Has there been any evidence of the dynamic complexities of climate and human history in western and southeastern Iran from the last ice age to the late Holocene? Has climate change been evident on the Iranian plateau?

Since the early Neolithic (∼10,000 years ago), the Iranian Plateau has witnessed the development of sedentary human settlement facilitated by periods of favorable climatic conditions prompting gradual or sweeping changes. Climate factors significantly drove the hydroclimatic conditions in western and southeastern Iran, which varied in response to the Mid-Latitude Westerlies (MLW) and Indian Summer Monsoon (ISM). In addition, the input of dust and its eastward transport from the Arabian Peninsula and North Africa coincided with the North Atlantic cooling events. Peak wet conditions during the early Holocene in southeastern (c. 11.4–9.6 ka BP) and western Iran (c. 10.2–8.6 ka BP) indicate different timings in regional precipitation. The northward displacement of the Intertropical Convergence Zone at the beginning of the early Holocene caused the ISM to expand over southeastern Iran. At the same time, it strengthened the sub-tropical high-pressure and northward expansion over western Iran, resulting in dry conditions. Between 7.8 and 6.3 ka BP, gradual weakening and southward movement of the ISM and the decrease in intensity of the subtropical high-pressure systems over the Zagros region resulted in southeastern Iran becoming mild and the western region humid. Between 6.3 and 5.0 ka BP, a decrease in solar insolation ushered dusty and arid conditions on the Iranian plateau. Notably, human activities in the region started experiencing significant changes around the mid-Holocene. A concurrence exists during the wet (c. 5.0–4.5 ka BP) and dry (c. 4.2–3.2 ka BP) periods, coinciding with the rise and decline of multiple Bronze Age settlements. These settlements flourished in exchange and trade, pyro-technologies, and agro-pastoral production, demonstrating an increasing complexity in social organization and vulnerability to climate change. After transitioning into the Iron Age, southeastern Iran experienced relatively wet conditions c. 2.9 to 2.3 ka BP and 1.6 to 1.3 ka BP coincided with major territorial expansions and advancements under the Achaemenid and Sassanian dynasties. Merging the historical and archaeological data with palaeoenvironmental conditions indicates a concurrence of unfolding climatic and cultural changes, suggesting cascading effects that led to growth or settlement decline and abandonment.

The hydroclimate on the Iranian Plateau exhibits intricate dynamics resulting from the interplay between several atmospheric systems, namely, the Mid-Latitude Westerlies (MLW), the southwest Indian Summer Monsoon (ISM), and the northeast Siberian Anticyclone, showcasing the complexity of the climate systems. (Figure 1A). These complex interactions have significantly influenced the climate and hydrological patterns across the Iranian Plateau since the end of the Last Glacial Maximum (LGM) c. 20 ka BP (Sharifi et al., 2015; Vaezi et al., 2019). Moreover, changes in fluvial discharge and fluctuations in lake levels played a major role in shaping the bio-climatic zones on the Iranian Plateau (Djamali et al., 2010a). These changes are argued to have influenced the emergence and subsequent development of urban societies stretching from Western Asia to the Indus Valley (Staubwasser and Weiss, 2006; Sharifi et al., 2015; Petrie, 2011; Matthews, 2013; Weiss, 2016; Petrie and Weeks, 2018; Sinha et al., 2019; Marchetti et al., 2024). However, this relationship remains complex and the subject of ongoing debate, underscoring the need for further research (Middleton, 2018; Middleton, 2024). The climates of western and southeastern Iran, the two focus regions in this review, exhibit significant differences due to their distinct geographical and topographical features. Western Iran, characterized by the mountainous Zagros range, experiences a Mediterranean climate with hot, dry summers and cool, wet winters, receiving 400–800 mm of annual precipitation mainly from October to April. This climate supports diverse ecosystems, including forests and grasslands. In contrast, southeastern Iran is defined by arid desert landscapes, primarily the Lut Desert. It features a deserttype climate marked by extreme summer heat exceeding 50 °C with minimal rainfall (50–250 mm annually), resulting in limited biodiversity dominated by drought-resistant vegetation. Overall, western Iran has higher humidity and more consistent rainfall, while southeastern Iran is mainly dry and subject to erratic precipitation patterns. The palaeoenvironmental reconstruction of Iran, particularly the Zagros region, has played a pivotal role in providing valuable insights into the ongoing discussion surrounding the influence of climate change on the Neolithic transition in West Asia. The unique features and significant role of the Zagros region underscore its importance in the influence of climate change on human history (Petrie and Weeks, 2018; Fazeli Nashli and Matthews, 2013; Matthews and Fazeli Nashli, 2022). It helps understand the societal changes during the early Holocene when humans transitioned from mobile hunter-gatherer lifestyles to largely sedentary farming and herding communities. The Chalcolithic societies of the seventh and sixth millennia BP, through their consolidation of the lifeways of their Neolithic forebears, established denser, sometimes hierarchical settlement systems. These were built on integrated agro-pastoral production and the development and mastery of specialized crafts producing ceramics and metals (Matthews and Fazeli Nashli, 2022). These significant developments laid the foundations for the emergence of the Early Bronze Age (EBA) settlement on the Iranian Plateau towards the end of the sixth millennium BP. The EBA settlements, with their crucial role in exchange and trade, agriculture, and cultural advancements, were instrumental in shaping the so-called ‘Middle Asian Interaction Sphere’ (MAIS; Possehl, 2007) that linked societies from Mesopotamia to the Indus Valley and from the shores of the Persian Gulf to Central Asia in the fifth millennium BP (Madjidzadeh and Pittman, 2008; Mashkour et al., 2013; Mutin and Minc, 2019; Beni et al., 2021; Matthews and Fazeli Nashli, 2022). Around 4 ka BP, the MAIS began to disintegrate, ushering in a period characterized by de-urbanization and, in some places, the near-total disappearance of sedentary sites during the fourth millennium BP (Petrie and Weeks, 2018). The subsequent Iron Age witnessed the (re-) emergence of towns and cities. These urban centers integrated under the imperial Persian dynasties and were the catalysts of significant historical and cultural events in this region (Staubwasser and Weiss, 2006; Mancini-Lander, 2009; Sharifi et al., 2015; Cronin, 2021). Throughout these developments involving shifts in agropastoral strategies, alterations in social and political structures, advancements in water management techniques, alterations in land use, and partial or largescale population migrations, the Iranian Plateau experienced ongoing climatic changes and phytogeographic shifts which greatly influenced the potential for human settlement in this region (Mashkour et al., 2013; Sharifi et al., 2015; Gurjazkaite et al., 2018).The transformation in the area dramatically impacted the environment and social structures. This concept has been highlighted in several pre-modern societies, such as the Mayans, Norse Greenlanders, and Easter Islanders, affected by environmental and climate change (Diamond, 2005). Weiss and Bradley (2001) focused on droughts triggering societal disruptions in Mesopotamia and the Near East. Such disruptions are often labeled as instances of societal ‘collapse,’ defined as a substantial, relatively rapid, and lasting decline in an established level of socio-political complexity, indicated by reduced population, social stratification, economic specialization, centralized control, and trade/exchange networks (Tainter, 1988; Diamond, 2005). Current discussions of ‘collapse’ recognize the challenges in defining the term and highlight the significant elements of continuity that characterize societies once described as having collapsed. These debates emphasize the need for multifactorial and multi dimensional explanations influenced by environmental, cultural, and political dynamics and historical contingencies (e.g., Middleton, 2018; Middleton, 2024). The idea that adverse climate events universally triggered a cultural collapse in premodernity does not match the available archaeological evidence from Western Asia, or indeed across the globe, and represents an oversimplification of collective events that were always multi-faceted and nuanced (Butzer and Endfield, 2012; Haldon et al., 2018). Nevertheless, the literature review indicates that despite claims that greater social complexity supported an increasing decoupling of demographic and climatic trends since the mid-Holocene (Palmisano et al., 2021), the variable environmental settings of ancient agro-pastoral communities across Western Asia regularly exposed them to climate change. This seemingly affected social resilience and influenced urban and demographic cycles during the later periods (Fleitmann et al., 2022; Marchetti et al., 2024). In Western Asia, researchers have examined abrupt events to explore the causality between climate and historical/social changes (e.g., Staubwasser et al., 2003; Thompson, 2006; Sharifi et al., 2015; Marsh et al., 2018; Jones et al., 2019; Sinha et al., 2019; Lawrence et al., 2022). Until recently, studies in Iran.FIGURE 1 Major climate systems in West Asia (Sharifi et al., 2015; Vaezi et al., 2019) and the location of Bronze Age settlements in SE Iran. (A) The red and blue lines in panel A depict the estimated present-day position of the Intertropical Convergence Zone (ITCZ) and its location during the Early Holocene, respectively. The abbreviations ISM, MLW, and SH correspond to the Indian Summer Monsoon, Mid-Latitude Westerlies, and Siberian High Anticyclone, respectively. Black stars indicate the locations of palaeoenvironmental studies, primarily along the Zagros Mountains and southeastern Iran. (B) Map of the Iranian Plateau showing the locations of archaeological sites: 1- Godin Tepe, 2- Tepe Giyan, 3- Baba Jan, 4- Kalleh Nisar, 5- Bani Surmah, 6- Mir Kheir, 7- Kunji Cave, 8- Deh Luran, 9- Susiana, 10- Tepe Kopandeh, 11- Deh Dumen, 12- Tal-i Malyan, 13- Tol-e Nurabad, 14- Tal-i Bakun/Tal-i Mushki, 15- Hormangan, 16- Tom-e Gavan, 17- Shahr-i Sokhta, 18- Shahdad/Tepe Dehno/Mokhtarabad, 19- Tal-i Iblis, 20- Tal-i Atashi, 21- Jiroft/Konar Sandal/Keshit, 22- Tepe Gav Koshi, 23- Hajiabad Varamin, 24- Tepe Gaz Tavilla, 25- Tepe Yahya, 26- Shamsabad. Principal regions mentioned in the text are marked with black circles. Modern provinces are labeled with grey letters and indicated with five-sided polygons.focused primarily on western Iran, especially the Zagros region (Djamali et al., 2008; Kehl, 2009; Nicoll and Küçükuysal, 2013), resulting in a lack of comprehensive geographic coverage. Additionally, excavations of prehistoric and early archaeological records face persistent challenges due to the varying survival rates of artifacts, including provenance and inferences drawn from various categories of archaeological evidence (Petrie and Weeks, 2018). Consequently, when examining different sites within a broader geographical scope, diverse growth patterns, decline, settlement, or abandonment can confound, tracing the subtle societal changes that have occurred over time (Petrie and Weeks, 2018; Eskandari, 2021a). This situation is further complicated by the proportionally low and uneven coverage of excavated archeological sites within Iran, making it challenging to build a broad, integrated, and reliable picture of settlement patterns. In particular, it is difficult at some sites to distinguish the effects of human activities and climate on palaeovegetation histories (Djamali et al., 2010b; Roberts et al., 2011; Petrie and Weeks, 2018). Nevertheless, some high-resolution multi-proxy palaeoclimate studies conducted in the Zagros region and southeastern Iran are particularly significant due to the presence of early human settlements that originated in these regions (Djamali et al., 2009a; Djamali et al., 2009b; Jones et al., 2015; Sharifi et al., 2015; Gurjazkaite et al., 2018; Petrie and Weeks, 2018; Safaierad et al., 2020; Vaezi et al., 2019; Vaezi et al., 2022; Matthews and Fazeli Nashli, 2022; see Table 1; Figures 2, 3). These studies are valuable for reconstructing palaeoclimate conditions and their potential to align these climate records with archaeological findings, providing valuable insights into the historical and cultural contexts of early human settlements along the Iranian borderlands. Reviewing these palaeoclimate studies in western and southeastern Iran could help synthesize spatiotemporal changes over larger geographic areas, providing a foundation for understanding the emergence of urban settlements and the development of exchange, trade, and cultural networks during the Bronze and Iron Ages. 2 Methods By gathering palaeoclimate, palaeoenvironment, and palaeovegetation data focusing on western and southeastern Iran, we aim to construct a regional framework for climate change. This region witnessed the rise and fall of multiple early complex societies (Petrie and Weeks, 2018; Matthews and Fazeli Nashli, 2022; Figure 1). To understand these dynamics, we aim to build a regional framework incorporating data on climate change, settlement scale and density, social complexity, economy, and subsistence, allowing us to explore these interactions. The review focuses on the climatic conditions that prevailed/varied during the post-LGM and Holocene, setting the stage for human migration, cultural development, and the occurrence of early sedentary communities that flourished around the mid-to-late Holocene. The dynamics of this rich cultural heritage have been linked to climate change through a mosaic of interdisciplinary evidence gathered from the multi-proxy records of climate variability and climate-related historical events in archaeology, geochemical proxies (e.g., biomarkers, stable isotopes, elemental ratios), pollen records, sedimentology, and palaeoclimate modeling efforts (e.g., Griffiths et al., 2001; Stevens et al., 2006; Jones and Roberts, 2008; Djamali et al., 2009a; Djamali et al., 2009b; Djamali et al., 2010a; Djamali et al., 2010b;Jones et al., 2013;Jones et al., 2015; Sharifi et al., 2015; Gurjazkaite et al., 2018; Carolin et al., 2019; Safaierad et al., 2020; Safaierad et al., 2023; Vaezi et al., 2019; Vaezi et al., 2022). Certain periods and locations have been investigated extensively using multiple proxies, allowing for a more comprehensive understanding and enabling researchers to draw richer insights and detailed inferences. The proxies have been interpreted to provide valuable insights into the palaeoenvironmental changes (see Figures 2, 3), e.g., 1) indication of low lake level or lake level decline = dry conditions; 2) high lake level or lake level increase = wet conditions; 3) frequent floods = wet conditions; 4) negative δ18O in speleothems and ostracod shells = wet conditions; 5) positive δ 13C in speleothem = dry conditions; 6) high percentage of oakderived pollen = wet conditions; 7) high percentage of Artemisia pollen = dry conditions; and 8) aeolian layers in lacustrine and marine sedimentary profiles = dry conditions (Sharifi et al., 2015). However, the varying thresholds and sensitivities to moisture and temperature in different climate proxies and locations pose challenges to predicting regional palaeoclimate interpretations or generalizations. Additionally, we examined the impacts of external climate shifts on subsistence, settlement patterns, and political structures associated with the mid-to-late Holocene, enhancing the empirical discourse. In doing this, the discussion delves into the intricate tapestry of climatic fluctuations, incorporating data from various sources to provide a nuanced understanding of historical climate dynamics and their impact on human settlements spanning millennia. We unravel the interplay between climate variations and cultural transitions toward more advanced phases by synthesizing evidence from existing literature on climate change and human settlements in the region and the prevalence of famines and droughts linked to climatic shifts in archeological and palaeoclimate studies (Figure 3; Table 2). By expanding the scope of regional data beyond the western and southeastern Iranian plateau, we aim to gain deeper insights into whether droughts, famines, and wet/dry conditions were widespread across the entire Middle East or primarily driven by localized factors. 3 Results and discussion 3.1 Chronological review of the evidence. 3.1.1 Terminal Pleistocene (dry conditions) The time frame in this interval extends from the latter part of the LGM, c. 20 ka BP, until 14.6 ka BP. Pollen records from Lake Zeribar (Van Zeist and Wright Jr, 1963; Van Zeist and Bottema, 1977), Lake Urmia in NW Iran (Bottema, 1986; Djamali et al., 2008), and Lake Van, Turkey (Wick et al., 2003) show similar assemblages, indicating the prevalence of the Iranian Turanian mountain steppe vegetation along the Zagros Mountains. A new multi-proxy palaeoenvironmental record from the Hashilan wetland in the central Zagros Mountains, western Iran (Safaierad et al., 2023), is consistent with previous evidence of pollen dominance of an Artemisia-Amaranthaceae dry steppe vegetation in the Zagros region (Van Zeist and Bottema, 1977; Djamali et al., 2008; Djamali et al., 2010a; Djamali et al., 2010b). These records indicate prevalence of dry conditions in southwest Iran during this period (Soleimani et al., 2023). More positive δ18O records from Lake Zeribar (Stevens et al., 2001) and Soreq Cave (Bar-Matthews et al., 1999; Bar-Matthews et al., 2003) imply that dry conditions prevailed in these areas, too. Likewise, the Lake Neor peat sequence in northwestern Iran was impacted by the MLW and indicated dusty and dry conditions over the region between 12.3 and 11.2 ka BP (Sharifi et al., 2015). The highest sedimentation rate correlates with the deposition of coarse sand and high magnetic susceptibility between 12.7 and 11.8 ka BP, indicating a highenergy environment and extensive erosion (Vaezi et al., 2019). Severe arid and dusty conditions during the Younger Dryas were present in southeastern Iran, as indicated by the high Zr/Al, Si/Al, and Ti/Al ratios (Vaezi et al., 2019; Safaierad et al., 2020). The weak ISM and low precipitation during this period were inferred based on the well-dated δ18O record in marine sediments from the western Arabian Sea (Sirocko et al., 1993). Similar arid conditions prevailed in other ISM-dominated regions, such as the Arabian Sea, Bay of Bengal (Chauhan, 2003), and the IndoGangetic plains (Sharma et al., 2004). In Iran, the arid conditions highlighted by previous workers (Mayewski et al., 1997; Sharifi et al., 2015) coincided with the cold Younger Dryas in northwest Europe (Dansgaard et al., 1989). The North Atlantic cold events caused by the release of freshwater and the breakup of icebergs from the North American ice sheets (Hoff et al., 2016; Ng et al., 2018) impacted the input of dust over the Iranian Plateau (Sharifi et al., 2015; Safaierad et al., 2020). It is suggested that the enhanced interhemispheric temperature contrast during these cold events (Clark et al., 2012; Singarayer et al., 2017) forced the Atlantic meridional overturning (Denton et al., 2005) and ushered a weak ISM (Wang et al., 2001; Stager et al., 2011). This eventually resulted in dry conditions over the Iranian Plateau and reduced surface air temperature over the ice-covered ocean (Kutzbach et al., 2014).Conclusion The high-resolution palaeoclimate records spanning the LGM and post-LGM events provide a valuable and comprehensive overview of environmental changes sweeping the Iranian Plateau. Records spanning the last 20 ka provide distinct insights into the region’s temporal variability of climate and hydrological patterns. In addition, these studies advance our knowledge about palaeoclimate changes and their potential impacts on past human settlements and cultural transformations. This multifaceted approach sheds light on the complex interactions between environmental conditions and human societies, contributing to a holistic comprehension of the historical trajectory since the Neolithic period. The strong correlation between the dust input over the Iranian Plateau with North Atlantic cooling events, e.g., Heinrich Stadial one and the Younger Dryas, supports the idea that atmospheric teleconnections most likely existed between these regions. Peak wet conditions during the early Holocene in southeastern Iran extended from 11.4 to 9.6 ka BP, whereas in western Iran, they occurred from 10.2 to 8.6 ka BP. During the early Holocene, high solar insolation played a significant role in causing changes in atmospheric circulation. Firstly, it led to the northward displacement of the ITCZ and the subsequent strengthening and northward expansion of the ISM. This caused increased rainfall and wet conditions in certain regions. On the other hand, the high solar insolation strengthened the descending air in sub-tropical high-pressure systems, leading to their northward expansion over the western Iranian Plateau. This caused dry conditions in the area, counteracting the wet conditions caused by the northward expansion of ISM. Neolithic settlements emerged in Iran around the mid-11th millennium BP. They were founded on a mixed economy exploiting domesticated plants and animals, which allowed both seasonal and permanent sedentary settlements. First established in the Zagros mountains, Neolithic settlements were notably scarce in southeast Iran until later millennia. In eastern Iran, Neolithic settlements were strategically established around alluvial fans, allowing access to fertile soils and groundwater alongside irrigation by surface runoff capture. Dusty, arid conditions covered the Iranian plateau c. 8.2 ka BP and subsequent cold and dry events coincided with the injection of icebergs into the North Atlantic Ocean. Between 7.8 and 6.3 ka BP, western Iran experienced a period of high humidity coinciding with the gradual expansion of oak forests. This is due to the decreased intensity and duration of the sub-tropical high-pressure systems over the Zagros region. During this period, southeastern Iran experienced a mild climate and increased precipitation. However, this region received less rainfall compared to the early Holocene.Between 6.3 and 5.0 ka BP, a decrease in solar insolation ushered in an arid period. The reduction in insolation impacted the amount of summer monsoon rains in southeast Iran, and the region shifted from the ISM to a MLW-dominated system. However, it is essential to note that a stable climate did not characterize the late Holocene period. Instead, the region experienced fluctuations in its climatic conditions, which were attributed to the irregularity in the atmospheric circulation of the MLW during the midHolocene period. Notably, there was a significant change in environmental conditions over the Iranian Plateau c. 5.0 to 4.5 ka BP. During this period, aeolian activity decreased, followed by a corresponding increase in rainfall, indicating a shift towards wetter conditions. The beginning of this wet phase coincided with Early Bronze Age settlements flourishing in southeastern Iran, suggesting improved environmental conditions may have been beneficial. This period coincided with the consolidation of an integrated oasis-based agricultural system, a fundamental adaptation for all future human settlements in eastern Iran. Concurrent developments in technologies and exchange systems marked this period as the first instance of intense intercultural contact, representing a transformative period that experienced substantial growth in socioeconomic complexity. Subsequently, the region encountered arid conditions between 4.5 and 4.0 ka BP, which was evident from increased aeolian input and dry conditions, particularly c. 4.2 ka BP in Western Asia. This downturn in climate towards the end of the Early Bronze Age is coeval to the observed decline of large urban sites that characterized eastern Iran from the early fourth millennium BP. This dramatic change in environmental conditions is associated with the widespread decline of Early Bronze Age settlement across many (but not all) regions from Mesopotamia to China. The environmental changes likely impacted agricultural productivity and are correlated with the abandonment or relocation of settlements on the Iranian Plateau. Except for a wet period in the western region and a semi-wet phase in southeastern Iran from c. 4.0 to 3.4 ka BP, overall arid conditions prevailed in both regions from c. 4.5 to 2.9 ka BP.The driest conditions centered c. 3.2 ka BP coincided with the regional Late Bronze Age cultural decline across many areas of Iran, Western Asia, and the Mediterranean. Consistent with various studies, we believe favorable climatic conditions contributed to increased socioeconomic stability, which seems more than coincidental and likely played a role in the success of the Persian Empires. The wet conditions from c. 2.9 to 2.3 and 1.6 to 1.3 ka BP, aligned with the expansive reigns of the Achaemenids and Sassanians. Despite arid conditions, the impressive agricultural productivity during the Parthian rule is attributed to cultural adaptation to the dry conditions and advancements in farming practices. Droughts during specific historical periods likely affected agricultural productivity, access to water resources, and overall food security, which could have destabilized these empires. Nevertheless, the interplay between environmental changes and the rise of organized human settlements is a complex and multidimensional phenomenon that calls for a comprehensive and more nuanced understanding of the historical, archaeological, and climatological evidence.