Varying contributions of fast and slow responses cause asymmetric tropical rainfall change between CO2 ramp-up and ramp-down

Tropical rainfall is important for regional climate around the globe. In a warming climate forced by rising CO₂, the tropical rainfall will increase over the equatorial Pacific where sea surface warming is locally enhanced. Here, we analyze an idealized CO₂ removal experiment from the Carbon Dioxide Removal Model Intercomparison Project and show that the tropical rainfall change features a stronger pattern during CO₂ ramp-down than ramp-up, even under the same global mean temperature increase, such as the 2 °C goal of the Paris Agreement. The tropical rainfall during CO₂ ramp-down increases over the equatorial Pacific with a southward extension, and decreases over the Pacific intertropical convergence zone and South Pacific convergence zone. The asymmetric rainfall changes between CO₂ ramp-down and ramp-up result from time-varying contributions of the fast and slow oceanic responses to CO₂ forcing, defined as the responses to abrupt CO₂ forcing in the first 10 years and thereafter, respectively, in the abrupt-4xCO2 experiment. The fast response follows the CO₂ evolution, but the slow response does not peak until 60 years after the CO₂ peak. The slow response features a stronger El Niño-like pattern, as the ocean dynamical thermostat effect is suppressed under stronger subsurface warming. The delayed and stronger slow response leads to stronger tropical rainfall changes during CO₂ ramp-down. Our results indicate that returning the global mean temperature increase to below a certain goal, such as 2 °C, by removing CO₂, may fail to restore tropical convection distribution, with potentially devastating effects on climate worldwide.