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The rate of CO₂ assimilation by plants is directly influenced by the concentration of CO₂ in the atmosphere, c_a. As an environmental variable, c_a also has a unique global and historic significance. Although relatively stable and uniform in the short term, global c_a has varied substantially on the timescale of thousands to millions of years, and currently is increasing at seemingly an unprecedented rate. This may exert profound impacts on both climate and plant function. Here we utilise extensive datasets and models to develop an integrated, multi‐scale assessment of the impact of changing c_a on plant carbon dioxide uptake and water use. We find that, overall, the sensitivity of plants to rising or falling c_a is qualitatively similar across all scales considered. It is characterised by an adaptive feedback response that tends to maintain 1 − c_i/c_a, the relative gradient for CO₂ diffusion into the leaf, relatively constant. This is …