Evidence of localised Amazon rainforest dieback in CMIP6 models

This is the final version. Available on open access from th European Geosciences Union via the DOI in this recordCode and data availability: The CMIP6 model output datasets analysed during this study are available online at https://doi.org/10.22033/ESGF/CMIP6.4507 (EC-Earth Consortium, 2019), https://doi.org/10.22033/ESGF/CMIP6.8473 (Krasting et al., 2018), doi10.22033/ESGF/CMIP6.6435 (Wieners et al., 2019), https://doi.org/10.22033/ESGF/CMIP6.10861 (Bethke et al., 2019), https://doi.org/10.22033/ESGF/CMIP6.7782 (Park and Shin, 2019), https://doi.org/10.22033/ESGF/CMIP6.9702 (Lee and Liang, 2020), and https://doi.org/10.22033/ESGF/CMIP6.5792 (Tang et al., 2019). Code used for analysis is available at https://doi.org/10.5281/zenodo.7038389 (Parry et al., 2022).Amazon forest dieback is seen as a potential tipping point under climate change. These concerns are partly based on an early coupled climate–carbon cycle simulation that produced unusually strong drying and warming in Amazonia. In contrast, the fifth-generation Earth system models (Phase 5 of the Coupled Model Intercomparison Project, CMIP5) produced few examples of Amazon dieback under climate change. Here we examine results from seven sixth-generation models (Phase 6 of the Coupled Model Intercomparison Project, CMIP6), which include interactive vegetation carbon and in some cases interactive forest fires. Although these models typically project increases in area-mean forest carbon across Amazonia under CO2-induced climate change, five of the seven models also produce abrupt reductions in vegetation carbon, which indicate localised dieback events. The northern South America (NSA) region, which contains most of the rainforest, is especially vulnerable in the models. These dieback events, some of which are mediated by fire, are preceded by an increase in the amplitude of the seasonal cycle in near-surface temperature, which is consistent with more extreme dry seasons. Based on the ensemble mean of the detected dieback events we estimate that 7±5 % of the NSA region will experience abrupt downward shifts in vegetation carbon for every degree of global warming past 1.5 ∘C.European Research Council (ERC)European Union Horizon 2020Engineering and Physical Sciences Research Council (EPSRC