Precipitation Reconstruction in the Asian Monsoon Region During the 4.2 ka Event With Paleoclimate Data Assimilation

For a long time, the 4.2 ka event was considered sufficiently significant to mark the onset of the Meghalayan Age, the latest stage of the Holocene. However, the spatial distribution and temporal evolutions of precipitation changes remain controversial in many regions, especially in monsoon Asia. In this study, combining the high‐resolution speleothem calcite oxygen isotope ratio value (δ 18 O c ) records and the transient simulation of an isotope enabled earth system model, we reconstruct the spatiotemporal evolution of δ 18 O c and precipitation in the Asian monsoon region during the 4.2 ka event (4.3–3.9 ka BP) with 10‐year temporal resolution and 2.5° × 1.9° spatial resolution, using an offline ensemble Kalman filter. Based on these reconstructed results, we analyze the characteristics of the 4.2 ka event over monsoon Asia from two aspects. On the centennial scale, the event shows a drought pattern in northern China and South Asia, and a wet pattern in southern China. On the multi‐decadal scale, frequent wet‐dry excursions occur in the region. Notably, both the centennial‐scale changes and multi‐decadal excursions largely fall within the range of Holocene natural variability. These results challenge the conventional view of the 4.2 ka event as a global megadrought and instead suggest that, even within monsoon Asia, it was expressed as a spatially heterogeneous hydroclimatic anomaly rather than a spatially uniform or exceptionally extreme event.