Impacts of photosynthetic capacity acclimation and adaptation to temperature and CO2

The response of photosynthesis to temperature and CO₂ remains poorly represented in land surface models, contributing to significant uncertainty in land carbon sink estimates. Here, we incorporate photosynthetic capacity adaptation and acclimation to temperature into the JULES land surface model, and we investigate the sensitivity of photosynthesis to CO₂ acclimation. Using an RCP8.5 climate scenario, we quantify the impact of these processes on Gross Primary Productivity (GPP). Simulations accounting for adaptation and acclimation to temperature and CO₂ increases modelled global GPP by 2050. Temperature acclimation in the extra tropics enhances GPP, but adaptation in the tropics weakens the CO₂ fertilisation response decreasing GPP. CO₂ acclimation down-regulates photosynthetic capacity, causing a universal decline in the rate of GPP enhancement across biomes. Our findings emphasize the need for models to incorporate temperature adaptation and acclimation to avoid underestimating global carbon uptake and to better capture spatial variability in responses to rising temperatures. In addition, improving our understanding of CO₂ acclimation across biomes and its integration into models is critical for reducing uncertainties in future carbon cycle predictions.