Complex interplay of forcings drives Indian vegetation and summer monsoon variability during MIS 11

Marine Isotope Stage (MIS) 11 has long been considered a unique Quaternary interglacial stage due to its orbital similarities with the Holocene, persistent high atmospheric CO 2 concentrations and its extended duration, which triggered an unusual polar ice-sheet loss. Despite its importance, variability of the Asian summer monsoon, as well as its impacts on vulnerable tropical forests, during MIS 11 remain unexplored. Here, we document, for the first time, the vegetation and monsoon changes in eastern peninsular India and their underlying forcings using pollen analysis from IODP Site U1446, strategically retrieved from the Bay of Bengal to record changes in the summer monsoon rainfall in the core monsoon zone of India. Our results reveal the distinct roles of insolation, CO 2 , ice volume, and millennial-scale variability in driving vegetation and summer monsoon changes, depending on the changing boundary conditions throughout MIS 11. Site U1446 pollen data, supported by model simulations, indicate that the maximum tropical forest expansion and strongest monsoon activity during the interglacial period (MIS 11c) followed boreal summer insolation, revealing the dominant role of insolation under a warm climate state with high CO 2 and reduced ice volume. Conversely, during the late MIS 11 (MIS 11b-a), the tropical forest decreased despite high insolation, indicating that expanding ice sheets and lower CO 2 overshadowed the insolation influence on the summer monsoon. Millennial-scale variability during late MIS 11 likely played an additional role in counteracting the response of the summer monsoon to insolation. Abrupt forest contractions, tied to low monsoon rainfall and southward shifts of the intertropical convergence zone, are attributed to ice sheet dynamics and ocean circulation disturbances. These events were rapidly followed by pronounced forest expansions, linked to northward shifts of the intertropical convergence zone, reinvigoration of deep-ocean circulation, and CH 4 overshoots. Conspicuously, the first and most severe forest setback interrupted the full interglacial conditions of MIS 11, suggesting that extreme summer monsoon weakening could also occur under similarly warm future conditions. Our findings provide new insights into monsoon behavior during MIS 11, highlighting its high sensitivity to climate changes in the context of projected summer monsoon intensification and its effect on the tropical forest, which is a key component of both global carbon and methane cycles.