Hysteretic Behavior of Global to Regional Monsoon Area Under CO2 Ramp‐Up and Ramp‐Down.

When projecting future monsoon changes by carbon dioxide (CO2) pathway, most studies have analyzed precipitation responses without considering monsoon area (total area of monsoon domain, MA) variations. However, how MA responds to CO2 removal remains uncertain. This study evaluates MA variations and impacts in idealized CO2 ramp‐up (toward CO2 quadrupling), ramp‐down, and stabilized simulations using the Community Earth System Model version 1. Global MA negatively overshoots (i.e., recovery with decreasing tendency beyond the original MA) during the ramp‐down period due to reduced or rapidly recovered MA in several regional monsoons, including Northern and Southern Africa, South and East Asia, and South America, showing hysteresis when comparing CO2 ramp‐up and ramp‐down periods despite similar global warming levels. These non‐linear regional MA variations come from distinct regional summer and winter precipitation variations, which are found to be mostly associated with Intertropical Convergence Zone movements and El Niño‐like response. Further, regional monsoon precipitation characteristics also vary through CO2 ramp‐up and ramp‐down periods consistently with overall hysteresis. Changes in total monsoon precipitation resemble the distinct responses of MA. Our results suggest that regions characterized by a monsoonal climate may experience reduced seasonal rainfall variations under net‐negative CO2 emissions. Plain Language Summary: Many studies have researched how increasing carbon dioxide (CO2) affects monsoon. Most have focused on precipitation changes without considering how monsoon area (i.e., total area of monsoon domain, MA) will change. Furthermore, it is unknown how global and regional MA responds to CO2 removal. In this study, we first investigate how global to regional MA changes under CO2 concentration increase, decrease and then equilibration using climate model simulations. After CO2 removal, the global MA is largely reduced below the present‐day state, as MA decreases over several monsoon regions. Further, when we compare MA at the same global warming level in CO2 ramp‐up and ramp‐down periods, many individual monsoon regions show reduced MA during the ramp‐down period. As MA is defined based on local contrast of summer and winter precipitation, we investigate hemispheric summer and winter precipitation responses and how they affect MA. The regional changes in MA are found to be due to distinct variations of local summer or winter precipitation, which are commonly associated with large‐scale atmospheric and oceanic variations. Further impacts of changes in MA with CO2 removal on the hydrological cycle more generally, are identified. Key Points: Non‐linear responses in global and regional monsoon areas to CO2 pathway are investigated using a coupled climate modelDistinct hysteresis in monsoon area variation is identified over global to regional monsoons between CO2 ramp‐up and ramp‐down periodThe hysteresis emerges due to distinct precipitation changes with large‐scale atmospheric and oceanic responses [ABSTRACT FROM AUTHOR]