Our case-study of the MW from Hungary illustrates the incidence of even small climate fluctuations upon a major historical event. In this respect it is necessary, first, to discuss the progress of the invasion in military terms and the measures taken by the Mongols to consolidate their occupation. Secondly, we consider the ways in which climate probably impacted the effectiveness of the Mongol army, based on specific characteristics and limitations in a certain region (which might have been different in other regions, e.g., drought versus temperature limited environments). Finally, we argue how environmental factors may have entered the decision-making process of the Mongol leadership (at a specific location and within a short period).

During their invasion in 1241, the Mongols moved swiftly across the country and were initially successful in taking various fortresses, destroying any resistance they encountered. The population bore the brunt of the subsequent occupation and villages were left empty as the people were killed or fled into the forests and hills. In some cases they were captured, enslaved and drafted into the Mongol army. Significant elements to consider in this scenario are: First, no serious armed resistance hindered the Mongol occupation after the battle of Mohi. Second, plundering was extensive and third, the Mongols set up a rudimentary administrative system in which several villages were placed under the control of Mongol leaders7, presumably for fiscal and civil order purposes. We should also take note of the fact that the Mongols, as per Roger’s testimony7, did not burn the fields and took care not to harm the harvest or fruit-bearing trees. Attention to productivity indicates that the Mongols did not intend to turn agricultural land into pasture, which would have presumably been the case had grass been insufficient.

The Mongols did not cross the Danube into the western part of Hungary until January or February of 1242, when several sources attest that the river froze, allowing the army to cross and engage in the devastation of western Hungary. Moreover, abundant snowfalls are recorded for that time. The Mongols who reached Croatia in pursuit of King Béla IV also found swampy conditions and eventually withdrew without being able to inflict much damage on the cities. In contrast to the surrounding hilly terrain where Luvisols dominate, much of the Hungarian plain is covered by relatively young soils without distinct profile development, so-called Cambisols25. Dark Chernozems are widespread throughout the Carpathian basin and represent the most fertile soil type. River valleys that have developed on stratified sediments are characterized by so-called Fluvisols, whereas Arenosols have developed on windblown sands deposited after the end of the last ice age. Most of the lower elevation soil types in Hungary are particularly prone to stagnant moisture and ponding. These areas may turn quickly into marshy terrain when increased snowmelt and rainfall coincide at the seasonal transition from winter to spring. Soil wetness, in turn, not only delays the onset of the vegetation period but also reduces the overall productivity of the extensive agricultural and natural grassland habitats in Hungary.

The military situation in the late winter and spring of 1242 therefore seems to have changed. The Mongols encountered considerable difficulties in taking fortified castles and citadels, for instance. While stone fortresses offered greater resistance, the Mongol failure is explicitly attributed in the sources to swampy terrain. It is possible that these conditions made it much more difficult to operate siege engines and to keep a cavalry force in the surrounding areas. Moreover, the early spring thaw may have severely reduced the amount of grazing land, with horses already weakened by the winter and thus limited the movements of the Mongols to areas with available pasture. The reduced population, abandonment of fields and devastation of large parts of the country must have also reduced the Mongols’ ability to procure victuals. The combined effects of the war and a less favorable climate may have also caused the failure of the harvest of 1242 and the ensuing ‘great famine’ in Hungary17. It should be further noted that military operations of inner Asian nomads, to which the Mongols were no exception, were normally executed in autumn and continued through the winter, while the spring and summer were seasons in which they were at their weakest and most vulnerable. Military seasonality is part and parcel with the pastoral economy upon which the Mongols depended and with the management of its resources, primarily horses. According to contemporary sources, the Mongols did not provide forage for the horses but allowed them to graze freely in the grassland26. This indicates an obvious vulnerability in case no sufficient grass was available or in easy reach.

It is therefore under conditions of (i) reduced mobility and military effectiveness; (ii) reduced fodder for the horses; and (iii) reduced victuals for the army, which in the late spring of 1242 the Mongols left Hungary. The main Mongol army withdrew towards east following the southern course of the Danube (Fig. 1), thus crossing Serbia into Bulgaria, where they obtained the submission of the king Kaliman I at Tarnovo, before crossing Wallachia and Moldavia and returning to the steppes in the lower Volga region. A secondary army under Qadan that had travelled to Dalmatia in pursuit of King Béla followed the same route, joining the main army. Some minor contingent, such as the troops that had captured Roger may have proceeded to return through Transylvania. It is difficult to say why Batu chose to return by a southern route, but it is possible that the army moved to overall dryer and higher ground along the Carpathian foothills to avoid marshy conditions. In this respect, we may recall that years later the Mongol khan Hülegü mentioned in a letter that it is the Mongol habit, when the summer is hot, to move to the fresh and snowy mountains and because during his campaign in Syria the grass and food had been for the most part used up, some troops were sent to the mountains of Greater Armenia15. The southern withdrawal route may therefore have been suggested by the drier conditions of a higher altitude, while the Hungarian plains were marshy and devastated by famine.

In our ‘environmental hypothesis’ (Fig. 5), we join paleoclimatic with historical information to show the possible impact of even small climatic fluctuations and time-specific environmental conditions on a set of historical circumstances. In so doing, we demonstrate that relatively modest changes can have a decisive impact. In this manner, for instance, we replace the static relationship between environment and human response – based on a constant notion of carrying capacity of the land – with a dynamic concept that estimates actual year-by-year conditions and thus allows us to perceive the precise constraints under which decisions were taken. The data presented here support the view that the climatic conditions that occurred in Hungary between 1241 and 1242 had a considerable impact on the productivity of the land as well as on the suitability of the terrain for military operations of the type performed by the Mongols (see also details above), relying on high mobility and swift storming of fortified places. In the winter of 1241–42 lower temperatures initially favored the movement of the Mongols by allowing them to overcome frozen rivers, but later the thawing of the snow and ice made the land wet and marshy. The failure of the Mongols to take fortified castles may be at least partially attributed to such unfavorable conditions. The alternating environmental circumstances probably made it difficult for the horses to find grass in sufficient quantity and for commanders to concentrate their forces in sufficient numbers to practice their usual tactics (Fig. 5). Moreover, the Mongols no longer fed their prisoners other than meager scraps of food7, which indicates that the great famine that was soon to hit Hungary could be foreseen, placing an additional obstacle on the Mongols’ occupation. Our ‘environmental hypothesis’, therefore, argues that small climatic changes from 1241–42 were though sufficiently extensive to alter the conditions under which the Mongols first invaded Hungary. These changes would not allow them to function effectively as an occupation army, thus forcing them to withdraw.

Figure 5 The Mongol withdrawal. (A) June-August (JJA) temperature anomalies (°C with respect to the 1961–1990 climatology) averaged from the two independent tree ring-based summer temperature reconstructions from the Austrian Alps12 and Romanian Carpathians11. (B) Map of the OWDA showing June-August (JJA) soil moisture anomalies for 1242 CE. (C) The putative interplay of various environmental factors, resulting in more or less favorable conditions that either increase or decrease the number of options for the Mongol army during their successful invasion of and sudden withdrawal from Hungary, respectively. The map was reproduced and modified from18. Full size image

The Mongol conquest operated according to multiple models and conditions specific of the several places and objectives. The initial campaign in Khurasan lasted several years and resulted in recruitment of local administrators and elites, which provided a basis for later campaigns and consolidation of Mongol rule in that area. Different phases of the conquest corresponded to different, sometimes limited objectives. The early campaigns in northern China mostly aimed to impose tributes and other demands, evolving only later into a model of territorial conquest. Therefore, we can see that the Mongol empire changed considerably from the tributary demands imposed by Chinggis Khan, to the phase of actual conquest that took place under Ögödei and later imperial expansion under Möngke. The campaign in Hungary belongs to the second phase, under the khanship of Ögödei, which focused more closely on territorial occupation and can regarded more similar and indeed an extension of the Russian campaign. This was not accomplished in multiple waves, but in a single multi-year campaign aimed to obtain the quick surrender of the local towns and imposition of tributes, while Mongols settled in areas that were ecologically suitable to their economy, lifestyle and military needs (the Volga basin). The Hungarian branch of the campaign was one of the western campaigns under Batu. That the Mongols stayed in southern Russia and did not seriously attempt to invade eastern Europe again (with the exception of a short-lived invasion of Poland in 1259) has not been so far an object of historical inquiry. However, this paper raises the possibility that the vulnerability of the Hungarian plains to even relatively short-term climate events made it obvious that the region was unsuitable for military occupation by a large army relaying mostly on horses. It is worth noting that the Hungarian river system was prone to flooding and to creating marshlands and only much later it became drier, thanks to drainage work undertaken by the Hapsburgs in the 19th century27,28. Explanations for the Mongol strategies have to take into consideration local conditions as well as the specific objectives of different phases of the conquest. We may also recall that the absence of a repeated Mongol invasion can be compared with the two failed invasions of Japan in the late-13th century, which were not followed by further attempts. Our paper shows that a possible reason why the Mongols who occupied Russia under Batu and his successors did not make further attempts to expand westward may have depended on the realization that local conditions would not have supported a prolonged occupation. While the reasons why the conquest of the West halted in southern Russia have to remain speculative in the absence of proper documentation, we should consider environmental conditions on a par with political ones, such as the civil war that engulfed the Golden Horde and the Il-Khanate in following decades and cultural ones, such as the Turkicization and Islamization of the Golden Horde, which transformed the Mongol leadership. Moreover, economic changes brought about by the development of trade also caused a change of attitude towards the West, whereby Venetian and Genoese trading towns were established on the Black Sea, in the territory under Mongol rule in the early-14th century.

We finally conclude that the advantages deriving from a joint analysis of historical and paleoclimatological data are especially relevant to the reconstruction of a more articulate and richer context, rather than to the construction of causal chains. Further insight should be produced, if available, from archaeological and other disciplines, including both natural sciences and the humanities. It would be, for instance, extremely important to examine the remains of the castles and fortifications that the Mongols stormed or failed to take. Studies on historical changes in the hydrology of Hungary would also provide relevant information about the geography and ecology of Hungary in the 13th century. Moreover, the extent to which war, devastation and famine impacted the local population may also surface through epidemiological studies, coupled with isotopic analysis to investigate dietary changes. Finally, the extraction of aDNA would inform us of the extent to which Mongol and Cuman invasions altered the genetic makeup of Hungary and its surrounding regions. While caution must be exercised when extrapolating complex mechanisms of human-environment interaction over space and time24, the MW constitutes an excellent, though perhaps still preliminary, case-study to explore the role that relatively minor environmental factors may have played on major socio-cultural, political and economic phenomena. The example of the MW also stresses the benefits of creating a science-informed and data-rich context for seeking explanations of historical events24.