Global Cooling Hiatus Driven by an AMOC Overshoot in a Carbon Dioxide Removal Scenario

The reversibility of global mean surface temperature was examined by a transient CO2 reversibility experiment using an Earth system model. The results showed that after CO2 ramp‐up toward CO2 quadrupling and ramp‐down returned to the present‐day level, the global mean surface temperature kept decreasing but stopped to change for ∼40 years in the early net‐zero CO2 emission period. This period, referred to a cooling hiatus, resulted from a compensation between Southern Hemisphere cooling and Northern Hemisphere warming. The Northern Hemisphere warming was centered over the North Atlantic. This localized warming was caused by an excessive heat advection by a delayed and surpassed Atlantic Meridional Overturning Circulation (AMOC) to CO2 forcing. During the progression of CO2 change, the meridional salinity gradient between subtropic and subpolar regions was enhanced, and the oceanic stratification in subpolar North Atlantic was reduced due to accumulated heat and reduced vertical salt import in the deeper ocean. As AMOC started to recover, consequently, the enhanced salt advection feedback and the relaxed buoyant force resulted in AMOC overshoot. Plain Language Summary Whether Earth's climate will be recovered after accomplishing a target goal of Paris Agreement is questionable. Here, we performed an Earth system model simulation to explore the reversibility of earth climate, in which a CO2 concentration level gradually increases to four times CO2 of the present‐day level and then returns to the present‐day level. The CO2 change leads to change in global mean surface temperature (GMST). However, even after the complete return to the present‐day CO2 level, GMST remained as about 1° higher than the present‐day. Furthermore, the cooling trend of GMST halts for about 40 years from a net‐zero emmission of CO2. This cooling hiatus is driven by a compensation between the Northern Hemisphere warming and Southern Hemisphere cooling. The Northern Hemisphere warming is likely due to an excessive thermal transport by ocean current. This ocean current, as an upper branch of Atlantic Meridional Overturning Circulation (AMOC), overly responds to CO2 forcing than it would do. Such overshoot of AMOC is likely caused by the stronger salt advection of enhanced salinity difference between subarctic and subtropics and the much easier downward motion of surface dense water under a reduced oceanic vertical density difference in a changing CO2 environment. Key Points In CO2 removal