The 4.2 ka event in the Northern Hemisphere: Spatial heterogeneity and driving mechanisms of hydroclimatic change

The 4.2 ka event (~4300–3900 yr BP), marking the boundary of the middle-late Holocene and the onset of the Meghalayan stage, is traditionally associated with global megadroughts and significant social changes. However, debates continue with respect to its spatial distribution (worldwide versus regional), hydroclimatic change (drying versus wetting), and driving mechanism (North Atlantic versus Pacific forcing). This study established a new dataset including 200 high-quality Northern Hemisphere records focused on the 4.2 ka event, selected based on chronological control points, temporal resolution, and proxy significance. According to the amplitude and duration of hydroclimatic change of individual records within the 4.2 ka event, 130 of the 200 records experienced hydroclimatic excursions. While slight overall drying (73 out of 130 records) is observed in the Northern Hemisphere, evident spatial heterogeneity of hydroclimatic change is detected, with North-South dipole patterns over the East Asian monsoon and Europe-circum Mediterranean regions, as well as an East-West dipole pattern over North America. This emphasizes that drying and wetting both represent hydroclimatic characteristics of the 4.2 ka event. Furthermore, this study assessed the intensity of hydroclimatic change during the 4.2 ka event across different regions in the Northern Hemisphere for the first time, which weakened from western to eastern Eurasia. Given that the Pacific forcing shifted only after the onset of the 4.2 ka event, we propose that the event was initially triggered by North Atlantic forcing, with Pacific forcing subsequently amplifying its impact. These two forcings jointly contributed to the pronounced spatial heterogeneity of hydroclimatic change during the event, highlighting a fundamental characteristic of hydroclimatic variability. Our results significantly differ from the conventional perspective of a uniform “low-latitude drought” or “global megadroughts”, highlighting the importance of hydroclimatic spatial heterogeneity in understanding the characteristics, mechanisms, and influence on civilizations.