Heatwaves push tropical forests past photosynthesis limits, threatening carbon sink capacity

A new EPFL study published in PNAS shows that tropical forests are increasingly exceeding the temperature thresholds where photosynthesis starts to fail. Researchers combined species-specific thermal limits with satellite data from 2001 to 2020 and found that the area of tropical forest experiencing canopy temperatures above the critical threshold grew from 43 million to 57 million hectares over that period. That is an area larger than France. By 2100, that area could reach 160 million hectares, larger than South Africa. The study covers about 200 tree species and warns that as photosynthesis declines, tropical forests absorb less CO2 and release less water vapor. That weakens the planet's largest terrestrial carbon sink and could accelerate global warming. The researchers plan to expand the work to more species and to other regions including Mediterranean forests in Europe. The findings are a concrete reminder that heat stress on ecosystems has direct consequences for carbon budgets and climate projections.