The role of atmospheric fast adjustment in the distinct precipitation response between increase and decrease of CO2 concentrations

Apparent hydrological sensitivity, the change in global-mean precipitation per degree of surface warming, is an important metric of the climate response to radiative forcing. While previous studies have suggested that hydrological sensitivity is approximately constant with increasing CO 2 concentrations, here we show that this behavior does not hold when comparing increases and decreases of CO 2 concentrations. Using idealized climate model experiments with abrupt CO 2 halving and doubling from pre-industrial (PI) conditions, we find that the apparent hydrological sensitivity is higher for CO 2 halving, indicating an asymmetry between CO 2 halving and doubling. This asymmetry arises almost entirely from the fast atmospheric adjustment, whereas the slow, temperature-mediated component remains nearly symmetric. A decomposition of the fast adjustment shows that the response in longwave radiative cooling, particularly its clear-sky component, is a primary contributor to the asymmetry. Applying this framework to abrupt carbon dioxide removal (CDR) simulations highlights the role of the fast adjustments in the previously reported hysteresis in CO 2 ramp-up and then ramp-down transient simulations. Moreover, a weaker but statistically significant asymmetry, analogous to that found in PI-based CO 2 doubling and halving, persists in the CDR abrupt simulation.