Asian summer monsoon orbital variability directly paced by CO2 and precession, not eccentricity

Abstract

Abstract The pacing and intensity of the Asian summer monsoon have long been debated. Geological records indicate either a dominant precession or eccentricity variability, while modelling studies have largely emphasised monsoon intensity rather than its temporal pacing. Here, we investigate both the temporal variability and changes in intensity of the Asian summer monsoon using a unique set of four series of full-complexity climate model equilibrium simulations spanning the last 800 kyr at 4-kyr resolution. The simulations have differing insolation, greenhouse gases, and ice sheet forcing. Here, we show that variations in greenhouse gases exert the dominant control on the 100-kyr variability via the thermal forcing of the Asian summer monsoon circulation, while precession directly impacts the 20-kyr variability. In contrast, the direct contributions of eccentricity forcing and ice sheet variability are smaller. Spatial heterogeneity in monsoon dynamics reveals different sensitivities to imposed forcings, which may help reconcile conflicting proxy records.