Empirical studies have shown that plant photosynthetic responses to environmental change can vary over time due to acclimation, but acclimation responses are often not included in Earth System Models. Photosynthetic least cost theory can be used to develop models of photosynthetic acclimation that are simple and testable. The theory is based on the idea that plants will acclimate to minimize the ratio of carbon costs to photosynthetic assimilation rate (Prentice et al., 2014, https://doi.org/10.1111/ele.12211). Formulations of this theory have been developed for C3 plants, but not C4 plants, which account for over 20% of global photosynthesis and are over‐represented among widely grown crops. Here, we use photosynthetic least cost theory to derive a model for C4 photosynthetic acclimation to above‐ground abiotic conditions. We then compare our model's responses to a similar model of C3 photosynthetic acclimation and find that C4 photosynthesis has the highest simulated advantage over C3 photosynthesis in hot, dry, and low CO2 environments. We find that this advantage predicts C4 abundance globally, but that the shallower CO2 response of C4 as compared to C3 photosynthesis will reduce C4 plant competitiveness under future conditions, despite higher temperatures. We also show that an acclimated model predicts similar or faster rates of C4 under all conditions than a model that does not consider acclimation, suggesting that Earth System Models (ESMs) are underestimating future C4 carbon uptake by not including acclimation. Our model is designed for easy incorporation into such ESMs.

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