Multi-fold increase in rainforests tipping risk beyond 1.5–2 °C warming

Tropical rainforests invest in their root systems to store moisture in their root zone 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 the (mass-balance-based) empirical understanding of root zone storage capacity, we assess the future state of the rainforests and the forest-to-savanna transition risk in South America and Africa under four different shared socioeconomic pathway scenarios. We find that under the end-of-the-21st-century climate, nearly one-third of the total forest area will be influenced by climate change. Furthermore, beyond 1.5–2 °C 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.