Stomata on leaves mediate fluxes of carbon and water between terrestrial plants and the atmosphere. The marginal water cost of carbon reflects plant water use strategy and is governed by stomatal conductance (g_s). Many Earth system models (ESMs) represent water use strategy in the calculation of g_s through an empirical parameter, the stomatal slope (g_1M). Here we examine how water use strategy influences photosynthesis using coupled ESM simulations by perturbing g_1M to observed 5th (low water cost) and 95th (high water cost) percentiles for each plant type. "Low water cost’’ perturbations represent a strategy with more efficient water use for carbon gain and high water cost’’ represents less efficient water use. We find that high g_1M reduces photosynthesis nearly everywhere, while low g_1M  has regionally dependent responses. Under fixed atmospheric conditions, low g_1M  increases photosynthesis in the Amazon and central North America but decreases photosynthesis in boreal Canada. These responses reverse when the atmosphere responds interactively due to spatially differing sensitivity to increases in temperature and vapor pressure deficit. Water use strategy also influences photosynthetic response to changes in atmospheric CO₂, with lower and higher g_1M modifying total global response to elevated 2x preindustrial CO₂ by 6.4% and -9.6%, respectively. Our work demonstrates that assumptions about plant water use strategy in ESMs significantly affect photosynthesis and its response to climate. Further, photosynthetic responses to water use strategy depend on which components of the model are interactive, so its impacts on historical or future photosynthesis cannot be generalized across model configurations.