(PDF) The dahliagram: An interdisciplinary tool for investigation, visualization, and communication of past human-environmental

S C I E N C E A D VA N C E S | R E S E A R C H R E S O U R C E SOCIAL SCIENCES Copyright © 2023 The Authors, some The dahliagram: An interdisciplinary tool for rights reserved; exclusive licensee investigation, visualization, and communication of past American Association for the Advancement human-environmental interaction of Science. No claim to original U.S. Government Works. Distributed Michael Frachetti1,2*, Nicola Di Cosmo3, Jan Esper4,5, Lamya Khalidi6, Franz Mauelshagen7, under a Creative Clive Oppenheimer8, Eleonora Rohland 9, Ulf Büntgen5,8,10,11* Commons Attribution License 4.0 (CC BY). Investigation into the nexus of human-environmental behavior has seen increasing collaboration of archaeol- ogists, historians, and paleo-scientists. However, many studies still lack interdisciplinarity and overlook incom- patibilities in spatiotemporal scaling of environmental and societal data and their uncertainties. Here, we argue for a strengthened commitment to collaborative work and introduce the “dahliagram” as a tool to analyze and visualize quantitative and qualitative knowledge from diverse disciplinary sources and epistemological back- grounds. On the basis of regional cases of past human mobility in eastern Africa, Inner Eurasia, and the Downloaded from https://www.science.org at Princeton University on November 22, 2023 North Atlantic, we develop three dahliagrams that illustrate pull and push factors underlying key phases of pop- ulation movement across different geographical scales and over contrasting periods of time since the end of the last Ice Age. Agnostic to analytical units, dahliagrams offer an effective tool for interdisciplinary investigation, visualization, and communication of complex human-environmental interactions at a diversity of spatiotempo- ral scales. INTRODUCTION proxy records can have an inadvertent effect of being too narrow Bringing evidence of environmental and climatic changes into dis- or idiosyncratic to provide effective explanations of more general course for understanding human history and behavior is not new trends throughout antiquity (18). Interdisciplinary attempts to (1). However, recent years have seen more concerted efforts to engage data from history, climate science, archaeology, and promote consilience through dialogue between the sciences and hu- ecology to understand past socio-environmental interactions manities (2, 3), as well as the application of genetics to questions of often face mismatched metrics, inspiring new approaches that aim past human geography and demography (4, 5). Such studies are to compile, corelate, and visualize diverse lines of evidence at a generally published in scientific journals (6–9), arguably limiting range of scales and uncertainties in robust scientific manner (3, their influence on historians for whom monographs and a small 19). While such methods allow for productive conceptual cross-as- number of history journals remain vital academic currency (10– sessment and insight, they require a comparative and qualitative 13). Multidisciplinary studies of the entanglements between tool to standardize the varied scales and resolutions of data and climate and society, generally, have also not been without their diverse research sources used to explain complex socioenvironmen- critics (14), and there is a thin line between over- and underinter- tal behaviors, such as population movement. pretation of direct and indirect linkages between human behavior Today, more than 280 million people or roughly 4% of the and environmental factors. world’s population live outside their country of birth (20). With increasingly refined proxy reconstructions and model sim- Despite decades of intensive research concerning the overlapping ulations of Holocene climates (15), as well as new insights into socioeconomic and environmental factors that may motivate or human organization and mobility during the past millennia (16, force human mobility (21), explanations of (pre)historic (and 17), there are now rich opportunities to explore the interplay of modern) population movements often remain restricted to mono- social, political, economic, and environmental factors on human causal explanations, such as climate, conflict, or economy (22). behavior through time and space. Yet, the current pivot toward Overly simplistic views, however, can create a circular narrative “small-scale” case studies that leverage high-resolution climate between the causes and consequences of movement and the practic- es and policies proposed as sustainable solutions (23). Given the heightened attention to the implication of past climate variation 1 Department of Anthropology, Washington University in St. Louis, 1 Brookings for historic human migration (among other socioenvironmental be- Drive, CB 1114, St. Louis, MO 63130, USA. 2School of Cultural Heritage, Northwest haviors), we here offer an interdisciplinary tool to visualize the rel- University, Xi’an, China. 3Institute for Advanced Study, Princeton University, Prince- ton, NJ 08544, USA. 4Department of Geography, Johannes Gutenberg University, ative roles of environmental factors alongside social, political, and Becherweg 21, 55099 Mainz, Germany. 5Global Change Research Institute (Czech- economic influences. Moreover, we assess the impacts of a range of Globe), Czech Academy of Sciences, 603 00 Brno, Czech Republic. 6Université Côte factors on local- to large-scale mobility in three regional settings d’Azur, CNRS, CEPAM, 24 avenue des Diables Bleus, 06300 Nice, France. 7 Department of Social Anthropology, University of Bielefeld, 33615 Bielefeld, throughout the past ~12,000 years. Germany. 8Department of Geography, University of Cambridge, Cambridge CB2 3EN, UK. 9Department of History, University of Bielefeld, 33615 Bielefeld, Germany. 10Swiss Federal Research Institute (WSL), 8903 Birmensdorf, Switzerland. 11 Department of Geography, Faculty of Science, Masaryk University, 613 00 Brno, Czech Republic. *Corresponding author. Email:

(M.F.);

(U.B.) Frachetti et al., Sci. Adv. 9, eadj3142 (2023) 22 November 2023 1 of 13 S C I E N C E A D VA N C E S | R E S E A R C H R E S O U R C E MATERIALS AND METHODS sufficiently broad to accommodate shifting definitions over time The dahliagram for interdisciplinary investigation and space. The dahliagram represents a qualitative tool for synthesizing and For example, “technology” might refer to the emergence of novel visualizing knowledge drawn from a wide range of disciplines and materials, such as bronze (in deeper antiquity) or the improvement spatiotemporal scales (Fig. 1). Although the dahliagram is a univer- of sailing vessels (in the 15th century CE) each of which influenced sal device for socioenvironmental research, we focus on human mo- human movements in their own contextual setting. In our case, bility as a behavioral response in three pivotal regions of world dahliagrams also accommodate diverse data sources and scholarly history: eastern Africa, Inner Eurasia, and the North Atlantic. Out- traditions that shape how knowledge is gathered and used to weight lined for select phases of pronounced mobility since the end of the the impact of factors on human movements. By considering and last Ice Age, the resulting dahliagrams represent easily accessible rating multiple factors in a single image, comparable to the and customizable visualizations that enable qualitative comparison concept of planetary boundaries (24), the intersectional social and of the relative influences of diverse social and environmental factors environmental framework of a dahliagram stimulates conceptual on human behavior. thinking and promotes critical engagement with the corollary con- In this study, “movement” is placed at the center of the dahlia- ditions surrounding complex human-environmental feedbacks, in- gram while different push and pull factors are represented in a sur- cluding climate. When applied diachronically within a regional rounding array of “petals.” Each factor is evaluated on the basis of context, a time series of dahliagrams can reveal phased transitions Downloaded from https://www.science.org at Princeton University on November 22, 2023 interdisciplinary synthesis of available research and ranked accord- within and between various socioenvironmental factors, encourag- ing to its influence from low to high, charted over three concentric ing specialized research into one or more of the pertinent domains. rings of increasing intensity. Factor categories can be customized The selected regional case studies for our three dahliagrams depending on the analytical focus, although they should be range in chronological scale from decades to centuries to millennia, framed according to historical or archaeological documentation of Fig. 1. Schematic template of the dahliagram. Push/pull factors are arranged in a ring surrounding the central research problem. Petal length for each factor is ranked at low, medium, or high on the basis of interdisciplinary assessment. Analytical scale is indicated in the lower left according to both spatial and temporal ranges considered. Frachetti et al., Sci. Adv. 9, eadj3142 (2023) 22 November 2023 2 of 13 S C I E N C E A D VA N C E S | R E S E A R C H R E S O U R C E human movement in each regional case (indicated in the lower-left occurred gradually and episodically, first in Yemen and Sudan at table of each dahliagram). Throughout our group’s ongoing inter- the Early-Middle Holocene transition and then in the Horn of disciplinary interactions, we have found dahliagrams to be as effec- Africa toward the end of the Middle Holocene (between 5000 and tive in assessing population movements that occurred within richly 4300 B.P.), with certain areas transitioning later. While the Middle documented historical timescales as they are for modeling more Holocene is characterized by the persistence of lakes and perennial protracted periods of human mobility, because in both cases, the rivers in much of Arabia and eastern Africa, the end of the African social and environmental factors can be qualitatively ranked accord- Humid Period brought aridification between 4.5 and 4.2 B.P. (30). ing to the available resolution of their underlying data, without re- In both regions, climate and environment influenced mobility, quiring a common unit of quantification. The dahliagram is though to a lesser extent than in earlier periods (Fig. 2C). particularly effective when considering lower-resolution archaeo- In eastern Africa, the transition to herding occurred at the end of logical data alongside more-detailed climate data because it facili- the Middle Holocene, between 5000 and 3000 cal B.P. (30, 38, 39) tates the interpolation of diverse data sources within one coinciding with major technological changes including the develop- visualization. Its graphical layout intentionally positions the rele- ment of local ceramic traditions, new storage, and food production vant factors with equal potential to influence the behavior in ques- techniques (ovens, grindstones, etc.) and backed pieces made on tion, although each factor can be ranked independently (as “low,” flakes rather than bladelets. Cattle, sheep, and goat were adopted “medium,” or “high”) according to its quantitative and/or qualita- into existing foodways that still drew on fishing, hunting, and for- Downloaded from https://www.science.org at Princeton University on November 22, 2023 tive measurements. Of course, applications of the dahliagram aging (40). should carefully consider and make transparent questions related By the Middle Holocene in Arabia, herding of cattle, sheep, and to data resolution, archival composition, and other source-related goats was well established, while foraging and hunting remained issues, depending on the evidential material relevant to each case. common (41). Herding posed new challenges for populations, re- quiring new mobility patterns distinct from those used by hunter- gatherers and social ties becoming invaluable as pasturage and RESULTS water holes became a routine need for pastoralists (42). Human movement in eastern Africa and southern Arabia The change and increase in distances covered by populations is The Horn of Africa and southern Arabia are regions whose deep evident through obsidian sourcing. In southern Arabia, obsidian and recent pasts are inextricably tied to population movement. Mo- from the Yemen highlands, as well as other materials such as bility has most likely been central to a range of behaviors and con- jade, were found as far as the Hadramawt, western Oman, and ditions that shaped the regions’ extensive linguistic, ethnic, and southern Saudi Arabia with the onset of the Middle Holocene genetic diversity. “Out of Africa” migrations of hominins including (43, 44). At about the same time on the Red Sea coast of Yemen, Homo sapiens were followed by “back to Africa” movements exogenous obsidian replaced local cherts used to make bifacial throughout the Pleistocene and Holocene, evident from both pale- points, suggesting Red Sea crossings and long-distance exchange. ogenetic and archaeological records (25–28). There is robust evi- This implies the development of new kinship ties and technologies. dence for human occupation before and after the Younger Dryas By ~5000 BP, these interactions led to African obsidian being (YD), a hyperarid period lasting from ~12,900 to 11,700 years brought over to the Red Sea shores of Yemen in increasingly large before present (B.P.) (29, 30). Hence, it has been suggested that quantities, along with a new tool type made in the same way as in the Late Pleistocene populations moved to refugia in well-watered high- African Horn (45, 46). Technology, innovation, connectivity, and lands, coastal plains, and oases (31, 32). These factors suggest that new subsistence strategies are therefore the most influential mobility (into refugia) was driven largely by extreme and rapid cli- factors for mobility during this time, eclipsing prior push factors matic shifts occurring on the order of centuries (33) and geograph- such as food resource availability and environment as populations ically focused by access to water and food (Fig. 2A). became more dependent on social networks and exchange for re- The Early Holocene (~11,700 to 8200 B.P.) is marked by the re- sources with outside groups (Fig. 2C) (47). surgence of the African Humid Period or Holocene Pluvial after the The Iron Age ushered novel political structures in eastern Africa YD event. The increased precipitation reactivated hydrological along with the rise of kingly political hegemonies, specialized pro- systems and substantially raised lake levels. It is also thought to duction workshops, and formalized trade partnerships among have facilitated movement across the deserts of Arabia and the distant kingdoms. By 2900 B.P., several Sabaean kingdoms were (Green) Sahara while serving to isolate other populations because vying for power and control of the incense trade routes in southern of abundant resource availability around newly developed lakes Arabia and had developed water storage systems for agricultural use and the flooding of major corridors of migration such as the Nile in arid zones (48). valley and East African Rift (30, 34). In the Horn of Africa, the same processes were in play during the In southern Arabia, this period is marked by resource abundance pre- and Aksumite periods. While throughout the pre-Aksumite and persistence of hunter-gatherer-fisher subsistence (35). The period, there was demographic continuity with populations that Early Holocene is characterized by lithic traditions distinct from preceded, Sabaean elements from the Daʿmat polity intermixed those found in the Horn of Africa (36, 37). Mobility was still with local innovations, eventually leading to a Semitic writing heavily influenced by subsistence strategies, such as fishing, system (Ge’ez), a pantheon, and architecture, that echoed those of hunting, or foraging. Given the increased availability of food re- the kingdoms in Yemen (49, 50). This period was key for the devel- sources at short range, long-distance mobility was likely associated opment of terraced agriculture in the highlands (51, 52), as well as with search for lithic resources (Fig. 2B), such as obsidian. the formalization of trade with the interior of the continent, and Major social changes took place in the Middle to Late Holocene with Arabia and northeast Africa, via the ports of the Red Sea (53). (~8200 to 3000 B.P.). Economic transitions toward pastoralism Frachetti et al., Sci. Adv. 9, eadj3142 (2023) 22 November 2023 3 of 13 S C I E N C E A D VA N C E S | R E S E A R C H R E S O U R C E Downloaded from https://www.science.org at Princeton University on November 22, 2023 Fig. 2. Dahliagram analyses of human movement in east Africa/southern Arabia. (A) Younger Dryas (YD) [~12,900 to 11,700 before present (B.P.)], (B) Early Holocene (~11,700 to 8200 B.P.), (C) Late Holocene (~8200 to 3000 B.P.), (D) pre-Aksumite period (~2800 to 2050 B.P. ), and (E) multiperiod composite dahliagram for east Africa. Frachetti et al., Sci. Adv. 9, eadj3142 (2023) 22 November 2023 4 of 13 S C I E N C E A D VA N C E S | R E S E A R C H R E S O U R C E Mobility as a result of colonization, conquest, and conflict is ev- perspective, the cumulative picture suggests that the Yamnaya mi- idenced throughout this period, as is mobility for maritime and gration to the Altai was likely prompted by conditions related to in- overland trade and the maintenance of social ties (Fig. 2D). The tergroup social dynamics, and predicated upon aspirations of tribal period saw many innovations, including the introduction of or political sovereignty among communities within the wider camels, horses, and other pack animals (54), and the development Yamnaya social arena, rather than being an adaptive response to of chariots and sailing craft. During this period, mobility and what changing environmental conditions or sparked by new technology motivated it changed considerably with polities increasingly con- per se (Fig. 3A). trolling territory, resources, and people. The importance of Roughly 1000 years later, regionally specialized mobility patterns climate and environment, of resource availability, and of subsis- among middle Bronze Age pastoralists reflect a significantly more tence strategies decreased in a very short time as a result of shifting localized geographic scale and character (Fig. 3B). Eurasian pasto- human-environment and social relations. ralists of the 4th millennium B.P. had engineered ecologically well- adapted, multispecies (agro-)pastoralist subsistence strategies (68) Human movement in Inner Eurasia and honed their mobility patterns to exploit both steppe and moun- Population movement has played a formative role in shaping soci- tain ecologies (69, 70). This era was also heavily influenced by the eties of the Inner Eurasian steppes from prehistory to the present growth of large-scale trade networks (71–73), which was not a pre- (55, 56). Whether one considers long-distance migratory episodes, dominant stimulus for the earlier Yamnaya migration. Widespread Downloaded from https://www.science.org at Princeton University on November 22, 2023 historical invasions, plague-related population displacements, or growth and transfer of technological innovation such as tin-bronze durable ecological patterns of seasonal mobility, archaeology and metallurgy, horse riding, and grain farming each played a height- history together document the paramount role of human move- ened role on mobility as they drew communities into regional ment on the ideological, economic, and political geography of the arenas of trade and exchange, and facilitated forays into novel envi- Eurasian steppe. Four key phases of human movement in Inner ronments (74, 75). Although paleoclimatic archives illustrate a Eurasia include the early and middle Bronze Ages, the Turkic era, broad climatic trend toward cooler, dryer conditions, there is little and the Mongol era (Fig. 3). to suggest an abrupt or large-scale environmental impulse that Human movement in the context of the Eurasian steppe has would have forced more extensive mobility after 4000 B.P. (76). always been respondent to both immediate and long-term Rather, human movement across (and within) Eurasia in the human-environment interactions (57, 58), operating at various geo- fourth millennium B.P. appears to be influenced most heavily by graphic scales. The seasonal mobility patterns of most Eurasian pas- novel technologies and network expansions for trade and interac- toralist communities, for example, are heavily influenced by the tion that were nascent in earlier millennia (77). availability of pasture, water, and other climatically responsive en- Historically documented Central Asian migrations have been vironmental resources, used to support their agro-pastoralist econ- discussed for centuries in relation to Huns, Avars, and other omies (59). However, the two prehistoric cases considered here—an peoples that appeared suddenly on the borders of the Roman early Bronze Age long-distance migratory event and pastoralist mo- empire and later hegemonies of Europe (78). Imperial formations bility patterns of the middle Bronze Age—differ in temporal lon- established by the 6th century CE in the eastern part of Eurasia gevity, in spatial extent, and in the combination of push/pull reflect a clear configuration of the forces underlying population factors underpinning them. movement, as political and economic transformations of the Iron The ancient genomic record of Eurasia documents a close ances- age evolved to shape diverse modalities of mobility up to the tral tie between west Eurasian “Yamnaya” populations and Altaic present (Fig. 3C). The end of the Eastern Türk empire and dispersal “Afanasievo” communities separated by thousands of kilometers, or relocation of Turkic peoples in eastern Inner Eurasia have been suggesting a rapid, long distance eastward migration by ancient connected with volcanic events that further intensified the general herders at the start of the 5th millennium B.P. (60–62). Subtle dif- cooling observed during the Late Antique Little Ice Age (8). ferences in the archaeological remains between regional communi- Environmental aspects and, in particular, the availability of ties (among the Yamnaya) and the migrants to the Altai suggests abundant pasture were essential to the imperial movement of pas- complex intracultural social dynamics that may have prompted pe- toral, horse-based people and their armies. Population scale and the riodic out-migration among select Yamnaya groups, both westward force of imperial warfare in the Mongol period (Fig. 3D) placed in- and eastward (63). Whether impelled by social or ideological creased stress on environmental resources, exacerbating the impact drivers, Yamnaya migration was partly facilitated by new technolo- of regional climate change evident across the eastern and central gies such as bullock carts and novel innovations such as horse riding Eurasian steppe zone (79, 80). Yet, comparatively, mobility on the and dairying (64, 65). There is little corresponding zooarchaeolog- part of the Turks, Uyghurs, and Mongols was likewise influenced by ical evidence for domesticated horse remains in Altaic Afanasievo nuanced differences in trade possibilities and commercial connec- settlements, perhaps indicating that horse riding was not overly in- tivity. The Uyghurs’ control of the silk trade (6th to 8th century CE), fluential in long-distance migration at this time, despite its potential for example, likely had greater relevance to their mobility, whereas (65, 66). Regardless, we must assume that a considerable degree of conflict can be considered as more influential in the Mongol impe- transregional knowledge and expertise was needed to navigate the rial movements. The dissolution of the Uyghur empire, which also vast steppe environment, whether moving by foot, by cart, or by led to the relocation of nomads from Mongolia to northern China horse. Paleoclimatic evidence for environmental amelioration sug- and to the Tarim Basin, in today’s Xinjiang, may also have been due gests slightly improved pasture quality at this time in prehistory, but to long-term climatic variation in combination with political trans- early Bronze Age herders had long established a resilient adaptive formation (81). strategy to drought and other climatic stressors (67). Therefore, For Turko-Mongol polities, centralized political organization when we consider the wider array of factors in comparative and social structures, including kinship, ethnic, and class Frachetti et al., Sci. Adv. 9, eadj3142 (2023) 22 November 2023 5 of 13 S C I E N C E A D VA N C E S | R E S E A R C H R E S O U R C E Downloaded from https://www.science.org at Princeton University on November 22, 2023 Fig. 3. Dahliagram analyses of human movement in Inner Eurasia. (A) Yamnaya long-distance migration [~5000 to 4900 before present (B.P.)], (B) middle Bronze Age steppe herders (~4200 to 3200 B.P.), (C) early Turkic expansion (6th to 7th centuries CE), (D) Mongolian expansion (~13th to 14th centuries CE), and (E) multiperiod composite dahliagram for Inner Eurasia. Frachetti et al., Sci. Adv. 9, eadj3142 (2023) 22 November 2023 6 of 13 S C I E N C E A D VA N C E S | R E S E A R C H R E S O U R C E stratification, drove their imposition of sovereignty over conquered was abandoned around 1450 CE. In the 1990s, synchronicity with lands. Their mobile lifestyle and associated technology (such as climate change combined with archaeological evidence established tents mounted on carts or collapsible tents carried on pack a narrative, according to which the “collapse” of the Greenland animals) also facilitated mobility. Continuous conflicts and faction- Norse was a case of ecological overshoot and maladaptation (89, al struggles developing within the Türk and Mongol empires con- 90). Supposedly, they had clung on to field farming, to increasing tributed to movement, together with policies, in the Mongol the size of their herds, and to a meat-based diet for too long and, empire, that aimed to remove and relocate entire population thus, missed the opportunity to successfully adapt to changing groups (82). conditions. While the comparative spatial scale of population movement un- However, this narrative has not withstood the test of time. Anal- derlying Turkic and Mongol imperial expansion was restricted or yses of carbon isotopes in human bone remains have shown that the enhanced by the availability of environmental resources (grass- proportion of marine proteins increased steadily between the 11th lands) in Eurasia, there were notable differences in terms of their and 15th centuries CE. The Greenland Norse had increasingly previous knowledge and of the speed and temporal range of the covered their needs from marine hunting and also adapted their movement. Türkic polities advanced throughout territories that agrarian strategies (91, 92). In sum, the Greenland Norse did not were largely unknown to them; the long-term effects of their move- fail to adapt to the challenges posed by the transition from a ments include a general expansion of Turkic languages across warmer to a cooler climate. If at all, they failed in spite of their adap- Downloaded from https://www.science.org at Princeton University on November 22, 2023 Central Asia and southwest Asia and into Anatolia. On the contrary, tative resiliency. the Mongols likely had prior information about at least some lands Even more important than such reinterpretations is the recent and tended to incorporate within their rank’s large numbers of con- discovery that the economic motivation for expansion to Greenland quered peoples—including Türks (broadly defined). Demographic came from Viking involvement in the European ivory trade patterns based on genetic analysis seem to indicate a higher-than- (Fig. 4A) (93). Their walrus hunting grounds shifted further normal reproduction rate of Mongols across Eurasia (83), which, north from Icelandic waters and as far east as the coast of New- however, did not result in large-scale linguistic changes but rather foundland. Thus, settling in precarious places, they had aims fostered cultural assimilation. Climatic elements such as droughts quite different from pushing agriculture to the limits. They or cooler periods have also been identified as possible push simply followed a lucrative prey and, in their efforts, they were tem- factors (84). porarily favored by relatively warm conditions. However, the Euro- While timescales, territorial extension, and historical relevance pean ivory trade phased out. Elephant tusks gradually flooded the of the two movements are different, the expansions of both Türk market and put the prices for ivory from North Atlantic walrus and Mongol empires have left indelible traces on the Inner Eurasian hunting under severe pressure. It is now widely believed that the linguistic, cultural, political, and genetic map. Their forms of move- Greenland Norse did not await death in their settlements but ment cannot be simply associated with climatic factors but need to simply abandoned them and returned to Iceland for more produc- be considered in relation to the more general forms of mobility un- tive economic endeavors. derlying nomadic ecology and their dynamic political-economic After 1492, entirely new routes of mass migration emerged in the systems. The respective dahliagrams for each era expose the Atlantic world (94). The accidental “discovery” by Christopher Co- nuanced balance of factors that need to be considered at any lumbus of land masses hitherto unknown to Europeans fueled given time in light of their specific archaeological, paleoenviron- desires among rulers, explorers, and conquerors to extract gold, mental, and historical circumstances. silver, and other natural resources (Fig. 4B). Mining and, soon af- terward, cash crops created a demand for labor among Spanish and Human movement across the North Atlantic Portuguese imperial forces initially met by enslaving indigenous Human movement across the North Atlantic has been an ongoing peoples. When natives died in great numbers from diseases and vi- process for more than 1000 years. As early as the late 10th century olence, deportation of human labor from Africa sparked one of the CE, Vikings expanded across the North Atlantic and even became most pronounced eras of forced human movement in human the first Europeans to reach (present day) Newfoundland and estab- history (95, 96). lish a small and short-lived settlement (L’Anse aux Meadows, 1021 First, the Spanish and Portuguese empires and then the French CE) (85).The Vikings reached Newfoundland from Greenland, set- and British empires in the Americas were erected on the economic tling there in 985 BCE through an expedition from Iceland. Esti- foundations of plantations and a demand by markets in Europe and mates suggest that the number of settlers ranged between 500 and Asia for their products (97). All highly profitable cash crops— 1500, split between “Eastern” and “Western” settlements. Their tobacco, indigo, sugar cane, etc.—required tropical or subtropical population apparently grew rapidly and peaked around the conditions and could not be grown in Europe. Hence, climatic middle of the 12th century CE, with up to 3000 people in both set- zones and plant geography were inseparably intertwined with econ- tlements combined (86, 87). omies in colonial America (98–100). This plantation economy Viking expansions were generally favored by conditions of the drove transatlantic movements of both African slaves and Europe- Medieval Climate Anomaly, which created significantly warmer ans, the latter crossing the Atlantic as indentured servants. Never- summers around the year 1000 (88). Moreover, the fate of the theless, most of the European emigrants made free choices about Greenland Norse has also been linked with the cooling that followed their movements, while this was not the case for enslaved Africans. later, in a transition to the Little Ice Age (LIA). A first phase of For this reason, the African Slave trade must be modeled as a sep- cooler temperatures set in after 1250. From 1400 CE, the cooling arate case and demands a more nuanced consideration than space became more marked and sustained. The western settlement was here allows. The dahliagrams presented here for the North Atlantic depopulated between 1350 and 1400, and the eastern settlement Frachetti et al., Sci. Adv. 9, eadj3142 (2023) 22 November 2023 7 of 13 S C I E N C E A D VA N C E S | R E S E A R C H R E S O U R C E Downloaded from https://www.science.org at Princeton University on November 22, 2023 Fig. 4. Dahliagram analyses of human movement in the North Atlantic region. (A) Greenland Norse expansion (~1000 to 1400 CE); (B) Spanish/Columbian explo- ration (late 15th to 16th century CE); Pre-1820 European Migration (17th to early 19th centuries CE); (D) post-1800 European migration (19th century CE); and (E) multi- period composite dahliagram for the North Atlantic. Frachetti et al., Sci. Adv. 9, eadj3142 (2023) 22 November 2023 8 of 13 S C I E N C E A D VA N C E S | R E S E A R C H R E S O U R C E therefore focus on emigration from Europe to the Americas, in the research design. So too, it exposes domains of knowledge that are 15th and 19th centuries, respectively. underdocumented in paleoclimatology, archaeology, or history or Estimates for European transatlantic migration after 1492 help which simply have not been considered directly as factors in identify two distinct periods (Fig. 4, C and D): Before century shaping behaviors like population movement. Hence, it requires 1820, numbers were low, while they went up rapidly afterward. the user to think beyond simply causality and consider the intersec- Only in the latter period did Europeans outnumber forced migrants tions of social, economic, political, and environmental conditions from Africa. Over the long term, the availability of arable land and for each case. The resulting visualizations further serve to expose labor remained important pull factors, while population growth in previously unexpected parallels among different regional cases Europe became a dominant push factor during the 19th-century in- that might otherwise never be viewed side by side and therefore dustrial transition, when in some parts of Europe, such as the open the possibility of multiregional comparative analysis. German territories (before 1871), industrial labor struggled to Through our exercise of modeling mobility in both the early keep up with demand (101). Chain migration (connectivity) was a Turkic era of Eurasia and the pre-Aksumite period of eastern common phenomenon already in the 18th century. People regarded Africa, for example, the resulting dahliagrams chart highly compa- themselves as members of a group—a nation, a religious communi- rable driving factors, with only minor shifts in the magnitude of ty, or family, etc.—both as emigrants and immigrants. Religious and impact across a few domains (Fig. 5). other forms of identity became key motives for leaving one place Although these cases are separated historically by hundreds of Downloaded from https://www.science.org at Princeton University on November 22, 2023 and for choosing another. The industrial transition saw major tech- years and situated thousands of kilometers apart, population move- nological innovations (e.g., railway and steam boats), which made ment within these emergent empires appears to have been rooted in long-distance migration generally more affordable and, thus, con- similar forces of political and social identity, as well as ambitious tributed to the rise in numbers. interests to acquire regional resources and stimulate trade and con- Only few authors have considered the role of the LIA in North nectivity. Environmental factors were an omnipresent concern but Atlantic migration and assessed that it “had a major impact” due to appear to be outweighed by factors such as conflict and sovereignty. cases of harvest failure (102, 103). However, the influence of LIA The historical implications of mobility within these formative fluctuations on agrarian production remains controversial among empires in their respective era and region are unique, but only economic historians (104, 105). Attributing meteorological events when visualized in the dahliagram do we see the shared correlations to the LIA is just one of the difficulties (106). On the other hand, across an array of factors that may produce fruitful onward investi- evidence for climatic anomalies causing crises in agrarian produc- gation into their behavioral similarities at the human-environmen- tion is overwhelming. Events such as the “Great Frost” in 1740 af- tal nexus. fected large parts of Europe and can be linked with emigration in At first glance, any given dahliagram may seem subjective and some cases, e.g., from Ireland (107). In the 19th century idiosyncratic, built for a particular case through interdisciplinary (Fig. 4D), Irish emigration to the United States peaked after the consensus among a given team of specialists. However, we consider Great Famine of 1846–1849. Although a potato blight was its prox- the dahliagram as a necessary rubric to establish common under- imate cause, neither the famine nor emigration can be reduced to standings of disparate analytical approaches and forms of evidential environmental factors. The Irish and Germans were the two support. The efficacy of the dahliagram lies first and foremost in its largest groups of immigrants to the United States after 1820, fol- capacity to spark dialogue and debate, because different research lowed by the British. German immigration peaked in 1816–1817, partnerships might generate alternative renderings of available 1846–1857, 1864–1873, and 1880–1893 (108). Some of these data. Accordingly, we do not propose that any particular dahlia- peaks [1816–1817 and 1846; (109)] coincided with bad weather, gram illustrated above must provide the final analytical word in harvest failures, and high prices for cereal crops. Others were dom- each case. Our goal, rather, is to demonstrate the efficacy of the dah- inated by political motives (after the failed Revolution of 1848). liagram to translate disciplinary topics as well as empirical and qual- While agrarian crises did form a recurring pattern in large parts itative data sources into a single visual illustration or historical time of Europe over the entire period of transatlantic migration until series that can be shared among researchers from different special- 1900, their overall contribution can hardly be assessed quantitative- izations and quickly communicated, understood, and incrementally ly. Parts of France, the Dutch Republic, Switzerland, or the Austrian modified. Empire were more or less equally affected by climatic anomalies and The main stimulus to create such a tool came from our own ex- extremes, often at the same time, and yet, they did not produce the perience with interdisciplinary collaborations as climate scientists, same peaks in the amounts of people that departed for the New archaeologists, and historians. The technical complexities pertain- World. The importance of climatic and/ or (other) environmental ing to each of these disciplines and deep epistemic cleavages circumstances varied in time and place, and they were always inter- between them, make approaches to the past fraught with uncertain- twined with political, social, and economic factors. ties. The dahliagram visualization thus emerges as an interdisciplin- ary response to ongoing challenges of socio-environmental studies, allowing nuanced connections to be drawn between historical DISCUSSION phases of climatic stress and the assumed rise and fall of human set- Given what is known about the diverse factors that shape human tlements, empires, and social cohesion (110). behaviors such as population movement (21), the dahliagram facil- Within the discipline of climate science similar qualitative visu- itates multidisciplinary researchers to correlate their knowledge and alizations have been produced for the express purpose of providing data in a paradigmatic fashion while remaining agnostic to incon- an immediate and synthetic representation of how the planet is gruent units of measure, data scale, resolution, and source. The tool being affected by anthropogenic climate change and environmental remains pliable and adaptable to the specific needs of a given degradation. One tool, referred to as “planetary boundaries,” is used Frachetti et al., Sci. Adv. 9, eadj3142 (2023) 22 November 2023 9 of 13 S C I E N C E A D VA N C E S | R E S E A R C H R E S O U R C E to translate diverse quantifications of atmospheric change, soil and water chemistry, and attendant environmental transformations in relation to sustainability “boundaries.” Planetary boundaries has been widely adopted as an effective way to communicate the risks and impact of human and natural forcing on the planet’s ecosyste- mic health and sustainability (24, 111–113). The power of such tools lies in their capacity to promote scientific consensus around accept- able human influence on the environment and long-term climate dynamics. Given its ease of use, we hope that the dahliagram will be adopted to stimulate further explorations of complex human behav- iors by leveraging multidisciplinary team building and consensus, especially concerning studies of the past. 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