We explored an inverse modeling technique to interpret global atmospheric measurements of CO₂ and the ratio of its stable carbon isotopes (δ¹³C). We detected the possible underestimation of drought stress in biosphere models after applying combined atmospheric CO₂ and δ¹³C constraints. This study highlights the importance of improving the representation of the biosphere in carbon–climate models, in particular in a world where droughts become more extreme and more frequent.