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Cultural Responses to Climate Change During the Late Holocene

Modern complex societies exhibit marked resilience to interannual to decadal droughts, but cultural responses to multidecadal to multicentury droughts can only be addressed by integrating detailed archaeological and paleoclimatic records. Four case studies drawn from New and Old World civilizations document societal responses to prolonged drought, including population dislocations, urban abandonment, and state collapse. Further study of past cultural adaptations to persistent climate change may provide valuable perspective into possible responses of modern societies to future climate change.

Reference: deMenocal, P.B., Cultural Responses to Climate Change During the Late Holocene, Science, 292, pp. 667-673 (2001) (PDF).

Introduction

In the spring of 1785, the geologist James Hutton presented a lecture to the Royal Society of Edinburgh that changed scientific inquiry into natural processes. The essence of his view was simple enough: The present is the key to understanding the past. Hutton recognized that slow geologic processes such as erosion or uplift could produce sedimentary strata or mountain ranges. In 1795, he wrote that "we find no vestige of a beginning -no prospect of an end ... Not only are no powers to be employed that are not natural to the globe, no actions to be admitted of except those of which we know the principle and no extraordinary events to be alleged in order to explain a common experience..."(1). This view was not accepted by most natural scientists at the time because it required full acceptance of the expanse of geologic time and rejection of the prevalent views of a young earth. Future generations of scientists, however, most notably Charles Darwin half a century later, were encouraged by this new way of thinking to interpret their observations based on what they knew of modern processes.

To understand how and why climates change we have to invoke a corollary to Hutton's view: The past must be used to understand the present. Modern instrumental records are sufficiently long to document climate phenomena that vary on an interannual time scale, such as El Niño, but too short to resolve multidecadal-to century-scale climate variability, which we know to exist from detailed tree ring, coral, and lake sediment records spanning the last 500 to 1000 years (2,3). Similarly, the socioeconomic impacts of recent El Niño/La Niña events are well documented (4), but little is known about the societal impacts of longer-period climatic excursions. Without knowing the full range of climatic variability at time scales of a few decades to a few millennia, it is difficult to place our understanding of modern climate variability, and its socioeconomic impacts, within the context of how Earth climate behaves, both naturally and as a result of anthropogenic increases of greenhouse gasses (3).

Historic and Pre-Historic Drought in North America

Excellent examples of the value of past climate records can be gleaned from the history of drought in the United States. Water availability, rather than temperature, is the key climatic determinant for life in semiarid expanses across the planet. Drought often conjures up images of the Dust Bowl drought of the 1930s, which lasted approximately six years (1933-1938) and resulted in one of the most devastating and well-documented agricultural, economic, and social disasters in the history of the United States. The drought was triggered by a large and widespread reduction in rainfall across the American west, particularly across the northern Great Plains (5). It displaced millions of people, cost over $1 billion (in 1930s US dollars) in federal support, and contributed to a nascent economic collapse. Subsequent analysis of the Dust Bowl drought has revealed that its tremendous socioeconomic impact was, in part, due to wanton agricultural practices and over-capitalization just prior to the drought when rainfall had been more abundant (5). A subsequent decadal-scale drought in the 1950's (Fig. 1A and 1 B) was also severe but less widespread, mainly impacting the American southwest where improved land use practices and disaster relief programs mitigated its effects.

How did the 1930s and 1950s droughts compare with other historic and pre-historic droughts? In a comprehensive analysis of hundreds of tree-ring chronologies from across the United States, Cook et al (1999) established a network of summer drought reconstructions extending back to 1200 AD (6,7) (Fig. 1A).This reconstruction documents much more persistent droughts prior to the 1600s (7).These so-called megadroughts were extremely intense, persisted over many decades, and recurred across the American southwest roughly once or twice every 500 years (Figs. 1A to 1D). Reconstructed conditions during the largest of these multidecadal droughts far surpassed those during droughts recorded within the last ~150 years (the period for which extensive instrumental data are available). Evidence for these and other megadroughts has been found in detailed lake sediment records (8), with additional evidence for even longer, century-scale droughts in California prior to 1350 AD and 1110 AD (9).

Figure 1. Drought history [1200-1994 AD (7)] for the American southwest reconstructed using a spatial network of tree ring chronologies across the United States (A). This network, based on the Palmer Drought Severity Index (PDSI), provides a detailed history of calibrated drought severity over a region roughly including Arizona, Utah, Nevada, Colorado, New Mexico, and west Texas. The reconstructed spatial drought index (PDSI) patterns are shown for the ~6 year Dust Bowl drought of the 1930 's, which was largely restricted to the northern and western Great Plains (B), the ~26 year southwest drought of the 1580s (C), and the ~26 year southwest drought of the 1280s (D). Relatively short (multiyear) but intense droughts such as the 1930 's Dust Bowl period (B) were found to recur roughly once or twice per century (6), whereas multidecadal droughts occur only a few times in a millennium.

The most severe drought in the southwestern US within the last eight hundred years spanned a ~22 year period between 1572-1593 AD (Fig. 1C) (7). The reconstructed spatial drought pattern at the peak of this dry period in 1583 AD shows extreme drought conditions extending across the American southwest (Fig. 1C). Dry conditions extended eastward and persisted into the early 1600s as far east as coastal Virginia (10). Based on a 700-year tree-ring chronology from northeastern Virginia, Stahle concludes that the intervals spanning 1587 to 1589 AD and 1606 to 1612 AD were the driest periods in the last 700 years (10). In August of 1587 AD, the first English colonists arrived and settled in Roanoke,Virginia. This small group of settlers became known subsequently as the Lost Colony because the entire Roanoke settlement had vanished by the time the English resupply ships returned four years later. Originally attributed to poor planning and inadequate supplies, the failure of the Roanoke settlement is now understood within the context of this severe drought, which began, to their monumental bad luck, just when the settlers arrived (10). A larger colonial settlement was established subsequently in Jamestown, Virginia in April, 1607 AD, and the settlers also suffered greatly. Within 25 years, over 80% of the population died, mainly of malnutrition (10).

Although appreciably less severe than the drought of the 1580 's, the 26-year "Great Drought" of the 1280s (11) was similarly prolonged and widespread (Fig. 1D). By the time of this drought, the Anasazi, ancestors of modern Pueblo Indians, had long established elegant stone and adobe villages in the semiarid highlands and canyons of the American southwest. Archeological investigations of Anasazi settlements have documented that many sites were abandoned abruptly near the end of the 13th century AD. Cited reasons for the collapse of the Anasazi include emergent balkanization, warfare, and religious turmoil within the region, as well as the onset of severe drought conditions and regional deforestation (11,12).Whether the multidecadal drought of the 1280s was the determining factor in the collapse of the Anasazi continues to be debated (13,14), but current archeological evidence firmly implicates drought as a contributing destabilizing factor (12, 14).

Modes and Mechanisms of Holocene Climate Variability

The relatively recent droughts described above persisted from a few years to a few decades. Complex societies can, and do, adapt readily to interannual to decadal fluctuations in water availability, but more persistent droughts present a different set of challenges and coping strategies. Multidecadal to multicentury scale droughts are now known to have punctuated the warm climate of the Holocene epoch [the last 11,700 calendar years before present (BP)]. Furthermore, transitions into and out of these climate shifts can be very abrupt, occurring in less than a decade (15). The Holocene was once believed to have been climatically stable (16), but detailed and well-dated paleoclimate records now show that Holocene climate was punctuated by several widespread cooling events, which persisted for many centuries and recurred roughly every 1500 ±500 years (17-22)(Fig. 2).

Figure 2. Millennial-scale climate variability during the Holocene. Oxygen isotope data from Greenland ice cores initially suggested that Holocene climates were stable (A) (16), but detailed and well-dated Holocene records of subpolar (B) and subtropical (C) North Atlantic sea-surface temperatures (19,21) document synchronous cooling events recurring at 1500 ±500 year intervals throughout the Holocene (and late Pleistocene). Analysis of surface temperature records has suggested that solar irradiance variability accounts for much of the observed temperature variability over the last millennium (30,31), although variations in solar irradiance spanning the entire Holocene [as partially represented by the production-corrected atmospheric 14C record (D) (32,33,36)] do not match the paleoclimate records. (E) Boreal summer (JJA) solar radiation variations at 20°N.

Paleoceanographic data indicate that these events were associated with changes in subpolar (19)and subtropical (21,22) surface ocean circulation (Fig. 2), as well as marked changes in terrestrial climates (17,18,23,24). These events appear to have occurred synchronously across the North Atlantic (21), with possible antiphase behavior in the NW Atlantic (25). Deep sea sediment evidence for deep ocean circulation changes associated with these Holocene events is, at present, equivocal (19,26), although other supportive evidence has been presented (20,27,28).The millennial-scale pacing of Holocene climate variability implicates mechanisms with long time constants such as thermohaline circulation or ocean-atmosphere coupling (26,29), which govern modern climate stability.

Analysis of the most recent millennial-scale Holocene climate cycle, the Little Ice Age (ca. 1300 to 1870 AD) and the preceding Medieval Warm Period (ca. 800 to 1300 AD), suggests that the primary factors affecting global temperature variability over the last millennium were variations in solar irradiance and volcanism, which together account for 40 to 60% of the reconstructed temperature variability (30). Climate models require an additional forcing agent, the anthropogenic rise in greenhouse gases, to account for 20th century warmth (30,31). Strong correlations of solar irradiance variability (32,33) with surface temperature (30,31) and regional drought (34,35) over the last millennium implicate solar variability as a significant factor influencing global climate over multidecadal to multicentury timescales. However, its role in forcing the full suite of millennial-scale climate variations during the Holocene (and the last glacial) is complicated by the absence of diagnostic 1500 ±500 year variability in the atmospheric 14C record (36), and its generally inconsistent match with Holocene climate anomalies (Fig. 2).The 14C record does exhibit significant variance at periods near 2200-2500 years [the Hallstadzeit cycle (36)].

Cultural Responses to Late Holocene Climate Variations

How did past cultures respond to the longer, multicentury-scale climate changes which punctuated late Holocene climate? Placing the archeological record of cultural change within the context of detailed and well-dated Holocene paleoclimate records presents opportunities to examine how large, complex societies responded to long-term, persistent changes in climate. At some times during the late Holocene, whole empires collapsed and their people were diminished to much lower subsistence levels, whereas in other cases populations migrated and adapted to new subsistence modes. In all cases, the observed societal response reflects an interaction between random human elements (socioeconomic, political, and secular stresses) and persistent multi-century shifts in climate. Four case studies drawn from the joint archeologic and paleoclimate histories of the New and Old World illustrate past cultural responses to late Holocene climate change: The collapse of the Akkadian (ca. 4200 years BP),Classic Mayan (ca. 1200 years BP), Mochica (1500 years BP),and Tiwanaku (ca.1000 years BP) empires.

Akkadian Collapse (Mesopotamia, ca. 4200 years BP)

Under the rule of Sargon of Akkad, the world 's first empire was established between ca. 4300 and 4200 years BP on the broad, flat alluvial plain between the Tigris and Euphrates Rivers (37). Akkadian imperialization of the region linked the productive but remote rain-fed agricultural lands of northern Mesopotamia with the complex city-states of the south. After about 100 years of prosperity, however, the Akkadian empire collapsed abruptly at around 4170 ±150 calendar years BP (37,38). Archeological evidence documents broad abandonment of the agricultural plains of northern Mesopotamia (37) and dramatic influxes of refugees into southern Mesopotamia where populations swelled (37,39).A 180 km-long wall, the "Repeller of the Amorites", was built across central Mesopotamia to stem nomadic incursions to the south. Resettlement of the northern plains by smaller, sedentary populations occurred near 3900 years BP, some 300 years after the collapse (37).The collapse horizon at Tell Leilan, northeast Syria, is overlain by a thick (~100 cm) accumulation of wind-blown silts which were devoid of artifacts (37), suggesting a sudden shift towards more arid conditions. Social collapse evidently occurred, despite archeological evidence that the Akkadians had implemented grain storage and water regulation technologies to buffer themselves against the large interannual variations in rainfall that characterize this region (37).

Fig.3. Excavated sample of residential occupation (600 sq.m2) within the lower town of Tell Leilan, NE Syria (100 ha.)during the terminal Akkadian empire occupation.Abrupt climate change (ca.2200 B.C.)forced the Akkadiann abandonment of rain-fed agriculture plains of northern Mesopotamia. The diagonal surface extending across the image (lower left to upper right) is a former road passing through the town, walls and occupation structures are seen to either side of the road. Photo credit:H.Weiss/Yale University (copyright)

Using a deep-sea sediment core from the Gulf of Oman, Cullen et al. (40) reconstructed a detailed record of Holocene variations in regional dust export based on mineralogic and geochemical tracers of windborne sediments from Mesopotamian sources (Fig. 3). Closely dated by a sequence of calibrated radiocarbon dates, the Gulf of Oman core documents a dramatic ca. 300 year increase in eolian dolomite and carbonate, which commenced at 4025 ±125 calendar years BP. Isotopic (87/86Sr) analyses demonstrate that the increased eolian dust was derived from Mesopotamian sources (40)(Fig. 3). Geochemical similarity of volcanic tephra shards found at Tell Leilan and in the deep-sea sediment core provided further evidence that Akkadian collapse and climate change events were synchronous (40). Enhanced regional aridity at this time is also indicated by increased eolian quartz deposition in nearby Lake Van at the headwaters of the Tigris River (41) and paleoclimate records from the Levant (42). The combined archeological and paleoclimate evidence strongly implicates abrupt climate change as a key factor leading to the demise of this highly complex society.

Figure 4. Mesopotamian paleoclimate and the collapse of the Akkadian empire. Detailed radiocarbon dates of archeological remains at Tell Leilan, northeast Syria document the abandonment and incipient collapse of the Akkadian empire near 4170 ±150 calendar years BP (37). A late Holocene record (40) of Mesopotamian aridity was reconstructed by quantifying wind-borne sediment components in a deep-sea sediment core from the Gulf of Oman, which is directly downwind of eolian dust source areas in Iraq, Kuwait, and Syria. The severalfold increase in eolian dolomite and calcite commencing at 4025 ±125 calendar years BP reflects a ~300-year interval of increased Mesopotamian aridity. A Mesopotamian provenance for this dust peak is indicated from detrital fraction Sr (and Nd) isotopic analyses, which show a marked shift toward the measured Mesopotamian (Tell Leilan abandonment) composition (40). Geochemical correlation of volcanic ash shards (tephra) found both in the dust peak and at the Akkadian collapse horizon at Tell Leilan suggests synchroneity between the collapse and drought onset.

The onset of sudden aridification in Mesopotamia near 4100 years BP coincided with a widespread cooling in the North Atlantic (19,21). During this event, termed Holocene Event 3 (Fig. 2), Atlantic subpolar and subtropical surface waters cooled by at least 1 to 2 °C (19,21). The headwaters of the Tigris and Euphrates Rivers are fed by elevation-induced capture of winter Mediterranean rainfall. Analysis of the modern instrumental record shows that large (50%) interannual reductions in Mesopotamian water supply result when subpolar northwest Atlantic SSTs are anomalously cool (43). The aridification of Mesopotamia near 4100 years BP may thus have been related to the onset of cooler sea surface temperatures in the North Atlantic.

Classic Maya Collapse (Yucatán Peninsula, ca. 1200 years BP)

The Preclassic Maya culture occupied vast lowland and highland regions of Mesoamerica from the second millennium BC to ca. 250 AD. The onset of the early Classic period after 250 AD marks the rapid growth of a more complex, stratified, and intellectually and artistically prolific empire. The hallmark accomplishments of Early (250 to 550 AD)and especially Late (550 to 850 AD)Classic Maya cultures include the development of trade networks spanning Mesoamerica, expansive urban centers, erection of monumental stelae, and advances in astronomy and mathematics (44).

Fig.5. Structures emerge from the surrounding tropical forest at the Maya archaeological site of Tikal in Petén,Guatemala.At its peak in the Late Classic Period (~ca.800 A.D.),this urban center in the southern Maya lowlands supported some ~60,000 inhabitants.It was largely depopulated after following the ninth-century A.D. demographic collapse. Photo credit:M. Brenner (copyright)

The Classic Maya empire collapsed at the peak of their cultural development between ca. 750 and 900 AD, based on the number of sites engaged in monument construction across Mesoamerica at any given time (45). Following the apex of monument construction in 721 AD, signs of collapse began to show between 750 and 790 AD. Construction effectively ceased throughout the region after 830 AD, and in 909 AD, the last monument, in southern Quintana Roo, Mexico, was inscribed with the Maya Long Count date (44). Many reasons for the collapse have been cited, including overpopulation, deforestation and soil erosion, social upheaval, warfare, and disease, as well as natural phenomena such as climate change (46,47). Deforestation, erosion, and overpopulation are well-documented in many regions prior to the collapse (46,48,49).

The first unambiguous evidence for the role of climate change in the collapse of the Classic Maya (50) came from lake sediments, which documented an abrupt shift to more arid conditions in central Yucatán (Mexico)between 1300 to 1100 years BP (800 to 1000 AD)(Fig. 4). Sediment composition and stable isotopic analyses of ostracode shells preserved in sediment cores from the closed-basin Lakes Chichancanab (50) and Punta Laguna (51) in central Yucatan indicate that the region was subjected to a ca. 200-year period of persistently arid and highly evaporative conditions centered near 1200 years BP (900 AD) (Fig. 4).

Figure 6. Mesoamerican paleoclimate and the Classic Maya collapse. Incipient collapse of the Classic Maya civilization began near 750-790 AD, and the last Maya stela or monument construction was dated at 909 AD based on Maya "Long Count" inscriptions (44,45). Well-dated sediment cores from Lakes Chichancanab (50)and Punta Laguna (51) (central Yucatán, Mexico)document an abrupt onset of more arid conditions spanning ~200 years between 800-1000 AD, as evidenced by more evaporative (higher) 18O values and increases in gypsum precipitation (elevated sulphur content). A century-long dry period coincides with the "Maya Hiatus " centered near 580 AD which documents a period (530-650 AD) of marked curtained monument construction (44,45,48). Windborne particle concentrations from the annually-dated Quelccaya ice core in the Peruvian Altiplano are also shown (53).

The densely populated southern lowlands of the Yucatán peninsula relied heavily on surface water supplies for human and agricultural needs, and it was these regions that were most acutely affected during the 800 to 1000 AD drought. Archeological excavations estimate that lowland population densities decreased from about 200 persons/km2 at the peak of the Late Classic period to less than 100 persons/km2 by ~900 AD; by 1500 AD, many watersheds had been completely abandoned (48). An additional dry period predating the collapse was noted at 580 AD in the higher resolution core record from Lake Punta Laguna (51)(Fig. 4).This century-scale dry period coincides with the Maya Hiatus at the Early/Late Classic Maya boundary, when monument construction was briefly curtailed (from ca. 530 to 650 AD) (44,45,48).

Moche IV-V Transformation (Coastal Peru, ca. 1500 years BP)
Precolumbian coastal and highland Peruvian civilizations offer exceptional insight into past linkages between culture and climate change because they sustained densely populated, complex, agrarian cultures in very challenging environments. The Peruvian coast is extremely arid and requires high reliance upon irrigation to sustain agriculture, yet these regions sustained large populations for many centuries.

Known for their sophisticated metallurgy and monumental adobe brick structures, the Mochica polity established urban centers and controlled the entire northern Peruvian coastline south of the Sechura desert from about 300 to 500 AD (early Moche IV period) (52).One such locality, the capital site of Moche, boasts the largest adobe structure in the New World, the Huaca del Sol (52).This immense coastal site and the cities it served were very abruptly abandoned near 600 AD. Archeological evidence shows that main irrigation channels had been overrun by sand dunes at the time of abandonment. The subsequent Moche V culture was reestablished inland, near the confluence of highland rivers draining the Andean foothills where runoff was more dependable, between 600 and 750 AD. The so-called Moche IV-V Transformation was unprecedented in scope, scale, and rapidity (52).

Figure 7. Peruvian coastal, highland, and altiplano paleoclimate and the Moche IV-V Transformation and Tiwanaku collapse. The Mochica civilization imperialized the desert coast of Peru between 300-500 AD (Moche IV period), but then abruptly abandoned coastal urban centers and moved to better-watered highland valleys between 600-750 AD (Moche V period) (52). The Moche IV-V transformation was evidently just one of a series of oscillatory population migrations between the arid coast and the more humid highlands (54). The annually-dated Quelccaya ice core in southern Peru documents large changes in regional climate spanning the last 1500 years (53), notably multicentury shifts in precipitation that coincide with the coastal-highland settlement dislocations. On the shores of Lake Titicaca in the Peruvian altiplano, the Tiwanaku civilization established large urban centers and populations between 300BC and 1100 AD. Tiwanaku urban centers and cultivation fields were abruptly abandoned after 1100 AD, and the Tiwanaku culture had completely collapsed by ca. 1400 AD (55,56). The Tiwanaku abandonment and collapse coincides with a multicentury interval of reduced precipitation based on the ice accumulation record at Quelccaya, only ca. 200 km southwest of Lake Titicaca (56).

An annual resolution record of regional precipitation changes from the Quelccaya ice core (Peru) firmly implicates climate change as a leading factor underlying the Moche IV-V transformation (52). Variations in oxygen isotopes, accumulation rate, and insoluble particle concentration in this ice core document large changes in regional climate spanning the last 1500 years, which can be used to place the cultural records within their paleoclimatic contexts (52,53). Comparison of the paleoclimatic and cultural histories indicates that the Moche IV-V Transformation near 600 AD was immediately preceded by a ~30 year period of reduced regional precipitation (lower ice accumulation between 563 and 594 AD) and corresponded with a ~60 year interval of increased windborne particles in the ice (Figure 5 ).The loss of Moche IV coastal irrigation channels to encroaching sand dunes and the population migration to the better watered highland valleys are consistent responses to the enhanced regional aridity indicated by the ice core (52). Paulsen (54) recognized several coastal-highland population shifts throughout the first and second millennium AD (Fig. 5), noting a general seesaw relationship between the rise and fall of coastal and highland agrarian cultures in both Peru and Ecuador. As discussed by Thompson et al. (53),these ancient coastal-highland population shifts closely corresponded with the largest ice accumulation (precipitation)changes recorded in the Quelccaya ice core record (Fig. 5). Of particular paleoclimatic interest is the evidently synchronous onset of arid conditions in the tropics of both hemispheres (Peru and the Yucatán) near 900 and 600 AD (51)(Figs. 4 and 5).

The Tiwanaku culture thrived for nearly 1500 years (300 BC to 1100 AD) in urban and rural agrarian settings surrounding Lake Titicaca in the southern Peruvian altiplano (~4000m elevation) (55,56).Through the ingenious use of raised field cultivation, which promotes efficient nutrient recycling and uses irrigation canals to thermally buffers crops against killing frosts, the Tiwanaku were able to sustain an urban complex with an estimated population of nearly half a million (55). The massive urban center at Lake Titicaca served as the capital of an expanding state society that eventually exploited regions extending to the Peruvian coastal desert and foothills.

The densely settled Tiwanaku urban centers and raised fields were abandoned abruptly near 1100 AD (55, 56). Full collapse of the Tiwanaku state occurred over the 12th to 15th centuries. The Quelccaya ice core was drilled just 200 km northeast of Lake Titicaca (53)and thus provides valuable insight into the paleoclimatic contexts of the Tiwanaku abandonment and collapse. Comparison of the Tiwanaku cultural changes with the Quelccaya isotopic and ice accumulation records shows a close coincidence between the abandonment and the onset of increasingly arid conditions (lower ice accumulation rate) (Fig. 5). Conditions markedly drier than today persisted for several centuries, commencing after 1040 AD (53). Sediment cores from Lake Titicaca document a ~10m drop in lake level at this time (56). It has been proposed that the sudden onset and multicentury persistence of more arid conditions would have dramatically impacted the productivity of the raised field agriculture system and, consequently, its ability to sustain swelling Tiwanaku urban and rural populations (53,55,56).

Past Cultural Responses to Climate Change

What can be learned from these ancient cultural responses to prolonged drought? The climatic perturbations associated with these societal dislocations were extreme in their duration and intensity, far surpassing droughts recorded during the modern instrumental period. As shown in Fig. 1A, interannual droughts occur many times within a given generation, and decadal droughts recur infrequently across many generations. Multidecadal to multicentury scale droughts are much rarer but are nonetheless integral components of natural climate variability. Well-dated and detailed paleoclimate records from climatically-sensitive locations bear witness to the occurrence and severity these multidecadal to multicentury droughts (Figs. 2 to 4).

For the examples discussed above, available paleoclimate and archeological data show that societal collapse and prolonged drought were coincident within respective dating uncertainties. Coincidence alone cannot demonstrate causality; indeed, each of these cultural collapses had been at one time interpreted solely in terms of human factors unrelated to natural climate variability such as warfare, overpopulation, deforestation, and resource depletion. However,joint interpretation of the paleoclimatic and archeological evidence now underscores the highly significant role of persistent, long-term drought in the collapse of the Akkadian (37,39,40), Maya (46,47,48,50), Moche (52,53),and Tiwaniku (53,55,56) civilizations. These examples show that challenged by unprecedented environmental stresses, cultures can shift to lower subsistence levels by reducing social complexity, abandoning urban centers, and reorganizing systems of supply and production (39).

Recalling James Hutton 's uniformitarian premise, what makes these ancient events so relevant to modern times is that they simultaneously document both the resilience and vulnerability of large, complex civilizations to environmental variability. Complex societies are neither powerless pawns nor infinitely adaptive to climate variability. As with modern cultures, the ancients adapted to and thrived in marginal environments with large interannual climate variability. As with ancient cultures, modern civilizations (regrettably) gauge their readiness to face future climate extremes based on what is known from historical (oral or instrumental) records. What separates these ancient cultures from ourselves is that they alone have witnessed the onset and persistence of unprecedented drought that continued for many decades to centuries. Efforts to understand past cultural responses to large and persistent climate changes may prove instructive for assessing modern societal preparedness for a changing and uncertain future (57).

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