The East Asian summer monsoon, the Indian summer monsoon, and the midlatitude westerlies at 4.2 ka BP

The Delingha stable isotope tree-ring record (1) provides exquisite precision and accuracy measurement for the Holocene paleoclimate proxies of the northeast Tibetan plateau, where 90% of annual precipitation now derives from the East Asian summer monsoon (EASM) (2). But does the Delingha δ¹⁸O_dendro record correspond to other EASM (3) and globally distributed records for the 4.2 ka BP (∼2200 BCE) megadrought event (4), or does Delingha δ¹⁸O_dendro record a different climate event?

A trend-point analysis of the Delingha δ¹⁸O_dendro record defines a drying trend that “intensified between ∼2000 and ∼1500 BCE” and “thus arguably marks the transition from the mid- to the late Holocene Asian moisture regime” (1). The authors of the analysis conclude that their findings do “not support a significant transition in the hydroclimate … around ∼2200 BCE during the so-called ‘4.2-ka event’ … nor the notion that this rapid climate deterioration and associated global-scale megadroughts should be regarded as a generalized climatic transition from the mid- to late Holocene” (1).

The Delingha megadrought is, however, precisely coincident with the Mawmluh Cave, India, KM-A speleothem's δ¹⁸O four stages that define the 4.2 ka BP event’s global type stratum (4), and only differs in its first-stage magnitude (Fig. 1A). That is, the Delingha trend-point analysis’s cutoffs mark the abrupt high-magnitude δ¹⁸O increase at 2095 BCE, but not the event’s first stage more than 100 y earlier. The Delingha record is also congruent with the Hulun Lake, eastern Mongolia plateau 4.2 ka BP abrupt desertification record and the north China EASM 4.2 ka BP event δ¹⁸O record in the Dongshiya Cave speleothem (ref. 5 and Fig. 1B).

Fig. 1.

Four-stage 4.2 ka BP paleoclimate proxies compared to KM-A speleothem δ¹⁸O, Mawmluh Cave, northeast India (4): (A) Delingha tree rings (1), (B) Dongshiya Cave speleothem (5), (C) Gol-e Zard Cave speleothem (7), (D) Mt. Logan ice core (8).

The KM-A speleothem record, an Indian summer monsoon record, is precisely congruent with the Katlekhor Cave and Gol-e Zard Cave speleothems, western Iran, that document the midlatitude westerlies' 4.2 ka BP event (ref. 6 and Fig. 1C). The KM-A record is also precisely congruent with the Mount Logan glacial record for displacement of the Pacific Kuroshio Current at 4.2 ka BP (ref. 7 and Fig. 1D).

In the eastern hemisphere, this 4.2 ka BP event record extends from Spain to China, from north to south Africa, to Australia, and to more than 50 subpolar North Atlantic proxies (8). In the western hemisphere, the record extends across North America from Wyoming to Massachusetts, down the west coast of South America to Patagonia and Antarctica, and along the Atlantic coast to Brazil (9).

The societal collapse records of adaptive regional abandonments and habitat tracking synchronous with the 4.2 ka BP megadroughts extend from Spain to Mesopotamia, the Nile River and the Indus Valley, and Tibetan Plateau and China. The latter include the flooding and megadrought Liangzhu abandonments in the lower Yangtse delta and the megadrought late Longshan Haidai abandonments in modern Shandong province (10). However, megadrought proxy transfer functions and high-resolution spatiotemporal quantification of regional settlement and abandonment remain desiderata for explaining the societal adaptations to the global 4.2 ka BP event.