Contrasting Impacts of Acclimation and Adaptation of Photosynthetic Capacity to Temperature and CO2 Across Biomes

The response of photosynthesis to temperature and CO₂ is poorly represented in land surface models, contributing uncertainty to estimates of the land carbon sink. We assess the sensitivity of carbon uptake to temperature adaptation and acclimation of photosynthetic capacity and CO₂ acclimation of photosynthesis in the JULES model forced with an RCP8.5 climate scenario. Simulations show enhanced global gross primary productivity (GPP) when these processes are included, but over time the enhancement of GPP is weakened. In extratropical regions, temperature acclimation enhances GPP by aligning the photosynthetic temperature optimum with seasonal temperatures, allowing higher rates of carbon assimilation. In the tropics, temperature adaptation weakens the rate of global carbon uptake by reducing the CO₂ sensitivity of photosynthesis and limiting the CO₂ fertilization response, while acclimation sustains higher rates of photosynthesis as temperatures rise. Combined, our results suggest enhanced thermal resilience of modeled global GPP to warming. Downregulation of photosynthetic capacity in response to elevated CO₂ could substantially affect future GPP. However, this response remains uncertain, highlighting the need for improved understanding and representation of CO₂ acclimation across biomes, especially in tropical ecosystems where field data are scarce. Results suggest models omitting these processes may underestimate global carbon uptake and ignore important spatial variability in response to climate change.