1.1. Location and environment of Palmyra

Palmyra’s location, at an oasis within a vast steppe desert, determined its subsistence base. It lies at 150–200 km distance from perennial rivers and major regions of rain-fed agriculture. The city is located immediately south of a relatively low, yet steep mountain chain running from the outskirts of Damascus northeast towards the Euphrates, and opens to the Syrian Desert from all the remaining sides. It is located in a semi-arid environment, a dry steppe (bādiya) (Fig 1), between isohyets 150 mm and 100 mm with an average rainfall of around 125 mm per year. This is below the standard water requirements for cereals such as barley (200 mm) and wheat (250 mm), and significantly below the traditional limit between agricultural settlement and desert pasture in the arid regions of the Near East. Through most of pre-modern history this limit was closer to 400 mm, a level that ensures sufficient rainfall even in most of the regularly occurring dry years [25]. Thus, while the oasis has been a source of agricultural produce since the Neolithic [26], the area beyond it could not sustain rain-fed agriculture in average years, and was predominantly used to herd sheep, goats, and camels. However, even in arid landscapes like the Syrian desert it is feasible to farm crops if there are adequate water management strategies in place, such as cisterns or dams collecting rainwater in wadis and in larger basins. Such infrastructure is well attested in the hinterland of Palmyra [27].

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Fig 1. Palmyra’s surroundings.

Clockwise starting from the left: Position of Palmyra (Eivind Heldaas Seland), gravel-plains southeast of Palmyra (reproduced with permission from Jørgen Christian Meyer, original copyright 2006), limestone mountains north of Palmyra (Eivind Heldaas Seland).

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1.2. Food types and city’s subsistence base

Cereals have been the basis of most ancient diets, and barley has been the staple crop in the region due to its lower water requirements. Palmyrene funerary art includes banqueting scenes as do the numerous locally produced clay-tokens known as tesserae, which depict food and drink available in Palmyra, at least at certain points of the year, such as dates, pomegranates, grapes, bread/cakes, wine etc [28–33]. The oasis likely provided various fruits and vegetables, such as olives and dates, but the amounts would have been comparatively small [27]. The Tax Tariff of Palmyra [34, 35] dating to the mid-second century CE provides a list of goods brought into the city from the surrounding hinterland, which includes commodities such as wheat, wine, olive oil, animal fat, sheep, horses, mules, camels, and fodder. Produce from within the Palmyrene’s borders was exempt from taxes, indicating that the city provided incentives for agricultural activities in the area and that such production took place.

1.3. The base of the city’s economy

Roman Palmyra is known as one of the examples par excellence of a “caravan city” with trade considered its raison d’être. Palmyra’s geographical location at the key respite point at the major trade route but also at the frontier between the Roman and Arsacid (Parthian) later Sasanian empires enabled engagement in the lucrative long-distance trade between the Mediterranean and the Indian Ocean both including caravan and overseas trade. Palmyrene merchants operated caravans across the desert between Syria and Mesopotamia by the early first century CE, and from the second century onwards also maritime trade in the Persian Gulf and to the mouth of the Indus. By the third century Palmyrene merchants and ship-owners were also established in Egypt and the Red Sea, venturing into the Gulf of Aden and the western Indian Ocean [36, 37]. Palmyrene merchants settled across the eastern Roman border and held important positions in local communities there [38–40]. Although there is limited evidence regarding the type of goods being traded, it is generally accepted that these were likely to be low-weight, high-value goods, such as silk, aromatics, spices or precious stones, rather than bulky foodstuffs [37, 41, 42]. Palmyra had a textile production sector of some importance [43] and other crafts must have been present to serve the local needs. While under Roman control, Palmyra retained considerable political and fiscal autonomy [44, 45].

1.4. Historical outline

The Palmyrene polity seems to have emerged from a tribal framework of the nomadic population in the surrounding desert, facilitating the operation of caravan trade and reducing the need for investment in military presence and infrastructure in the hinterland and along this section of the Roman border [46–50]. The start of the rapid urbanisation of Palmyrene can be traced in archaeological, epigraphic and historical records to the first century BCE [10, 11, 15]. The city grew in both prominence and size during the first and the second century CE and a new city centre was laid out on a monumental scale north of the earliest urban nucleus [10]. Palmyra was integrated into the Roman Empire at an unknown date before the early first century CE [51, 52]. It received the honorary name of Hadriane in connection with emperor Hadrian’s visit to the city 129 CE and officially received the status of a Roman colony at the beginning of the third century during the reign of Septimius Severus (193–211) or Caracalla (211–217 CE) [53, 54]. Throughout this period the population of the city gradually grew. The new district north of the wadi, which comprises the bulk of the presently visible ruins of Palmyra received most trappings of a prosperous Roman city, including an agora (forum) a theatre, several temples a, nymphaeum (fountain), two long colonnaded streets, a bath and perhaps even an amphitheatre [10].

A sudden change came around the mid-third century when Septimius Odaenathus became the ruler, followed by his wife Zenobia. They dramatically expanded Palmyra’s realm so that, at its peak, it stretched from Egypt to Anatolia [55, 56]. After the defeat at the Battle of Edessa, 260 CE, when emperor Valerian was taken captive by the Sassanids, Odaenathus and the Palmyrene army drove the invaders back and sacked the Persian capital of Ctesiphon. This victory and subsequent military campaigns gave Palmyra military control over the eastern part of the Roman world. In 271/272 CE Zenobia even claimed the imperial purple on behalf of her son Vaballathus. Palmyra’s dramatic ascension to hegemony, however, did not last long. A Roman punitive intervention put an end to the Palmyrene empire in 272 CE. After a brief rebellion in 273 the city was sacked. It survived, but reduced to a minor provincial town [57].

During the third quarter of the third century, Palmyra was arguably one of the most important cities of the eastern Roman Empire. The city attracted artists, philosophers, along traders, artisans, soldiers, builders and slaves charged with ensuring the smooth functioning of this urban centre. The large population of the city thrived in the inhospitable environment of the Syrian Desert. It is clear that this city must have had a robust food production and distribution system in its hinterland that ensured its population food security levels necessary to survive grow and thrive. By combining data and methods from several disciplines, we can now reconstruct the subsistence basis of the city, assess the robustness of their food security and track how climatic changes over the first three centuries CE influenced the city’s resilience.

2.1. Spatial range of Palmyra’s hinterland

The details for this and the next two steps are collected in S2 File. The catchment area of Palmyra is represented as three concentric distance envelopes, which encompass the area of one, three and five days travel from the city (Fig 2). This site-catchment spatial analysis followed standard methods [59, 60], adjusted to capture the specificity of camel movement across the Palmyrene arid landscape. The two main factors used in the cost calculations were topography and water availability.

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Fig 2. Map of the catchments around Palmyra.

It shows areas within one day (green), one to three days (yellow) and five days (red) camel travel from the city. The numbers indicate each soil unit from the HWSD [61]. The striped areas are the maximum extent of fields in each of the main agricultural areas. Landsat 8–9 aerial imagery courtesy of the USGS, SPOT CIB-10 imagery was used for the analysis [62]. Water sources from Seland [63]. Palmyra territory’s boundary from Matthews [34].

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The slopes were generated from a reprojection of the USGS EROS SRTM 1 arc-second global elevation raster [64–67]. We squared the value of slopes, therefore exaggerating those with the highest values to counteract the well-known issue of the systematic underestimate of the impassibility of steep slopes [68], which is particularly pronounced in the low but steep hills north of Palmyra (Fig 1). We also used the 1/500000 provisional "Goings" map of Syria [69], which displays the difficulty of travel to amend the baseline topographic map with areas deemed impassable. The resulting slope raster was then used as an input for a version of Tobler’s Hiking function [70] modified to work with slope expressed in degrees [71].

Lack of reliable water sources severely impedes the ability of caravans to travel through arid areas such as the Syrian Desert. Thus, cells’ friction was scaled depending on the presence of a water source and the reliability of the water source (spring, well, cistern, etc) [data based on Soviet military maps, collected by E. H. Seland, 63]. Assuming six hours of march per day the calculated distances average to about 30 km on even terrain which is consistent with values known for caravans with pack animals, such as camels [72]. The three-day travel envelope also matches Palmyra’s frontiers, as documented by inscribed stone markers [34, 52] (Fig 2).

2.2. Baseline carrying capacity of Palmyra’s hinterland

The details for this step are collected in S3 File. The carrying capacity of the hinterland is based on barley cultivation, which has been the staple crop in the area since the Neolithic thanks to its low water requirements (200 mm). Nevertheless, rain-fed agriculture is not reliable in the Syrian Steppe so traditionally barley is grown on irrigated fields [73]. Other cereal crops or horticulture are feasible in the area, but they require more water. While not arguing that the population of Palmyra sustained itself exclusively on barley [74], it is the most cost-effective cereal in terms of water use and therefore the best proxy for the maximum amount of calories that can be extracted from the available land and water resources.

Crop yields were calculated using the AquaCrop software—a widely used software tool developed by the United Nations Food and Agriculture Organisation for calculating agricultural yields [75], particularly suited for annual cereals in areas where water is the limiting factor [76–78]. The required inputs include soil type, field management strategy, and climate data, such as monthly precipitation, maximum and minimum temperature, evapotranspiration and CO₂. The soil data were obtained from the FAO/UN Harmonized World Soil Database v. 1.2 [61, 79], which contains the required data on soil compositions and depth. Since there are several soil types within each soil unit, the weighted average of productivity was used as the productivity of the entire soil unit.

Climate data, i.e., precipitation and temperature, was taken from WorldClim 2.1 [80], evapotranspiration data from Brunel et al. [81] and for atmospheric CO₂ the global average was used. We assumed simple field management strategies, consistent with local agricultural techniques [73], such as weed management, and water management described below for each area. We tested two irrigation levels, 200 or 400 mm, to see if irrigating a smaller area but with more water would be more productive but the total yield was on average three times higher for 200 mm (S3 File, step 3.6). The 1-year fallow cycle was part of traditional cereal agriculture in Syria in the twentieth century [73] and we assume that the same practice was followed in Roman times. The weighted average productivity was multiplied by the area of the corresponding soil unit within each of the three distance envelopes and for each of the four identified agricultural areas (S3 File). This pipeline produced a total output in tonnes of barley under ideal conditions, i.e., consistent amount of rain, no losses to pest infestations, no losses in storage and distribution, and perfectly efficient water distribution.

2.3. Agricultural areas in Palmyra’s hinterland

As mentioned, large-scale rain-fed agriculture is not feasible in the hinterland of Palmyra [82]. While opportunistic small-scale rain-fed barley farming was sometimes possible in some wadi beds as a supplementary source of grain, it would not constitute a food production strategy efficient enough to reliably supply a large urban centre. To achieve that, dedicated agricultural areas with water management infrastructure were necessary. Meyer [20] distinguished four main agricultural areas that fall within the three-day travel envelope: the Bir Arak oasis, the At-Tarfa depression, the fields north of Harbaqah and the Oasis of Palmyra (Fig 1). Each of them has archaeological evidence of occupation and agricultural activity during Roman times:

• In Bir Arak, the Qanats and remnants of buildings, as well as an inscription in an altar that mentions “Yaribol (a Palmyrene deity), the irrigator of the earth” [16].
• In At-Tarfa, three Palmyrene altars thank deities for the rain and include agricultural imagery [83].
• A Latin inscription and an analysis of construction phases strongly suggest a Roman origin for the Harbaqah Dam [20, 84–86, Contra 87]. Roman architectural fragments found close to the Umayyad Garden in Qasr al-Heir al-Gharbi, north of the Dam, suggest an earlier Roman occupation [88]. We assume that just like in Umayyad times, the water of the Dam was used for irrigation there.

The mountain range north of Palmyra has been traditionally considered an important source of food for Palmyra [20, 27, 89], and surveys have produced evidence of agricultural estates and small irrigation systems. However, access from Palmyra was difficult due to the topography as indicated by the region falling outside the catchment envelopes (Fig 2). In addition, the evidence suggests only small-scale irrigation and farming there. Large-scale irrigation and agricultural production would have also been difficult in higher areas, due to the difficulty of capturing runoff water and establishing irrigated fields on the steep slopes at any larger scale.

In all of the four areas, traces of contemporary or abandoned fields are clearly visible from aerial imagery. While the extent of ancient fields around Palmyra is unknown, it is safe to assume that the fields opened in the 1960s as part of Bedouin settlement and agricultural development efforts [90] occupied the most favourable lands for farming and represent the maximum practical extent of non-opportunistic cereal agriculture.

Across the region, rainfall needs to be complemented with water management techniques to reach the level required for barley to grow [20]. Evidence for water management in the area indicates that runoff could have been redirected with temporary earthen embankments [91], small walls and dams such as those identified in wadis north of Palmyra [89] and rock-cut channels, or collected in cisterns. Additionally, water could have also been acquired from wells thanks to water-lifting devices, such as animal-powered Saqiyya or similar devices, which were fairly common across the Roman world, especially towards the Late Roman period [85, 92, 93]. For Bir Arak and the At-Tarfa depressions, we used their catchment areas to estimate the rainfall available during the growing period. The effectiveness of the water management systems was set to between 5 and 15% for At-Tarfa, and 30% for Bir Arak where a more stable source of water—Qanats are still visible in aerial imagery. The capacity of the Harbaqah Dam of 5,000,000 m3 [20, 27, 94–97] was used for the fields of Habarqah. Finally, the Oasis of Palmyra does not have an estimate of the water that could be available for agriculture before the introduction of water pumps, so its maximum extent irrigated at 200 mm was used as a baseline.

With this input, we simulated the productivity of all four areas. The weighted average productivity of each farming area, calculated in AquaCrop, was multiplied by the irrigated area to give an output in tons of barley. All of the above estimates need to be considered as a “reasonable best-case scenario” and are likely to be overestimating water and land availability for agriculture, thus they represent the maximum feasible carrying capacity of Palmyra’s hinterland.

2.4. Population numbers

The translation of barley yields into the number of people that could be sustained is based on the average consumption figures of 360–510 kg of barley/person/year [20]. These values come from Cato the Elder’s De agri cultura, where he details cereal requirements for a slave tasked with hard manual labour (Agr. 56). The figures match estimates of yearly consumption for Seleucid peasants made by Aperghis [98]. According to his numbers, 320–370 kg of hulled barley/person/year would be needed for a 1900 kcal a day 100% barley diet rising to 515–590 kg of barley/person/year for a 3000 kcal diet appropriate for manual labourers. 1900 kcal a day is a low caloric intake, and most likely mean intakes were higher. Scheidel and Friesen criticized estimates of 250 kg of wheat equivalent as insufficient for human survival, instead accepting higher estimates around 315 kg [99]. This means that the lower end of our carrying capacity estimates is likely closer to the truth than the upper estimates.

The population size numbers necessarily include the rural population that would have been involved in producing the food destined for the city but which does not necessarily constitute Palmyra’s population. Workforce requirements would have been highest during harvest [73] but the local population might have been aided then by the nomads [100, 101] as the winter crops harvest coincides with the return of pastoralists from the desert to the better watered areas close to agricultural land [73, 102]. There is also evidence in other arid areas, like the Negev, of nomad-settled economic complementarity during the Roman period [103, 104]. While city inhabitants could have also helped the effort, given the distance between the city and the agricultural areas it was unlikely to be a significant addition.

The average labour requirements of harvesting, threshing and winnowing cereal (S6 File) were calculated following the methods by Laabs and Knitter [105] and complemented with the data from the Cologne Tableau [106, 107] a collection of econometric data for traditional economic activities collated from written, ethnographic and experimental sources.

2.5. Temporal patterns in the carrying capacity of Palmyra’s hinterland

Additional details for this step are collected in S4 File. The calculated baseline carrying capacity of Palmyra is based on modern rainfall figures. Since the climate has remained broadly similar during the late Holocene [108], we consider it to be an appropriate first approximation. However, the marginal nature of the Palmyrene environment means that even relatively modest fluctuations in water availability could have significant consequences for food production and security. We used paleoclimate data to evaluate the changes to food security in Palmyra during the first three centuries CE.

There are no high-resolution palaeoclimatic sequences for the area immediately surrounding the city, and climate proxies in the northern Levant, especially in the interior are limited [see 108 fig. 1,4,109 fig. 2]. We evaluated 24 distinct climate records from the wider region of the East Mediterranean. Most of them did not have the decades-long resolution or their values were affected by stochastic annual variations, while others mapped multiple factors making disentangling the climate proxies impossible (S4 File).

The Dead Sea rainfall records based on the reconstruction of the ancient shoreline [110, 111] is the most representative of the rainfall in the region of Palmyra because of its proximity and quality of the record. The Dead Sea is fed from a large catchment area, thus representing an averaged record of regional rainfall. While annual variation in the lake level occurs, the sedimentological record accumulates slowly, so it smooths these short-term changes thus making the reconstructed lake levels a reliable proxy for decadal and centennial trends in regional rainfall [110]. To corroborate the patterns against a higher resolution but more localised dataset the Dead Sea trends were contrasted against isotope data from speleothems in Soreq cave, Israel [112] and against several larger scale, long-term climate records from outside of the region (S4 File).

The Dead Sea sequence was then calibrated using 20-century precipitation data from the Palmyra Weather Station (S3 File, step 3.8; S9 File, step 23) [113] to extrapolate the average and standard deviation of rainfall in the Palmyra’s area during the first three centuries CE from the mean rainfall in the Dead Sea.

2.6. Data caveats

Several factors, currently difficult to estimate, could narrow our estimates down (but not up), in particular, the area under cultivation, fluctuations in rainfall and losses during distribution. First, the area assumed here as under possible cultivation was assigned generously and is thus much larger than the area farmed at present. Remote sensing and field survey could narrow the extent of arable land further.

Second, yearly fluctuations in rainfall are not accounted for in the model. They are significant in the region and limit the reliability of food production [82, 113] requiring storage solutions. There are no known large storehouses for grain (horrea) in Palmyra before 273 CE similar to those found in Rome, Ostia, and elsewhere. Traditional elite dwellings in the Syrian countryside, however, incorporate large storage rooms for grain, and the so-called khans (caravanserai) could have also included grain storage spaces [73]. The countryside around Palmyra holds a large number of such structures visible in aerial photographs and identified through archaeological surveys and excavation [15, 114]. Further research is needed, however, to better establish their possible function and role in the city’s food storage and supply system. Similarly, storage for seed would have been required, but due to the productivity of the area the amounts might have been as low as 50 kg/ha, 2,5% of the total grain, or even lower [73, 115, 116].

Third, the model assumes no losses during the distribution of grain. Traditional grain management, storage and transport under premodern conditions lead to considerable loss due to decomposition, parasites and pests. For example, in twentieth century Ethiopia this amounted to 25–38% of the harvest [117]. These losses depend on the methods and length of storage, climatic conditions, and other factors currently difficult to estimate for Roman Palmyra. However, these losses would have further lowered the amount of people that could be reliably fed in Palmyra. As stated above, a usual loss rate of 30% would have had a proportional impact on the city’s carrying capacity.

On the other hand, a higher population could be sustained if alternative sources of food were used. As discussed above, systematic large-scale long-distance transport of bulk foodstuffs over land from the mountains or the Orontes river Valley is unlikely due to its prohibitive cost but cannot entirely be ruled out, as these areas could have complemented local production in bad years [20, 27, 89, 118–120]. Alternatively, opportunistic dry farming in wet years could support grain production, however, at a significant risk due to the unpredictability of rainfall. We know that Palmyrenes had a more diverse diet than simply barley, including meat and animal products provided by nomads. However, they were unlikely to be given freely or without compensation. The exchange of animal products for grain is a common practice between farmers and pastoralists [121], thus not changing our estimates in a significant way.

3.1. Reasonable maximum population of Palmyra

The results indicate that a population of between 30,000 people could be comfortably supported by the hinterland of Palmyra at all times during its history (Fig 3). The maximum viable population falls between 65,000 and 90,000 (Table 1) people and assumes significant storage infrastructure to ensure food security despite annual fluctuations in agricultural production.

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Fig 3. Changes in the carrying capacity between 1–300 CE.

In the background, average rainfall levels needed to feed different population levels. Dashed lines show the decadal frequency of successful crop yields.

https://doi.org/10.1371/journal.pone.0273241.g003

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Table 1. Estimates of the maximum possible population fed in optimal conditions.

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While the soils in the immediate surroundings of Palmyra are highly fertile and could theoretically support hundreds of thousands of people (see S3 File), the available rainfall (on average 125 mm) is a strongly limiting factor restricting the land use to irrigable areas only. In the area within a day from Palmyra, only 13% of the land irrigated practically, and that would not be enough to feed a moderately large Roman city reliably. Within the boundaries of Palmyra’s territory these are available at the distance of up to three days’ travel from the city. This is a significantly larger hinterland compared to most ancient cities where agricultural hinterland was defined by the distance farmers could commute in a day [122]. Historical data also shows the limits on food transport. In eighteenth century India the correlation of grain prices between urban markets decreased markedly with distances over 35 km and was very weak above 70 km [123]. During the eighteenth century famine in Syrian Aleppo in the same period, market mechanisms were insufficient to supply the city from regions beyond its regular hinterland forcing authorities to bring in subsidized supplies from further afield [124]. While several Roman cities required food to be brought in from far away, all had maritime or fluvial ports. In contrast, the city’s isolated location forced Palmyrenes to develop an exceptional food security strategy of exploiting agricultural opportunities further afield.

The city depended on the four irrigated agricultural areas identified by Meyer [20] of The Palmyra Oasis, the At-Tarfa Depression, the Bir Arak Oasis, and the fields north of the Harbaqah Dam (Fig 2). The most productive area is the At-Tarfa depression (Table 1), still farmed extensively in the present day. It occupies a low point east of Palmyra and collects most of the runoff from the mountains north of the city. The Harbaqah Dam could store substantial amounts of water to serve fields of cereals [20]. The oases of Bir Arak and Palmyra had a lower potential for grain agriculture but thanks to the continuous and reliable water supply could be dedicated to gardens and tree groves. Measured in calories these crops outputs are less water-efficient than barley. Altogether, the four agricultural areas could produce around 33 thousand tonnes of barley a year, which could feed between 65,000 and 92,000 people. These yield outputs are in line with previous estimates for agricultural productivity in the ancient world [125; S7 File].

Included in this number is the agricultural workforce. Approximately 8800 labourers would be necessary during harvest (Table 2). With 17,587 hectares farmed this is around 0.50 workers per hectare, which is the upper limit of the range (0.33–0.5 people per ha) estimated for the workforce involved in food production around Pergamon by Laabs and Knitter [105]. Nomadic pastoralists aiding during the harvest could decrease this number [100, 101] but if other, more labour-intensive crops were also grown, the workforce requirements were higher.

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Table 2. Estimated maximum workforce requirements.

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The reported numbers of 33,000 tonnes of barley, and 65,000 to 90,000 people, represents the maximum reasonable estimate for food production in the hinterland of Palmyra, with a 6000–9000 strong agricultural workforce. The extent of irrigable land places a hard limit on food production meaning that additional rainfall would result in surplus food production only up to the threshold of 90,000 people (Fig 4). Nevertheless, more efficient water management strategies could increase the security of the food supply and the diversity of foodstuffs available to the local population.

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Fig 4. Carrying capacity in relation to rainfall.

Top: the relationship between population numbers and yearly rainfall levels. Bottom: a violin plot showing the distribution of yearly rainfall between 1934 and 1999 shows the range of feasible population sizes give twentieth century levels of rainfall [113].

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3.2. Food security over time

All of the above calculations are based on WorldClim’s 30-year average rainfalls. However, the region experiences significant annual variation in rainfall (Fig 4) [113]. The highest level of food production, enough to support 90,000 people, is feasible on average five out of every ten years. Reliable food security (nine out of every ten years) is achievable for a population of up to 45,000 people, including 6000 agricultural workers. Paleoclimatic records, however, indicate that the amount of rainfall in the early first Millennium CE would have been lower than these figures [111].

Rainfall reconstructions in the Dead Sea show a steady decrease in precipitation between the first and late third centuries. The lowest point falls in the second part of the third century during which the rainfall in the region fell below 70% of the modern levels. This is corroborated by a higher resolution but a more localised record of the Soreq cave although it also documents a significant dip in precipitation at the end of the first century, not noted in the Dead Sea records (Fig 5).

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Fig 5. Graph of rainfall proxies.

Shown are the reconstructed rainfall over the Dead Sea (table 4 in 111) and the 20-year rolling mean of Oxygen isotopes in Soreq [112]. Note the truncation of the y-axis, which starts at 60% of modern values.

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Recent work on historical climate-society interaction warns against assuming uniform climate periods and similar effects in different regions [126]. The trend of a generally drier but also more variable climate in the period following the so-called Roman Climate Optimum (200 BCE—150 CE) has, however, been documented across the northern hemisphere and in the region through several paleoclimatic (tree ring, ice cores, lake and other geological sequences) and historical sources [109, 127, 128]. For example, the Talmud [Yerushalmi Ta’ anit 3.4; Bavli Ta’anit 24a, as collected in 129] mentions a string of severe droughts that affected Palestine between 210–220 CE, 220–240 CE and 255–270 CE [130]. A dry event has been recorded around 250 CE in other palaeoclimatic records in Anatolia, such as Bereket Basin [131, 132] and the Gravgaz marsh [133]. It is uncertain whether the rebounce from dry to wetter conditions occurred in the fourth or fifth century, but the pattern of prolonged droughts and dry conditions dominating the Roman East in the third century CE is well-documented [126]. While keeping in mind that the aforementioned climatic data do not come from the immediate surroundings of Palmyra, and that the relationship between climate change and agricultural production is complex [134, 135] it is clear that any climatic downturn in such a marginal environment would have consequences for food security.

The paleodata indicates that while at the beginning of the first century up to 95,000 could be sustained by Palmyra’s hinterland five out of ten years, this number drops to 72,000 in the second half of the third century. In terms of robust levels of food security, Palmyra’s hinterland could easily support 43,000 people nine out of every ten years at the beginning of the city’s history, but by the mid third century this number dropped to 31,000, including the rural workforce. While the exact values may be contested given the inherent uncertainty over several factors used in the model (e.g., if storage and transport losses are included all of the above figures need to be reduced by 30%), the overall trend is clear: the capacity to ensure food security in the city would gradually decrease to values over 25% lower than the starting point (Fig 3). This process would have reached the critical point around the mid-third century when the city’s population was supposedly the largest while the hinterland’s food production was at its lowest.