Risks of unavoidable impacts on forests at 1.5 with and without overshoot

Regional climate impacts studies are usually predicated on output from fully-coupled Earth system models, which, due to computational constraints, can only simulate a limited number of scenarios and ensemble members. Using the PRIME system, we can simulate spatially resolved impacts quickly - emulating the response of 34 CMIP6 models, and generating ensemble members that capture the IPCC assessed range of equilibrium climate sensitivity (ECS). We assess the tail risks associated with high ECS simulations on critical tropical and boreal forest ecosystems over the 21st century and beyond, using three policy-relevant strong-mitigation IPCC WG3 emissions scenarios with different relationships to 1.5°C global warming. We quantify the future resilience and risk of dieback across these ecosystems, focus on the reversibility of loss using a temperature overshoot-and-return scenario and delineate hazardous climatic space for the Amazon basin, with climate-boundaries consistent with the literature. We show that despite using emissions scenarios which achieve 1.5 and 2 degrees Paris Agreement targets, uncertainty in ECS exhibits unavoidable risk of Amazon forest health decline and dieback, further highlighting the requirement for urgent, focused, global mitigative actions.