Crop models need improving for an accurate assessment of the impacts of climate change on crop productivity. Yin considers that most existing models are unable to accommodate photosynthetic acclimation to CO2 concentrations, and therefore tend to overpredict crop responses to elevated CO2. Strong evidence is provided for the need to quantify carbon–nitrogen interactions in order to simulate this acclimation mechanism. This would provide a basis for a mechanistic framework that models critical physiological processes and traits in response to other climatic factors and extreme weather events. In an accompanying article, Kimball comments on a methodological error in a previously published report of a free-air CO2 enrichment (FACE) study that is cited by Yin.