Europe British Isles In
Ireland, there is little definitive record of the 4.2 ka event outside of a brief isotopic excursion in some cave speleothem records. The manner in which this climatic event manifested itself in the region is thus unclear. In Great Britain as in Ireland, the nature of the 4.2 ka event is ambiguous and unclear. The
yew tree's abundance declined in eastern England.
Central Europe Analysis of sediments from Lake Spore reveals that in
Poland, winters became colder between 4250 and 4000 BP, with this cooling likely responsible for a
podzolisation (generation of boreal forest soil type) event around 4200 BP, whereas summer temperatures remained constant. Humidity levels were not affected by the 4.2 ka event.
Iberian Peninsula On the
Iberian Peninsula, in general the climate between 2800 and 1100 cal BC is quite stable and relatively humid. A reconstruction of precipitation shows two rapid, pronounced dry phases from 2350 to 2200 cal BC (4.3 - 4.15 ka BP) and from 2100 to 2000 cal BC (4.05 - 3.95 ka BP). The dry phases were followed by a shift towards wetter conditions, suggesting a more complex pattern of climate change than other regions during the 4.2 ka event. On the entire Iberian Peninsula, there is a slight decrease in settlement activity from 2500 cal BC, followed by a significant decline between 2300 and 2100 cal BC. In the southeast, and especially in the Evora region, a collapse of settlement activity has been documented, and in the following centuries settlement remained at a low level. The other is the El Argar phenomenon, which began to flourish at around the same time, although there was initially a slight collapse, too. However, it soon stabilised and was not negatively affected by the second dry period. It is conceivable, although not perfectly clear as M. Hinz and his colleagues stress, that the two developments of decreasing settlements in the west of the Iberian Peninsula and increasing settlement activities in the east are linked.
Italian Peninsula In the
Gulf of Genoa, mean annual temperature dropped, winters became drier, and summers became wetter and cooler, a phenomenon most likely caused by the southward retreat of the ITCZ in summer that weakened the high pressure and reduced ocean warming over the western Mediterranean, which led to retarded evaporation rates in the autumn and early winter. The 4.2 ka event appears to have wettened the climate in the
Alps. Lake Petit saw increased precipitation during the ice-free season, evidenced by an increase in δ18Odiatom. Southern Italy, in contrast, experienced intense aridification.
North Africa At the site of Sidi Ali in the
Middle Atlas, δ18O values indicate not a dry spell but a centennial-scale period of cooler and more humid climate. In c. 2150 BC, Egypt was hit by a series of exceptionally low
Nile floods that may have influenced the collapse of the centralised government of the
Old Kingdom after a famine.
Middle East The south-central
Levant experienced two phases of dry climate punctuated by a wet interval in between and thus the 4.2 ka event in the region has been termed a W-shaped event. Enhanced dust flux coeval with
δ18O peaks is recorded in
Mesopotamia from 4260 to 3970 BP, reflecting intense aridity. The aridification of Mesopotamia may have been related to the onset of cooler sea-surface temperatures in the
North Atlantic (Bond event 3), as analysis of the modern instrumental record shows that large (50%) interannual reductions in Mesopotamian water supply result when subpolar northwest Atlantic
sea surface temperatures are anomalously cool. The headwaters of the
Tigris and
Euphrates rivers are fed by elevation-induced capture of winter
Mediterranean rainfall. The
Akkadian Empire in 2300 BC was the second civilization to subsume independent societies into a single state (the first being ancient Egypt around 3100 BC). It has been claimed that the collapse of the state was influenced by a wide-ranging, centuries-long drought. Archaeological evidence documents widespread abandonment of the agricultural plains of northern Mesopotamia and dramatic influxes of refugees into southern Mesopotamia, around 2170 BC, which may have weakened the Akkadian state. A 180-km-long wall, the "Repeller of the
Amorites", was built across central Mesopotamia to stem nomadic incursions to the south. Around 2150 BC, the
Gutian people, who originally inhabited the
Zagros Mountains, defeated the demoralised Akkadian army, took
Akkad and destroyed it around 2115 BC. Widespread agricultural change in the
Near East is visible at the end of the 3rd millennium BC. Weiss suggests a figure of 300,000 displaced from the zone of uncertainty, Resettlement of the northern plains by smaller sedentary populations occurred near 1900 BC, three centuries after the collapse.
South and Central Asia The
Siberian High increased in area and magnitude, which blocked moisture-carrying westerly winds, causing intense aridity in
Central Asia. The Indian Summer Monsoon (ISM) and Indian Winter Monsoon (IWM) both declined in strength, leading to highly arid conditions in northwestern South Asia. The ISM's decline is evident from low Mn/Ti and Mn/Fe values in
Rara Lake from this time. The area around PankangTeng Tso Lake in the
Tawang district of
Arunachal Pradesh had cold and dry conditions and was dominated by subalpine vegetation. Though some proxy records suggest a prolonged, multicentennial dry period, others indicate that the 4.2 ka event was a series of multidecadal droughts instead.
Effects on the Indus Valley civilization In the 2nd millennium BC, widespread aridification occurred in the
Eurasian steppes and in
South Asia. On the steppes, the vegetation changed, driving "higher mobility and transition to the nomadic cattle breeding." , many scholars believed that drought and a decline in trade with Egypt and Mesopotamia caused the collapse of the Indus civilization. The
Ghaggar-Hakra system was rain-fed, and water supply depended on the
monsoons. The
Indus Valley climate grew significantly cooler and drier from around 1800 BC, which is linked to a contemporary general weakening of the monsoon. leading to erratic and less-extensive floods, which made inundation agriculture less sustainable. Aridification reduced the water supply enough to cause the civilization's demise, and to scatter its population eastward.
East Asia The 4.2 ka event resulted in an enormous reduction in the strength of the East Asian Summer Monsoon (EASM). the cooling of North Atlantic waters led to retardation of northward movements of the EASM and diminished rainfall on its northern margin. A stark humidity gradient emerged between northern and southern China because of the EASM's southward move. Northeastern China was strongly affected; proxy records from
Hulun Lake in
Inner Mongolia reveal a major dry event from 4210–3840 BP, δ18O values from Yonglu Cave in
Hubei confirm that the region became characterised by increased aridity and show that the onset of the event was gradual but that its end was sudden. In the Luoyang Basin, the 4.2 ka event was much less strongly felt; precipitation was low, but not extremely so. In central China, precipitation increased, meanwhile. In the Korean Peninsula, the 4.2 ka event was associated with significant aridification, measured by the large decline in arboreal pollen percentage (AP). On
Jeju Island, in contrast, the climate was humid, as evidenced by the presence of tychoplanktonic species found in the Sara-oreum and Muljangori-oreum wetlands. The
Sannai-Maruyama site in Japan declined during the same period; the growing
population of the
Jomon culture gradually turned to decline after that.
Rebun Island experienced an abrupt, intense cooling around 4,130 BP believed to be associated with the 4.2 ka event.
Effects on Chinese civilization The drought may have caused the collapse of
Neolithic cultures around Central China in the late 3rd millennium BC, or at least partially contributed to it. In the
Yishu River Basin (a river basin that consists of the Yi River (
沂河) of Shandong and
Shu River), the flourishing
Longshan culture was affected by a cooling that severely reduced rice output and led to a substantial decrease in population and to fewer archaeological sites. Around 2000 BC, Longshan was displaced by the
Yueshi culture, which had fewer and less-sophisticated artifacts of ceramic and bronze. The
Liangzhu civilization in the lower reaches of the Yangtze River also declined during the same period. The 4.2 ka event is also believed to have helped collapse the
Dawenkou culture. The Longshan culture of the less strongly affected Luoyang Basin, however, continued to develop and thrive.
Southeast Asia The 4.2 ka event substantially reduced ENSO variability in
Borneo, as evidenced by stalagmite
δ18O values. The reduction in ENSO variability that occurred was comparable only to the earlier 8.2 ka event.
Southern Africa Stalagmites from northeastern
Namibia demonstrate the region became wetter thanks to the southward shift of the ITCZ. The Namibian humidification event had two pulses.
Mascarenes No signal of the 4.2 ka event has been found in
Rodrigues. ==See also==