk, although used for domestic consumption, could also have been the currency that paid for the acquisition and transportation of food supplies within the Empire, thus alleviating possible food shortages. Presumably, a large city like Kara Balghasun had to rely on a supply system to supplement local production. Moreover, our findings offer at least a partial explanation for why the Khirgiz invaders did not settle in the Orkhon Valley. The moisture reconstruction presented herein identifies continued drought as a significant probable cause; the Orkhon Valley did not become an important political center until the Mongol capital of Karakorum was established there.24

Extensive trade relations, as well as such external sources of revenues as tribute, were necessary to support the complex political and military structures of the Uyghur and other steppe empires, even under neutral or favorable climatic conditions, but environmental constraints possibly reduced options for adjusting to new political realities and eroded existing capabilities. The destruction of the Uyghur Empire was accompanied by demographic, political, and cultural change, but it did not lead to the immediate disappearance of the Uyghurs, who moved to the oases of the Tarim Basin and established local kingdoms. All of these events and conditions make the case of the Uyghurs an important point of comparison with other instances of societal collapse in history.25

Notes

1 

Brendan M. Buckley et al., “Climate as a Contributing Factor in the Demise of Angkor, Cambodia,” Proceedings of the National Academy of Sciences, CVII (2010), 6748–6752; Cook et al., “Megadroughts in North America: Placing IPCC Projections of Hydroclimatic Change in a Long-Term Palaeoclimate Context,” Journal of Quaternary Science, XXV (2010), 48–61; Peter M. J. Douglas et al., “Drought, Agricultural Adaptation, and Sociopolitical Collapse in the Maya Lowlands,” Proceedings of the National Academy of Sciences, CXII (2015), 5607–5612; Gareth Jenkins, “A Note on Climatic Cycles and the Rise of Chinggis Khan,” Central Asiatic Journal, XVIII (1974), 217–226; Qiang Chen, “Climate Shocks, Dynastic Cycles and Nomadic Conquests: Evidence from Historical China,” Oxford Economic Papers, LXVII (2015),185–204; Cook, “Megadroughts, ENSO, and the Invasion of Late-Roman Europe by the Huns and Avars,” in William V. Harris (ed.), The Ancient Mediterranean Environment between Science and History (Leiden, 2013), 89–102; Pederson et al., “Pluvials, Droughts, the Mongol Empire, and Modern Mongolia,” Proceedings of the National Academy of Sciences, CXI (2014), 4375–4379; Ulf Büntgen and Di Cosmo, “Climatic and Environmental Aspects of the Mongol Withdrawal from Hungary in 1242 CE,” Scientific Reports, VI (2016), 25606.

2 

For a discussion of the taxonomy and typology of these empires, see J. Daniel Rogers, “The Contingencies of State Formation in Eastern Inner Asia,” Asian Perspectives, XLVI (2007), 249–274; idem, “Inner Asian States and Empires: Theories and Synthesis,” Journal of Archaeological Research, XX (2012), 205–256.

3 

Colin Mackerras, The Uighur Empire According to the T’ang Dynastic Histories: A Study in Sino-Uighur Relations 744–840 (New York, 1972; orig. pub. 1968); Zhenping Wang, Tang China in Multi-polar Asia: A History of Diplomacy and War (Honolulu, 2013); Christopher I. Beckwith, The Tibetan Empire in Central Asia (Princeton, 1987). For general surveys of the history of the Uyghur Empire, see Mackerras, “The Uighurs,” in Denis Sinor (ed.), The Cambridge History of Early Inner Asia (New York, 1990), 317–342; idem, “The Uighur Empire of Mongolia,” in Hans Robert Roemer (ed.), Philologiae et historiae Turcicae fundamenta. 1. History of the Turkic Peoples in the Pre-Islamic Period (Berlin, 2000), 187–204; Roman K. Kovalev, “Uyghur Khaganate,” The Encyclopedia of Empire, Wiley Online Library (2016), available at http://onlinelibrary.wiley.com/.

4 

Pederson, “Pluvials, Droughts, the Mongol Empire”; Aaron E. Putnam et al., “Little Ice Age Wetting of Interior Asian Deserts and the Rise of the Mongol Empire,” Quaternary Science Reviews, CXXXI (2016), 33–50; Vladimir S. Myglan et al., “A 2367-Year Tree-Ring Chronology for the Altai–Sayan Region (Mongun-Taiga Mountain Massif),” Archaeology, Ethnology and Anthropology of Eurasia, XL (2012), 76–83.

5 

Cook, “A Time Series Analysis Approach to Tree Ring Standardization,” unpub. Ph.D. diss. (Univ. of Arizona, 1985); idem and Kenneth Peters, “Calculating Unbiased Tree-Ring Indices for the Study of Climatic and Environmental Change,” The Holocene, VII (1997), 361–370; Thomas S. L. Wigley et al., “On the Average Value of Correlated Time Series, with Applications in Dendroclimatology and Hydrometeorology,” Journal of Climatology and Applied Meteorology, XXIII (1984), 201–213.

6 

Gerard van der Schrier et al., “A scPDSI-Based Global Dataset of Dry and Wet Spells for 1901–2009,” Journal of Geophysical Research: Atmospheres, CXVIII (2013), 4025–4048.

7 

Jianhui Chen et al., “Hydroclimatic Changes in China and Surroundings during the Medieval Climate Anomaly and Little Ice Age: Spatial Patterns and Possible Mechanisms,” Quaternary Science Reviews, CVII (2015), 98–111; Yinzhou Huang et al., “Historical Desertification of the Mu Us Desert, Northern China: A Multidisciplinary Study,” Geomorphology, CX (2009), 108–117; Sarah J. Fowell et al., “Mid to Late Holocene Climate Evolution of the Lake Telmen Basin, North Central Mongolia, Based on Palynological Data,” Quaternary Research, LIX (2003), 353–363; Avery L. Shinneman et al., “Late-Holocene Moisture Balance Inferred from Diatom and Lake Sediment Records in Western Mongolia,” The Holocene, XX (2010), 123–138.

8 

Xingqi Liu et al., “Glacier Fluctuations of Muztagh Ata and Temperature Changes during the Late Holocene in Westernmost Tibetan Plateau, Based on Glaciolacustrine Sediment Records,” Geophysical Research Letters, XLI (2014), 6265–6273; Bernhard Aichner et al., “High-Resolution Leaf Wax Carbon and Hydrogen Isotopic Record of the Late Holocene Paleoclimate in Arid Central Asia,” Climate of the Past, XI (2015), 619–633; Pederson et al., “Pluvials, Droughts, the Mongol Empire”; Hessl et al., “Khorgo Lava Pine Tree Ring Chronology” (2012), available at https://www.ncdc.noaa.gov/paleo/study/16773.

9 

Cook et al., “Asian Monsoon Failure and Megadrought during the Last Millennium,” Science, CCCXXVIII (2010), 486–489.

10 

Mackerras, Uighur Empire According to the T’ang Dynastic Histories, 14–50; Martin Slobodník, “The Early Policy of Emperor Tang Dezong (779–805) towards Inner Asia,” Asian and African Studies, VI (1997), 185; MacKerras, “Uygur–Tang Relations, 744–840,” Central Asian Survey, XIX (2000), 229–231; idem, “Sino-Uighur Diplomatic and Trade Contacts (744 to 840),” Central Asiatic Journal, XIII (1969), 215–240; Beckwith “The Impact of the Horse and Silk Trade on the Economies of T’ang China and the Uyghur Empire: On the Importance of International Commerce in the Early Middle Ages,” Journal of the Economic and Social History of the Orient, XXXIV (1991), 183–198; Sechin Jagchid, “The Uyghur Horses of the T’ang Dynasty,” in Walther Heissig and Klaus Sagaster (eds.), Gedanke und Wirkung, Festschrift zum 90: Geburtstag von Nikolaus Poppe (Wiesbaden, 1989), 175–188.

11 

The Sogdians were central Asians who dominated the trade routes from central Asia to China. For their presence on the Silk Road, see Valerie Hansen, “New Work on the Sogdians, the Most Important Traders on the Silk Road, AD 500–1000,” T’oung Pao, LXXXIX (2003), 149–161. Kovalev, “Uyghur Khaganate”; Mackerras, “Uighurs,” 318–319; Takao Moriyasu, “Qui des Ouigours ou des Tibétains ont gagnè en 789–792 à Beš-Balïq,” Journal Asiatique, CCLXIX (1981), 197, 198–199; Sergey G. Klyashtorny, “East Turkestan and the Kaghans of Ordubalïq: The Interpretation of the Fourteenth Line of the Terkh Inscription,” Acta Orientalia Academiae Scientiarum Hungaricae, XLIX (1988), 277–280; MacKerras, “Uighur-Tang Relations, 744–840,” 227.

12 

Huang et al., “Historical Desertification of the Mu Us Desert”; Sanping Chen, “Bai Juyi and Manna,” Central Asiatic Journal, LVIII (2015), 17–25.

13 

Eric Trombert, “The Demise of Silk on the Silk Road: Textiles as Money at Dunhuang from the Late Eighth Century to the Thirteenth Century,” Journal of the Royal Asiatic Society, XXIII (2013), 327–347; François Thierry, “La monétarisation de la société türke (VI e-IX e siècle): Influence chinoise, influence sogdienne,” in Étienne de La Vaissière and Trombert (eds.), Les Sogdiens en Chine (Paris, 2005), 397–417; Kovalev, “Uyghur Khaganate.”

14 

Vladimir Minorsky, “Tamim ibn Bahr’s Journey to the Uyghurs,” Bulletin of the School of Oriental and African Studies, XII (1948), 275–305; Jan Bemmann et al., “Biomarkers in Archaeology–Land Use around the Uyghur Capital Karabalgasun, Orkhon Valley, Mongolia,” Praehistorische Zeitschrift, LXXXIX (2014), 337–370; Christoph Grützner et al., “Improving Archaeological Site Analysis: A Rampart in the Middle Orkhon Valley Investigated with Combined Geoscience Techniques,” Journal of Geophysics and Engineering, IX (2012), S70–S80.

15 

Rashîd al-Dîn Tabîb (trans. John A. Boyle), The Successors of Genghis Khan (New York, 1971), 62–63.

16 

Yihong Pan, “Marriage Alliances and Chinese Princesses in International Politics from Han through T’ang,” Asia Major, X (1997), 121; Michael R. Drompp, Tang China and The Collapse of the Uyghur Empire: A Documentary History (Leiden, 2005), 120–122; Pan, “The Sino-Tibetan Treaties in the Tang Dynasty,” T’oung Pao, LXXVIII (1992), 116–161; Janos Szerb, “A Note on the Tibetan-Uighur Treaty of 822–823 A.D,” in Ernst Steinkellner and Helmut Tauscher (eds.), Contributions on Tibetan Language, History and Culture (Vienna, 1983), 375–387.

17 

Mackerras, “Sino-Uighur Diplomatic and Trade Contacts,” 239; Beckwith, “Impact of the Horse and Silk Trade,” 188; William Honeychurch and Chunag Amartuvshin, “Hinterlands, Urban Centers, and Mobile Settings: The New Old World Archaeology from the Eurasian Steppe,” Asian Perspectives, XLVI (2007), 48.

18 

Drompp, “The Uighur-Chinese Conflict of 840–848,” in Di Cosmo (ed.), Warfare in Inner Asian History (500–1800) (Leiden, 2002), 73–103; Rosanne D’Arrigo et al., “1738 years of Mongolian Temperature Variability Inferred from a Tree-Ring Width Chronology of Siberian Pine,” Geophysical Research Letters, XXVIII (2001), 543–546; Nicole K. Davi et al., “A Long-Term Context (931–2005 CE) for Rapid Warming over Central Asia,” Quaternary Science Reviews, CXXI (2015), 89–97; Moriyasu, “The Sha-chou Uighurs and the West Uighur Kingdom,” Acta Asiatica: Bulletin of the Institute of Eastern Culture, LXXVIII (2000), 28–48; Peter Zieme, “The West Uigur Kingdom: Views from Inside,” Horizons, V (2014), 1–29; James Hamilton, Les Ouïgours à l’époque des Cinq Dynasties d’après les documents chinois (Paris, 1955); Drompp, “Breaking the Orkhon Tradition: Kirghiz Adherence to the Yenisei Region after AD 840,” Journal of the American Oriental Society, CXIX (1999), 390–403.

19 

For studies that postulate a direct correlation between climatic events and nomadic history, see Ying Bai and James Kai-sing Kung, “Climate Shocks and Sino-Nomadic Conflict,” Review of Economics and Statistics, XCIII (2011), 970–981; Fang Jin-qi and Guo Liu, “Relationship between Climatic Change and the Nomadic Southward Migrations in Eastern Asia during Historical Times,” Climatic Change, XXII (1992), 151–168; Yun Su et al., “The Relationship between Climate Change and Wars Waged between Nomadic and Farming Groups from the Western Han Dynasty to the Tang Dynasty Period,” Climate of the Past, XII (2016), 137–150; Qing Pei and David Zhang, “Long-Term Relationship between Climate Change and Nomadic Migration in Historical China,” Ecology and Society, XIX (2014), 68.

20 

Larry Clark, “Manichaeism among the Uygurs: The Uygur Khan of the Bokug Clan,” in Jason D. BeDhun (ed.), New Light on Manichaeism (Leiden, 2009), 61–72; Moriyasu, “New Developments in the History of East Uighur Manichaeism,” Open Theology, I (2015), 316–333; Samuel N. C. L. Lieu, Manichaeism in Central Asia and China (Leiden, 1998). For the Sogdian merchants, see Valerie Hansen, “The Impact of the Silk Trade on a Local Community: The Turfan Oasis 500–800”; Jonathan Skaff, “Documenting Sogdian Society at Turfan in the Seventh and Eighth Centuries: Tang Dynasty Census Records as a Window on Cultural Distinction and Change”; Binglin Zheng, “Non-Han Ethnic Groups and Their Settlements in Dunhuang during the Late Tang and Five Dynasties”; Etsuko Kageyama, “Sogdians in Kucha: A Study from Archaeological and Iconographical Material,” in de la Vaissière and Trombert (eds.), Les Sogdiens en Chine, 283–310, 311–342, 343–362, 363–375, respectively. Bemmann, “Biomarkers in Archaeology”; Peter Golden, “Courts and Court Culture in the Proto-urban and Urban Developments among the Pre-Chinggisid Turkic Peoples,” in David Durand-Guédy (ed.), Turko-Mongol Rulers, Cities and City Life (Leiden, 2013), 21–73; Rogers, et al. “Urban Centres and the Emergence of Empires in Eastern Inner Asia,” Antiquity, LXXIX (2005), 801–818; Daniel C. Waugh, “Nomads and Settlement: New Perspectives in the Archaeology of Mongolia,” The Silk Road, VIII (2010), 97–124; Irina Arzhantseva et al., “Por-Bajin, An Enigmatic Site of the Uighurs in Southern Siberia,” The European Archaeologist, XXXV (2011), 6–11; Sergey G. Klyashtornyi, “Qasar-Qurug: Western Headquarters of the Uighur Khagans and the Problem of Por-Bazhyn Identification,” Archaeology, Ethnology and Anthropology of Eurasia, XL (2012), 94–98.

21 

De La Vaissière, Sogdian Traders: A History (Leiden, 2005).

22 

Drompp, Tang China and the Collapse of the Uyghur Empire, 33–38.

23 

Shurentuya Begzsuren et al., “Livestock Responses to Droughts and Severe Winter Weather in the Gobi Three Beauty National Park, Mongolia,” Journal of Arid Environments, LIX (2004), 785–796; Mukund Palat Rao et al., “Dzuds, Droughts, and Livestock Mortality in Mongolia,” Environmental Research Letters, X (2015), 074012; Karl W. Butzer, “Collapse, Environment, and Society,” Proceedings of the National Academy of Sciences, CIX (2012), 3632–3639.

24 

Drompp, “Breaking the Orkhon Tradition,” 400–403; di Cosmo, “Why Qara Qorum? Climate and Geography in the Early Mongol Empire,” Archivum Eurasiae Medii Aevi, XXI (2014/15), 67–78.

25 

For comparisons, see Georgina H. Endfield, “The Resilience and Adaptive Capacity of Social-Environmental Systems in Colonial Mexico,” Proceedings of the National Academy of Sciences, CIX (2012), 3676–3681; Crawford S. Holling, “Resilience and Stability of Ecological Systems,” Annual Review of Ecology and Systematics, IV (1973), 1–23; Donna M. Glowacki, Living and Leaving: A Social History of Regional Depopulation in Thirteenth-Century Mesa Verde (Tucson, 2015).