Amazon rainforest hydroclimatic stress under 2 °C global warming in response to progressive idealized deforestation simulated with MPI-ESM-HR

The Amazon rainforest (ARF) is one of the most important tipping elements on the planet and has been under increasing stress due to global warming and deforestation over the last few decades. Several studies have investigated the stability of the ARF, and the best available estimates indicate that the forest could surpass a tipping point after approximately 25% deforestation or 2–4 °C of global warming. However, there are significant uncertainties surrounding these estimates, and there is still a lack of understanding of how these different forcings (global warming and deforestation) interact and how their combined effects could accelerate a critical transition of large parts of the ARF to a tropical savanna state. In this study, we performed idealized experiments to investigate the hydroclimatic response of the ARF to deforestation-only and composite (global warming + deforestation) forcings, with the aim of better understanding the tipping point potential, how it affects the hydroclimatic stability of the forest, and how the two forcings interact. We therefore performed experiments with 0%, 25%, 50%, 75%, and 100% deforestation under a stable global temperature 2 °C warmer than pre-industrial conditions and analysed the response of four hydroclimatic stress indicators: annual precipitation, dry season length (DSL), mean climatological water deficit (MCWD), and top 10 cm soil moisture. We calculated the deviation of the long-term average of each of these variables from stable (no-deforestation) scenarios and classified the deviations using relative anomalies, defined with respect to the standard deviation of the distribution of the stable scenarios. Using these metrics, an anomaly of 0.5 (i.e., a deviation of the mean by half of the standard deviation of the stable scenario) qualifies as a significant moderate anomaly, while anomalies exceeding 2.0 and 2.5 are classified as severe and extreme anomalies, respectively. We found that deforestation of 25% of the ARF can expose approximately 70% of the rainforest to significant hydroclimatic stress according to at least one of the four indicators analysed. When the combined effects of 25% deforestation and 2 °C of global warming are considered, this fraction increases to 89% of the ARF. This composite effect is larger than the deforestation-only stress resulting from a 50% removal of forest cover (82% of the ARF). Among the indicators, increasing dry season length is the most pronounced response, affecting a larger fraction of the forest and with greater intensity than the other variables, and with effects not limited to the vicinity of deforested areas. Whether tree mortality occurs in areas under significant stress is likely to depend strongly on the ability of the vegetation to rapidly adapt to substantial changes in hydroclimatic conditions. Overall, these results reveal quantitative aspects of the potential for deforestation and global warming to trigger cascading effects in the ARF that could ultimately lead to its transition to a tropical savanna state.