Novel temperatures are already widespread beneath the world’s tropical forest canopies.

Tropical forest biodiversity is potentially at high risk from climate change, but most species reside within or beneath the canopy, where they are buffered from extreme temperatures, implying that forest canopies may reduce the severity of warming impacts. Using a mechanistic microclimate model, we model hourly below-canopy climate conditions of 300,000 tropical forest locations globally between 1990–2019. We show that while temperature extremes are buffered below canopy, recent small increases in beneath-canopy temperature (<1ºC) have led to highly novel temperature regimes across most of the tropics. This is the case even within contiguous forest, suggesting that tropical forests are sensitive to climate change. However, across the globe, some forest areas have experienced relatively non-novel temperature regimes and thus serve as important climate refugia. These areas require urgent protection and restoration. By conducting the first pan-tropical analyses of changes in below-canopy climatic conditions, we challenge the prevailing notion that tropical forest canopies reduce the severity of climate change impacts.