Ecosystems and Drylands Under Stress

The Amazon rainforest, the largest tropical forest on Earth and a major regulator of regional and global climate, is showing signs of stress that have led scientists, including the Brazilian climatologist Carlos Nobre, to warn of a potential tipping point beyond which large portions of the forest could shift irreversibly from rainforest to a drier, savanna-like ecosystem. This risk stems from a feedback loop: the forest generates much of its own rainfall by recycling moisture through evapotranspiration, so as deforestation removes trees, it also reduces regional rainfall, stressing the remaining forest and making it more vulnerable to fire and drought. Some field studies have already found that heavily deforested and degraded parts of the eastern and southern Amazon now release more carbon than they absorb, a striking reversal for an ecosystem long regarded as one of the planet's most important carbon sinks.