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Today the desert margins of northwest India are dry and unable to support large populations, but were densely occupied by the populations of the Indus Civilization during the middle to late Holocene. The hydroclimatic conditions under which Indus urbanization took place, which was marked by a period of expanded settlement into the Thar Desert margins, remains poorly understood. We measured the isotopic values (δ18O and δD) of gypsum hydration water in paleolake Karsandi sediments in northern Rajasthan to infer past changes in lake hydrology, which is sensitive to changing amounts of precipitation and evaporation. Our record reveals that relatively wet conditions prevailed at the northern edge of Rajasthan from ~5.1 ± 0.2 ka BP, during the beginning of the agricultural-based Early Harappan phase of the Indus Civilization. Monsoon rainfall intensified further between 5.0 and 4.4 ka BP, during the period when Indus urban centres developed in the western Thar Desert margin and on the plains of Haryana to its north. Drier conditions set in sometime after 4.4 ka BP, and by ~3.9 ka BP an eastward shift of populations had occurred. Our findings provide evidence that climate change was associated with both the expansion and contraction of Indus urbanism along the desert margin in northwest India.

Abstract. The plains of northwest South Asia receive rainfall during both the Indian summer (June–September) and winter (December–March) monsoon. Researchers have long attempted to deconstruct the influence of these precipitation regimes in paleoclimate records, in order to better understand regional climatic drivers and their potential impact on human populations. The mid–late Holocene transition between 5.3 and 3.3 ka is of particular interest in this region because it spans the period of the Indus Civilization from its early development, through its urbanization, and onto eventual transformation into a rural society. An oxygen isotope record of the surface-dwelling planktonic foraminifer Globigerinoides ruber from the northeast Arabian Sea provided evidence for an abrupt decrease in rainfall and reduction in Indus River discharge at 4.2 ka, which the authors linked to the decline in the urban phase of the Indus Civilization (Staubwasser et al., 2003). Given the importance of this study, we used the same core (63KA) to measure the oxygen isotope profiles of two other foraminifer species at decadal resolution over the interval from 5.4 to 3.0 ka and to replicate a larger size fraction of G. ruber than measured previously. By selecting both thermocline-dwelling (Neogloboquadrina dutertrei) and shallow-dwelling (Globigerinoides sacculifer) species, we provide enhanced detail of the climatic changes that occurred over this crucial time interval. We found evidence for a period of increased surface water mixing, which we suggest was related to a strengthened winter monsoon with a peak intensity over 200 years from 4.5 to 4.3 ka. The time of greatest change occurred at 4.1 ka when both the summer and winter monsoon weakened, resulting in a reduction in rainfall in the Indus region. The earliest phase of the urban Mature Harappan period coincided with the period of inferred stronger winter monsoon between 4.5 and 4.3 ka, whereas the end of the urbanized phase occurred some time after the decrease in both the summer and winter monsoon strength by 4.1 ka. Our findings provide evidence that the initial growth of large Indus urban centers coincided with increased winter rainfall, whereas the contraction of urbanism and change in subsistence strategies followed a reduction in rainfall of both seasons.

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The start and end of the urban phase of the Indus civilization (IC; c. 2500 to 1900 BC) are often linked with climate change, specifically regarding trends in the intensity of summer and winter precipitation and its effect on the productivity of local food economies. The Indus Village is a modular agent-based model designed as a heuristic “sandbox” to investigate how IC farmers could cope with diverse and changing environments and how climate change could impact the local and regional food production levels required for maintaining urban centers. The complete model includes dedicated submodels about weather, topography, soil properties, crop dynamics, food storage and exchange, nutrition, demography, and farming decision-making. In this paper, however, we focus on presenting the parts required for generating crop dynamics, including the submodels involved (weather, soil water, land, and crop models) and how they are combined progressively to form two integrated models (land water and land crop models). Furthermore, we describe and discuss the results of six simulation experiments, which highlight the roles of seasonality, topography, and crop diversity in understanding the potential impact of environmental variability, including climate change, in IC food economies. We conclude by discussing a broader consideration of risk and risk mitigation strategies in ancient agriculture and potential implications to the sustainability of the IC urban centres.

The 4.2-kiloyear event has been described as a global megadrought that transformed multiple Bronze Age complex societies, including the Indus Civilization, located in a sensitive transition zone with a bimodal (summer and winter) rainfall regime. Here we reconstruct changes in summer and winter rainfall from trace elements and oxygen, carbon, and calcium isotopes of a speleothem from Dharamjali Cave in the Himalaya spanning 4.2–3.1 thousand years ago. We find a 230-year period of increased summer and winter drought frequency between 4.2 and 3.97 thousand years ago, with multi-decadal aridity events centered on 4.19, 4.11, and 4.02 thousand years ago. The sub-annually resolved record puts seasonal variability on a human decision-making timescale, and shows that repeated intensely dry periods spanned multiple generations. The record highlights the deficits in winter and summer rainfall during the urban phase of the Indus Civilization, which prompted adaptation through flexible, self-reliant, and drought-resistant agricultural strategies.

Abstract. The plains of northwest South Asia receive rainfall during both the Indian Summer (June–September) and Winter (December–March) Monsoon. Researchers have long attempted to deconstruct the influence of both precipitation regimes in paleoclimate records, in order to better understand regional climatic drivers and their potential impact on human populations. The Mid–Late Holocene transition between 5.3–3.3 ka BP is of particular interest in this region because it spans the period of the Indus Civilization from its early development to eventual transformation. The oxygen isotope record of the surface-dwelling planktonic foraminifer Globigerinoides ruber from the northeast Arabian Sea provided evidence for an abrupt decrease in rainfall and reduction in Indus River discharge at 4.2 ka BP, which the authors linked to the decline of the urban phase of the Indus Civilization (Staubwasser et al., 2003). Given the importance of this study, we used the same core (63KA) to replicate the oxygen isotope profiles of a larger size fraction of G. ruber than measured previously and, in addition, we measured two other foraminifer species at decadal resolution over the interval from 5.4 to 3.0 ka BP. By selecting both thermocline-dwelling (Neogloboquadrina dutertrei) and shallow-dwelling (Globigerinoides sacculifer) species, we provide enhanced detail of the climatic changes that occurred over this crucial time interval. We found evidence for a period of increased surface water mixing, which we suggest was related to a strengthened winter monsoon with a peak intensity over 200 years from 4.5 to 4.3 ka BP. The time of greatest change occurred at 4.1 ka BP when both the summer and winter monsoon weakened, resulting in a reduction in rainfall in the Indus region. The earliest phase of the Mature Harappan period coincided with the period of inferred stronger winter monsoon between 4.5–4.3 ka BP, whereas the end of the urbanized phase followed the decrease in both the summer and winter monsoon strength by 4.1 ka BP. Our findings provide evidence that the initial growth of large Indus urban centers was coincident with increased winter rainfall, whereas the contraction of urbanism and change in subsistence strategies followed a reduction in rainfall of both seasons.