Global warming beyond 1.5–2⁰C multiplies the rainforests' tipping risk

Tropical rainforests invest in their root systems to store soil moisture from water-rich periods for use in water-scarce periods. An inadequate root-zone soil moisture storage predisposes or forces these forest ecosystems to transition to a savanna-like state, devoid of their native structure and functions. Yet changes in soil moisture storage and its influence on the rainforest ecosystems under future climate change remain uncertain. Using a mass-balance-based (empirical) understanding of root zone storage capacity, we assess the future state of the rainforests and the forest-savanna transition risk in South America and Africa. For this, we analyse the hydroclimatic estimates of 33 Earth System Models under four different shared socioeconomic pathway scenarios (i.e., SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5). We find that by the end of the 21^st century, nearly one-third of the total forest area will be influenced by climate change. Furthermore, beyond 1.5-2⁰C warming, ecosystem recovery reduces gradually, whereas the forest-savanna transition risk increases several folds. For Amazon, this risk can grow by about 1.5-6 times compared to its immediate lower warming scenario, whereas for Congo, this risk growth is not substantial (0.7-1.65 times). The insight from this study underscores the urgent need to limit global surface temperatures below the Paris agreement.