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Climate change mitigation through negative CO2 emissions has been recognized as a crucial strategy to combat global warming. However, its potential effects on terrestrial productivity and agricultural activities remain uncertain. In this study, we utilized large ensemble simulations with an Earth system model of full complexity to investigate the response of Gross Primary Production (GPP) to CO2 forcings. Our findings reveal a significant asymmetry in the GPP response to CO2 ramp-up and symmetric ramp-down model experiments, especially in Europe, suggesting that GPP declines rapidly as CO2 levels decrease. Remarkably, during the CO2 removal period, the North Atlantic Sea surface temperature experienced cooling due to a delayed recovery of the Atlantic Meridional Overturning Circulation (AMOC). This cooling led to precipitation and soil moisture deficits, resulting in a rapid reduction in GPP. This asymmetry in GPP response holds consistent across multi-model simulations. These results underscore the potential implications of delayed recovery in ocean circulation, which could unexpectedly accelerate terrestrial GPP reduction. These insights are crucial for policymakers, aiding them in projecting agricultural activity and formulating targeted GPP control policies specific to the European region. [ABSTRACT FROM AUTHOR]

Understanding regional hydro‐climatic extreme responses to CO2 pathways is fundamental to climate change mitigation and adaptation. This study evaluates responses of extreme precipitation frequency (R30mm: days with precipitation ≥30 mm) over East Asia to idealized CO2 forcing using the Community Earth System Model (CESM1). Under a symmetric increase (ramp‐up, +1% per year until quadrupling level) and decrease (ramp‐down, about −1% per year back to the present level) of CO2 concentrations, East Asian R30mm shows an asymmetric response with higher frequency during the ramp‐down period. This hysteresis behavior is found to be due to a northwestward propagating wave response to the El Niño‐like warming, which induces a three‐cell (positive‐negative‐positive) R30mm difference pattern from central equatorial Pacific to East Asia. Monthly analysis further reveals that this asymmetry has a seasonal locking, occurring during July–September only, constrained by the background precipitation climatology over the sub‐tropical western Pacific. Plain Language Summary: Considering tremendous damages caused by regional‐scale hydro‐climatic extremes under global warming, understanding their responses to CO2 forcing is fundamentally important but in‐depth analyses have been limited. This study provides a first examination of the responses of extreme precipitation over East Asia to CO2 forcing using an ensemble simulation of the coupled climate model Community Earth System Model version. When increasing CO2 concentration at a 1%‐per‐year rate from the present level to quadrupling level and then decreasing it symmetrically, extreme precipitation frequency is found to remain higher during the ramp‐down period. This hysteresis behavior occurs strongly during July to September, implying that East Asia can experience more frequent heavy precipitation events even after CO2 concentration is recovered. Composite analysis reveals that this irreversible response originates from the El Niño‐like warming pattern, which induces a tri‐pole atmospheric wave pattern from central equatorial Pacific to East Asia. We also find that the seasonal locking of this asymmetric response is related to the background precipitation state over the sub‐tropical western Pacific. Key Points: Heavy rainfall frequency over East Asia shows an asymmetric response to CO2 forcing, remaining higher during the ramp‐down periodThe hysteresis behavior is caused by a northwestward propagating wave response to the El Niño‐like equatorial Pacific warmingThis asymmetry is evident during July–September only, constrained by background precipitation state over the sub‐tropical western Pacific [ABSTRACT FROM AUTHOR]

The hydrological cycle has a significant impact on human activities and ecosystems, so understanding its mechanisms with respect to a changing climate is essential. In particular, a more detailed understanding of hydrological cycle response to transient climate change is required for successful adaptation and mitigation policies. In this study, we exploit large ensemble model experiments using the Community Earth System Model version 1.2.2 (CESM1) in which CO2 concentrations increase steadily and then decrease along the same path. Our results show that precipitation changes in the CO2 increasing and decreasing phases are nearly symmetrical over land but asymmetric over oceans. After CO2 concentrations peak, the ocean continues to uptake heat from the atmosphere, which is a key process leading the hydrological cycle's contrasting response over land and ocean. The symmetrical hydrological cycle response over land involves a complex interplay between rapid responses to CO2 and slower responses to ensuing warming. Therefore, the surface energy constraints lead to the contrasting hydrological response over land and ocean to CO2 forcing that needs to be verified and considered in climate change mitigation and adaption actions. [ABSTRACT FROM AUTHOR]