The 4.2 ka BP event: global rearrangement of ocean-atmosphere interaction patterns forced Eastern Hemisphere societal collapses

Consequent to excessive melt water discharge from Greenland forced by higher summer temperatures of the late Holocene Thermal Maximum, enhanced export of Labrador Current low-salinity waters into the Northwest Atlantic peaked ca. 4.2 ka BP, suppressing ocean deep convection and northward ocean heat transport, which implies changes in the Atlantic Meridional Overturning Circulation system.  At around 4.2 ka BP southward migration of the North Atlantic atmospheric Polar Front and the associated storm belt occurred in parallel with insolation-forced southward shift of the ITCZ and subsequent Southern Ocean warming. In the tropics, this rearrangement of global ocean-atmosphere interaction patterns led to temporarily reduced ENSO variability with a dominant La Nina regime. The marine and continental paleoclimate records are supported by present-day ENSO-La Nina climate features and teleconnections.

In the Eastern Hemisphere, the abrupt ~30% megadrought reductions of midlatitude westerlies’ precipitation coinciding with cooling and dust events, documented at decadal resolution, caused the cascading agro-production crises that led to adaptive societal collapses, regional abandonments, and refugia habitat tracking, from Chalcolithic Iberia to Early Bronze Greece and Levant, Akkadian Empire Mesopotamia, and Iran, beginning ~2250 - 2200 BCE. The synchronous four-phased Indian Summer Monsoon (ISM) megadroughts, known from congruent, sub-decadal/decadal precision East and West India lake sediment, marine and speleothem cores, forced adaptive abandonment of the five Harappan cities and habitat tracking eastward beginning ~2200 BCE. The abruptly reduced ISM Ethiopian precipitation, source of Nile River flow, and 4.2 ka BP megadroughts documented at tropical Lake Teli and Lake Turkana, caused abandonment of Old Kingdom Egypt delta settlement and seasonal riverine inundation lands, with violent collapse into First Intermediate Period polities ~2250 BCE. In the West Pacific, the Kuroshio Current was weakened by abrupt La Nina onset documented in Japan marine cores and in the Pacific Northwest, with sub-decadal resolution, in the Mt Logan ice core at 4.2 ka BP. Simultaneous reduction of the East Asian Summer Monsoon forced regional Longshan settlement collapses along the Yangtze River delta and in the East Haidai regions. Tropical Australia synchronously experienced megadrought documented at KNI-51 Cave and Black Springs, Kimberley.

Disruption of the North American Monsoon forced the 4.2 - 3.9 ka BP megadroughts observed in lake sediment cores from Idaho to Massachusetts and Maine.  The precipitation patterns of South America were disrupted from north to south at 4.2 ka BP, from tropical Andean Ecuador to Lake Titicaca and to southern Chile.  The disruptions included the Humboldt Current along the west coast, the continental monsoon along Brazil’s east coast, the ITCZ, and the South Atlantic Convergence Zone that crosses Brazil. The Patagonian series of abrupt 4.2 ka BP dry-cold events extended south to the Marguerite Bay, Antarctica, abrupt transition to colder, icier conditions. The cold snap at King George Island was followed by a 4.2 - 3.7 ka BP warm period. A wet west and dry east Andes are suggested by high resolution 4.2 ka BP event proxies and include the Patagonian 4.2 ka BP volcanic eruptions coeval with Hekla 4, Iceland, and Avellino, Italy.