Possible reverse trend in Asian summer monsoon strength during the late Holocene

Abstract

Holocene climate change is characterized as generally cooling in high latitudes and drying in tropical and Asian summer monsoonal regions, following the gradual decrease in northern hemisphere summer insolation over the last 12,000years. However, some recent high-resolution, well-dated monsoon reconstructions seem to suggest an abnormal increase in Asian summer monsoon strength during the late Holocene, against the generally weakening Holocene trend. Here, we synthesize marine and terrestrial moisture records from Asian monsoonal regions that span most of the Holocene period. Late Holocene strengthening of Asian summer monsoon identified from a wealth of the synthesized monsoon records appears to be a robust feature, which warrants further consideration of its possible causes. The possible reverse trend in Asian summer monsoon strength preceding insolation minima seems to have also occurred during previous interglacial periods, based on speleothem records. We further show a similar late Holocene reverse trend in tropical hydrological changes, suggesting that the Asian summer monsoon behavior might be internally linked to the movement of the average position of the ITCZ and ENSO variability during the late Holocene. On the other hand, we suggest that even though several Holocene temperature records indeed show a reverse trend in the late Holocene, the overall evidence for a link between the late Holocene reverse trend in Asian summer monsoon and global temperature changes is insufficient. The reverse trend in Asian summer monsoon during the late Holocene is difficult to be explained with the traditional boreal insolation-driven view. We suggest that this phenomenon might be linked to austral summer insolation changes and/or greenhouse gas increase. However, we caution that additional paleoclimate reconstructions and model simulations are needed to systematically study the spatial pattern and understand underlying mechanism of the late Holocene reverse trend in Asian summer monsoon strength.