57 million hectares of tropical forests lose photosynthesis efficiency due to rising heat: study implications for carbon sinks

A new study published in PNAS reveals that rising temperatures have pushed 57 million hectares of tropical forests past critical heat thresholds, making photosynthesis less efficient. Researchers from EPFL used satellite data from 2001 to 2020 and thermal tolerance information for 200 tropical plant species. The affected area grew from 43 million to 57 million hectares, roughly the size of France. When leaf temperatures exceed a critical point, proteins involved in photosynthesis degrade, slowing tree growth and reducing carbon uptake. If warming continues, the study projects that by 2050 the affected area could reach 93 million hectares, and by 2100 up to 160 million hectares. This means tropical forests, one of the world's largest natural carbon sinks, will absorb less CO2. The shift could also alter forest composition, with heat-tolerant species replacing others, affecting biodiversity. Reduced water vapor release from stressed forests may worsen droughts and extreme weather elsewhere. For carbon markets and climate policy, this is a concrete signal that nature-based carbon credits tied to tropical forest preservation may face increasing physical risk. Projects relying on forest carbon sequestration need to account for heat stress impacts. The study underscores that protecting forests is not enough if the climate they depend on is disappearing.