Increasingly severe thermal stresses on global photosynthesis using insights from observations of canopy temperature

Abstract

Temperatures could routinely exceed the optimal levels for photosynthesis as global warming intensifies, imposing thermal stress on the productivity of vegetation. We utilised satellite-derived canopy temperature and gross primary productivity (GPP) data from 2003 to 2024 to identify the ecosystem-level optimal canopy temperature (Tcan opt) for global photosynthesis and the extent of any thermal acclimation, which may offset warming impacts. Our findings indicated that across the globe, heat-induced restrictions on global photosynthesis are worsening, and areas subjected to thermal limitations have expanded by 1.7 billion hectares (57% increase) over the last 22 years. The number of days per year with high thermal suppression of photosynthesis during that period has increased sharply, averaging 28 days globally, and is especially high in tropical forests (117 days) and key agricultural regions (39 days). We demonstrate that vegetation acclimation to higher canopy temperature is partially mitigating emerging heat stress, but it is insufficient to keep up with the rate of global warming, with more than 90% of vegetated areas, and from locations across the globe, exhibiting partial acclimation. A key feature of our analysis is the use of canopy-level temperatures, which more accurately represent the actual temperatures that vegetation physiologically responds to, rather than previous research using air temperature. This difference accounts for our identified more rapidly intensifying vegetation response to warming than that estimated by other analyses. Overall, our canopy-level analysis reveals an escalating threat to global vegetation productivity and highlights the need for climate models to have refined land components, which often rely on air-temperature forcing and simplified acclimation schemes. Required are targeted ecosystem management strategies for adaptation to further global warming.