How quickly are canopy temperatures approaching their critical limit in the tropics?

Tropical forests host Earth’s highest biodiversity and act as global climate regulators more than any other biome. In a world where heat waves become increasingly severe and recurrent, especially in the Amazon, it becomes crucial to know how close tropical species are getting to their critical temperatures. Indeed, while plant leaves operate within a broad range of air temperatures, they must stay under a species-specific critical temperature (T_crit) to sustain their function. In this context, we study how quickly the canopy temperature (T_c) approaches the critical temperature of tree species in the tropics.

We specifically focus on the evolution of the thermal safety margin (Δ = T_c − T_crit) during the period 2001- 2020 across tropical forest biomes in South America, Southeast Asia, and Central Africa. Data analysis (~ 1km resolution) was conducted by combining (1) tree species distribution maps, (2) T_crit databases, (3) MODIS-derived maximal Land Surface Temperature per month, and (4) a dense vegetation map to obtain the T_c maps. Given that the exposure time to T_crit required to cause the leaves to die is short, we focus on the heatwaves and select the warmest month per year to study the Δ evolution. We fit a linear regression at each geographic coordinate to obtain trend maps based on their coefficients.

Our results indicate a consistent trend wherein T_c progressively approaches critical thresholds. On average over the three studied regions, the median increase in T_c is around 0.11 °C per year, with a median Δ of 9.5 °C in 2020.

More sensitive locations exhibiting initial proximity to T_crit show an accelerated rate of gap closure. For instance, in South America, while the median increase per year in T_c is about 0.1 °C, the 90^th percentile is about 0.19 °C: in 2001, 10% of the locations were 4.5 °C away from the critical temperature of their most sensitive species, dropping to 0.3 °C in 2020. Central Africa has a less severe but still concerning trend, with the hottest 10% experiencing a 0.16 °C increase per year and a thermal safety margin of 2.5 °C in 2020.

Our findings suggest that, although most areas in the tropical biomes still have a rather large safety margin before reaching the T_crit associated with their species, sensitive areas are getting dangerously close to these critical thresholds, suggesting enhanced vulnerability to global warming.