End of Green Sahara amplified mid- to late Holocene megadroughts in mainland Southeast Asia

Abstract

Between 5 and 4 thousand years ago, crippling megadroughts led to the disruption of ancient civilizations across parts of Africa and Asia, yet the extent of these climate extremes in mainland Southeast Asia (MSEA) has never been defined. This is despite archeological evidence showing a shift in human settlement patterns across the region during this period. We report evidence from stalagmite climate records indicating a major decrease of monsoon rainfall in MSEA during the mid- to late Holocene, coincident with African monsoon failure during the end of the Green Sahara. Through a set of modeling experiments, we show that reduced vegetation and increased dust loads during the Green Sahara termination shifted the Walker circulation eastward and cooled the Indian Ocean, causing a reduction in monsoon rainfall in MSEA. Our results indicate that vegetation-dust climate feedbacks from Sahara drying may have been the catalyst for societal shifts in MSEA via ocean-atmospheric teleconnections.

Fig. 1. Location of Tham Doun Mai cave (diamond) and other climate proxy sites mentioned in the text.

Locations of climate proxy records across East Asia showing relative hydroclimate changes at ≈4 ka as inferred from the synthesis in Supplementary Table 1. Background shading shows Global Precipitation Climatology Project (GPCP) average June-September (JJAS) rainfall anomalies during the large 1982/1983, 1998/1999, and 2015/2016 El Niño events.

Fig. 2. The multiproxy hydroclimate record from northern Laos and cultural shifts in mainland Southeast Asia for the Holocene.

a δ¹⁸O and b δ¹³C for stalagmites TM4 (cyan), TM5 (blue), and TM11 (orange), where values are expressed in per mill (‰) relative to Vienna Peedee Belemnite (V-PDB). Black line represents the composite record constructed by averaging data for the periods of overlap of each stalagmite record. Each record was interpolated to a common 10-year interval prior to averaging. Gray shading indicates the standard deviation for periods of overlap. For sections of the record with no overlap, the average standard deviation for overlapping periods was used. Yellow curve in panel a shows changepoints calculated using a Bayesian change-point algorithm that employs a probabilistic least-squares method to identify significant regime shifts. Color-coded circles indicate times when the various speleothems stopped and started growing. Prior to averaging, the δ¹⁸O curve of TM11 was offset by −0.6‰. c Mg/Ca and ¹⁴C-inferred dead carbon proportion (DCP) for stalagmite TM5. d Approximate timing of lifestyle changes in mainland Southeast Asia during the Holocene.

Fig. 3. Mainland southeast Asian hydroclimate and end of the Green Sahara.

a Northern Laos composite δ¹³C record (black line) and 1σ uncertainty (gray shading) from Tham Doun Mai speleothems. b–d δD_wax records from marine core P178–15P (Gulf of Aden), Lake Challa, and Lake Tanganyika. e Percent dolomite (light purple line) and carbonate (pink line) (expressed as standardized Z-scores) from the Gulf of Oman. Also shown is the dust record (aquamarine line) from Mt. Kilimanjaro ice core KNIF3. f Ca/Ti record of dust deposition in the Nile Delta. g Color-coded (cyan: Lake Challa; orange: Lake Tanganyika; black: Tham Doun Mai; blue: Gulf of Aden) probability density function (PDF) output from the Bayesian change-point algorithm. Vertical color bar indicates the transition from a wet to a dry Sahara between 5.5 and 3.5 ka^,.

Fig. 4. Mainland southeast Asian hydroclimate and ENSO variability.

a Northern Laos composite δ¹³C record (black line) and 1σ uncertainty (gray shading) from Tham Doun Mai speleothems. b, c Mg/Ca- and alkenone-inferred sea-surface temperature (SST) records from western Pacific marine cores MD76 and MD06–3040, respectively. d δ¹⁸O variance (var.) of individual G. ruber planktonic foraminifera from core V21–30 (blue triangles; eastern Pacific) and relative ENSO variance changes inferred from fossil coral δ¹⁸O [calculated from sliding 30-yr windows of the standard deviation (stdev) of the 2- to 7-year band, and plotted as percent (%) differences from 1968–1998 C.E. intervals of modern coral δ¹⁸O] in Fanning Island and Christmas Island (red circles) located in the central Pacific. Dashed lines show 6^th order polynomial regression. e El Junco (Galapagos) δD of botryococcenes (bot), interpreted to reflect shifts in ENSO variance. f Bulk titanium content of marine sediments from ODP site 1002 where lower values indicate drier conditions typical of El Niño events. Vertical color bar indicates the transition from a wet to a dry Sahara between 5.5 and 3.5 ka^,.

Fig. 5. Changes in precipitation, temperature, and Walker circulation between dry and wet Sahara.

a Changes in precipitation, b surface temperature, and c zonal stream function of the Walker circulation (contour lines indicate PI climatology with a contour interval of 2 × 10¹⁰Kgs⁻¹ from −14 to 14 Kgs⁻¹; 0 line in bold) for JJAS in the MH_PMIP simulation relative to MH_GS+RD. Shaded regions indicate significant values at the 95% level using a two-sided t-test. Red circles in panel a denote published proxy records showing a drying trend between 5 and 4 ka: 1, West African Margin; 2, Buca della Ranella; 3, Nile River Delta; 4, Gulf of Aden; 5, Gol-e Zard; 6, Gulf of Oman; 7, Mawmluh Cave; 8, Tham Doun Mai (this study).

Fig. 6. Seasonal shifts in the westerlies and precipitation across East Asia between dry and wet Sahara.

a, b Hovmöller diagrams of climatological U200 winds and precipitation for wet Sahara and its anomalies (contours: MH_GS+RD climatology; shading: difference between MH_PMIP and MH_GS+RD experiments). c Support for the model simulations is provided by the paleoclimate archives, which show drier conditions in North China (red) due to a southward shift in the westerlies (blue; inferred from ESR intensity of silt-sized quartz grains in sediments from the Japan Sea) and equatorward contraction of the ITCZ during boreal summer (black; this study) and austral summer (brown). Vertical color bar indicates the transition from a wet to a dry Sahara between 5.5 and 3.5 ka^,.