Reversibility after reversals? Hysteretic ecosystem stress responses under CO2 removal

Climate extremes are projected to increase with climate change, and have the potential to negatively impact terrestrial ecosystems with consequences for carbon- and water cycles. While the responses of ecosystems to increasing CO₂ concentrations and the resulting climate change are relatively well studied, the reversibility of ecosystem responses under forcing reversals remains less understood. Using the idealised CDRMIP experiment set-up, we assess the reversibility of simulated ecosystem stress and associated changes in physiological ecosystem resilience which we quantified using lag-1 autocorrelation. We identify Amazonia as a hotspot for hysteretic behaviour in ecosystem stress responses at a high model agreement level (six out of eight), characterized by generally stronger negative carbon flux anomalies at identical CO₂ levels during ramp down compared to ramp up. While previous studies have suggested localized tipping or abrupt responses in parts of Amazonia, we do not detect significant changes in physiological resilience throughout the CO₂ ramp up. However, we find reduced physiological resilience in South Amazonia comparing equivalent CO₂ levels during ramp down and ramp up, pointing to potential limits in the capacity of these ecosystems to recover from stress induced by global change.