Ancient farmers experienced climate change at the local level through variations in the yields of their staple crops. However, archaeologists have had difficulty in determining where, when, and how changes in climate affected ancient farmers. We model how several key transitions in temperature affected the productivity of six grain crops across Eurasia. Cooling events between 3750 and 3000 cal. BP lead humans in parts of the Tibetan Plateau and in Central Asia to diversify their crops. A second event at 2000 cal. BP leads farmers in central China to also diversify their cropping systems and to develop systems that allowed transport of grains from southern to northern China. In other areas where crop returns fared even worse, humans reduced their risk by increasing investment in nomadic pastoralism and developing long-distance networks of trade. By translating changes in climatic variables into factors that mattered to ancient farmers, we situate the adaptive strategies they developed to deal with variance in crop returns in the context of environmental and climatic changes.

The routes are indicated by arrows in the wheat panel. (A) Pathway of the Inner Asian Mountain Corridor ( 27 ). (B) Southern Himalayan route ( 29 ). (C) Northern Asia route ( 2 ). The ages of sites represented by triangles are estimated from summed radiocarbon dates including those on grains; the ages of sites represented by circles are as reported in the literature. See data file S1 and table S2 for chronometric data and estimated site occupation spans.

We use a high spatial and temporal resolution model of the changing thermal crop niche for six grain crops across Central, South, and East Asia ( Fig. 1 ) combined with a large (1187-entry) database that charts both the appearance and timing of use of broomcorn, foxtail millet, wheat, barley, buckwheat, and rice (see Materials and methods for details about the model and database). We use this niche model to examine the impact that changing temperatures had on human’s ability to subsist on these crops and to reveal the strategies they used to cope with these potential challenges. This model covers a wider geographic scope and temporal depth and more crop varieties than previously published thermal niche models ( 8 ), allowing us to make several general observations about the nature of grain crop farming across early-mid Holocene Eurasia.

Between 5000 and 1500 years ago, economic systems across Eurasia experienced major shifts as farming spread into areas far outside of its original centers of domestication. Throughout this period, humans also dealt with notable episodes of climatic change that affected agricultural returns. During this period of time, broomcorn and foxtail millet made their way from China to Central Asia, and wheat and barley moved from Central Asia to the Far East. Who spread these crops across Asia—when, how, and by what routes—has been a topic of increasing debate ( 2 , 3 ), as have the social and economic motivations behind their adoption ( 4 – 8 ). There is an increasing interest in how changes in precipitation and temperature may (or may not) have affected agricultural production in past societies ( 9 – 12 ). Where understanding the impact of climate in East Asian history has gained popularity ( 9 , 13 – 15 ), these approaches often simply correlate paleoclimate proxies with archaeological data—and often fail to reveal the adaptive strategies used by humans when faced with variation in agricultural returns ( 16 , 17 ).

Projected estimates for global warming are expected to pose serious challenges for existing systems of grain production around the globe, with some regions having predicted decreases in production as high as 70% ( 1 ). Understanding how farmers coped with past changes in the mean state of climate may be crucial for understanding how we must adapt to a rapidly changing world. Humans are able to tolerate varying levels of failure in their agricultural systems. But exactly how many years of a bad harvest were humans able to cope with before they modified their farming strategies? What type of strategies did they use when returns got bad?

RESULTS

Barley, wheat, foxtail, broomcorn, and buckwheat millet (Fig. 2 and movies S1 to S4) remain completely within the thermal niche across Central Asia during the early years of the transmission of these crops across Eurasia. Following the domestication of wheat and barley in the Fertile Crescent beginning c. 10,000 cal. BP (calibrated years before the present), they move into pre-Indus sites, such as Mehrgarh, as early as 9000 cal. BP (18, 19). Wheat and barley move deeper into Central Asia between 5450 and 4700 cal. BP (20–22). From this point, these crops move eastward between 4000 and 3600 cal. BP (23) and are rapidly adopted across the eastern Himalayas by 4000 cal. BP (8).

Fig. 2 The spread of wheat, barley, buckwheat, foxtail millet, broomcorn, and rice across Asia at the end of the Holocene Climatic Optimum (4030 cal. BP), 3550 cal. BP, 1690 cal. BP, and 1010 cal. BP. Sites that have evidence of a crop at a particular year are shown as black dots. See movies S1 to S6 for the full animations.

Following domestication c. 8500 cal. BP (24), millet farmers began a westward expansion that resulted in the spread of millet products to the eastern margins of the Tibetan Plateau between 5400 and 4400 cal. BP (movies S3 and S4) (25). Broomcorn millet (and potentially foxtail millet) begins to appear at sites throughout Central Asia following 4000 cal. BP (7).

These crops may have been exchanged along several different routes. A number of sites line up with a route that is consistent with the Inner Asian Mountain Corridor (2, 21, 26–28). Finds of wheat and barley cluster along this route, as do finds of broomcorn millet. For wheat and barley, a large and almost simultaneous cluster of sites across northern India makes a route to the south of the Himalayas route equally plausible—one that has been suggested by previous researchers as playing an important role (movies S1 and S2) (29). This route, however, is truncated by a lack of archaeobotanical research in Assam, Bangladesh, and Myanmar. A third route for these crops’ transmission has been proposed—a route to the north of the Inner Asian Mountain Corridor. While this area remained within the thermal niche during the early years of these crops’ transmission, there is no archaeological evidence in our database to support crops moving along this route. It is worth noting that this route is also not supported by genetic evidence on barley landraces (26).

Between 6000 and 4000 cal. BP, the probability of being in the thermal niche ranges between 100 and 90% for sites with millet and barley, suggesting that temperature did not form an impediment to their spread. During the early years of cultivation of all these crops, farmers exploited them conservatively and cultivated these crops in areas that had a high or, in most cases, almost certain probability of success (Fig. 3). This high probability of success is mirrored in the storage data from central China during this period of time. Individuals stored roughly half a year of grain crops—a quantity that a family (that was also reliant on foraging) might have consumed over the course of a year, reserving some seeds for planting for the following years (see Materials and methods).

Fig. 3 Calibrated radiocarbon ranges for each site in our database (x axis) and the probability of each of these sites being in the niche during that same phase of occupation (y axis). An interactive version of this figure that labels each of the cross-plots and enables zooming is available as data file S2.

A decline in the thermal niche of both broomcorn and foxtail millet takes place around 3800 cal. BP, particularly in the higher-altitude areas of the Tibetan Plateau, Tianshan and Altai mountains, and Inner Mongolia (Fig. 3). How did millet farmers cope with this high probability of failure following 4000 cal. BP? In the Liao River Basin, the numbers of millet remains present in archaeological sites decrease following 3000 BP and major changes in settlement distribution are seen during the Upper Xiajiadian period (30). On the southeastern Tibetan Plateau (SETP), the impacts of this cooling episode appear to have been catastrophic: Many sites in the region appear to have been abandoned during this period of time. However, in equally affected parts of the northeastern Tibetan Plateau (NETP), at one site in Central Tibet and two sites in Central Asia (Tasbas and Tuzusai), finds of millets persist despite being at only a 60 to 70% probability of being in the thermal niche (Fig. 3). What explains the continued presence of millets on the NETP and at select sites in Central Asia?

Wheat and barley appear on the SETP several hundred years after millet-producing sites had already been abandoned—not giving farmers an opportunity to integrate these new grains into the diet (8). On the NETP, however, wheat and barley arrived before the 4000 cal. BP cool down. Following 4000 cal. BP, millets begin to form a much smaller percentage of total grains found at a given site and are largely replaced by wheat and barley. We argue that faced with cooling climatic conditions, farmers on the NETP used a risk reduction strategy based on grain crop diversification to include cold-tolerant wheat and barley. Given the high probability of millet’s failure and the lack of available warm days for temporal diversification throughout the year, we argue that it is likely that farmers used a spatial diversification strategy—planting wheat and barley on cooler slopes and millets in warmer basins. This strategy allowed farmers to continue to cultivate millets on a small-scale basis even as their probability of failure rose sometimes to 30% (Fig. 3).

Researchers in Central Asia have pointed out that at Tuzusai and Tasbas (two Central Asian sites situated at about 70% probability of being in the thermal niche for millet), inhabitants of the sites grew millets, wheat, and barley as an attempt at diversifying crops to reduce risk (31–33). Households in this part of Central Asia also appear to have invested more in storage than those in early Holocene central China: Here, they appear to have stored a year and a half of crops (see Materials and methods), closely mirroring the probability of success of wheat and barley.

Millets could also have arrived at low probability sites via exchange with individuals occupying lower and warmer elevations. Archaeologists have often assumed that seeds found on an archaeological site were grown in the immediate vicinity of the site. It has been difficult for archaeologists to determine whether plant remains found on archaeological sites were locally grown because of a lack of understanding where the limits of their cultivation lay or how these limits changed with changing climates. When crops are locally cultivated, they are often found in association with crop-processing debris. In many societies, crop processing takes place on a daily basis, which results in the distribution of crop-processing waste across the site (34). A lack of systematic archaeobotany in China and reporting on crop-processing remains makes it impossible to comment on this issue for most sites located within China. However, at Tuzusai in Kazakhstan (2360 to 2100 cal. BP), where such data have been reported, no crop-processing debris was found at the site for wheat and barley. Spengler et al. (31) interprets this lack of chaff as being due to crop processing taking place off site; however, it is also possible that this indicates that the inhabitants may have derived their crops through increasing networks of trade and exchange that had begun to develop in the area. Crop-processing remains of millet can only be documented through the microbotanical record, and more work on this front is necessary to determine whether these crops were locally grown or rather traded into the area.

The fact that these earlier sites lie within or close to the borders of being in the thermal niche makes both crop diversification and exchange viable strategies for explaining the presence of these grains on archaeological sites. Systematic application of strontium isotopes and microbotanical analysis should be used to confirm or refute this explanation.

An event at c. 2000 cal. BP is the point when the largest changes take place in the crop niche for all six crops. For wheat and barley, large parts of the Tibetan Plateau become very cold and fall out of the thermal niche, as do the Tian Shan and Altai mountains (Fig. 2). When temperatures reach their lowest point, at about 1690 cal. BP (260 CE), almost the entire area corresponding to Mongolia falls outside of the thermal niche. Wheat and barley also experience difficulties in northern China. By 1670 cal. BP, broomcorn and foxtail millet completely lose their niche not only on the Tibetan Plateau but also across much of northwestern China (Outer and Inner Mongolia, Qinghai, Ningxia, Gansu, Heilongjiang, Liaoning, and North Korea); even parts of central China fall within the area that corresponds to the lower confidence interval (Fig. 2). Likewise for rice, the entire area north of the Yangtze River falls below the 70% probability of being in the niche (Fig. 2).

Systematic archaeobotany has not been carried out at large numbers of sites dating to the historic period; however, textual records show that this period also corresponds to a substantial retreat of the Han dynasty out of Central Asia. Between 1660 and 1633 BP (290 to 317 CE), a massive southward migration began after the Jin dynasty became too weak to resist the constant incursions of “barbarians” (“Hu”) along their northern margins (35). By 1633 BP (317 CE), the dynasty abandoned Chang’an and established itself in Nanjing (Jiankang) in Jiangsu—an area, while at the border of our lower confidence interval, that still fell within the niche not only for millets but also for wheat and barley. Xianbei pastoralists capture the north entirely by 1564 BP (386 CE) and rename themselves the northern Wei dynasty (36). An estimated one in eight farmers from northern China moved to southern China during this period of time. While warfare undoubtedly contributed to this movement, the changes in crop returns may have played a major role in convincing farmers (and the Chinese Jin court) to leave (37). It is precisely between 1659 and 1590 BP (291 to 360 CE) that historical records report several decades of particularly catastrophic harvests (37). The more variable returns experienced in crops after this date may have encouraged the migration of China’s core agricultural zone away from the Yellow River valley and toward the south. The creation of the Grand Canal during the Sui dynasty (1369 to 1332 BP or 581 to 618 CE) served the purpose of transporting grain produced in southern China from Hangzhou in south central China to Luoyang, Chang’an, and to the northern border near Beijing—an ingenious technological development for providing reliable access to food in areas marginal for grain production.

Despite these crops already falling out of their climatic niches in China’s higher-altitude margins after 4000 cal. BP, farmers in the warmer low-lying north-central China continued to rely heavily on millets. Wheat and barley remained minor components of the diet of north-central China up until 200 CE (4). However, following 200 CE, wheat and barley move from being minor crops to food staples in northern China. Rotary querns are also introduced to northern China during this period of time, suggesting that similar ways of processing wheat and barley also began to make their way over from China’s peripheries (4, 38). We argue not only that this shift in dietary strategies in northern China took place as a result of a gradual change in culinary preference (4) but also that increasingly higher risk associated with growing millet led farmers in northern China to experiment with the multicropping strategies and pastoralism that were already used on their margins. This shift was likely facilitated by contact with groups more heavily invested in pastoralism such as the Xianbei, who moved into this area and who may have already been engaged in low-level cultivation of these crops on China’s margins.

Our model demonstrates that after 2000 cal. BP is the first time that grains begin to appear at a number of sites (such as Mebrak, Kyung-Lung Mesa, Huang Niangniangtai, and several grottoes along the Silk Road), where probabilities of a successful harvest are less than 50% (Fig. 3). It is unlikely that the inhabitants of these sites attempted to grow crops in these areas, making trade and exchange a plausible solution for explaining the appearance of grain at these sites. This possibility is not surprising, particularly for several key sites on the Tibetan Plateau, like Kyung-lung Mesa and Ding-dun, where we know that nomadic pastoralism formed an important component of the economy (25). This represents another form of spatial diversification (exchange in items like salt, fur, hide, meat, and milk for grain with individuals situated in areas of lower altitude) that grew in importance in high-altitude and high-latitude Eurasia throughout time. In the ethnographic present, Tibetan pastoralists engage in networks of trade that bring products produced hundreds of kilometers away into their settlements (39). Following their arrival in East Asia during the fourth millennium BP, the mobility and economic diversification afforded by the exploitation of pastoral animals aided humans in further cementing their resilience.

We know that following 2000 cal. BP, China engaged in substantial trade and exchange with groups engaged in pastoralism along its borderlands (36, 40, 41). For instance, a Wei court document dating to 1430 BP (520 CE) writes that one group received “one thousand bushels of newly cooked rice, eighty bushels of parched wheat, fifty bushels of roasted nuts… and two hundred thousand bushels of grain” (36).