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Description: Vegetation modifies Earth’s climate by controlling the fluxes of energy, carbon, and water. Of critical importance is a better understanding of how vegetation responses to climate change will feedback on climate. Observations show that plant traits respond to elevated carbon dioxide concentrations. These plant trait acclimations can alter leaf area and thus productivity and surface energy fluxes. Yet, the climate impacts of plant structural trait acclimations remain to be tested and quantified. Here we show that one leaf trait acclimation in response to elevated carbon dioxide – a one third increase in leaf mass per area – significantly impacts climate and carbon cycling in Earth system model experiments. Global net primary productivity decreases (-5.8 PgC/yr, 95% confidence interval, CI95% -5.5 to -6.0), representing a decreased carbon dioxide sink of similar magnitude to current annual fossil fuel emissions (8 PgC/yr). Additional anomalous terrestrial warming (+0.3°C globally, CI95% 0.2 to 0.4), especially of the northern extratropics (+0.4°C, CI95% 0.2 to 0.5), results from reduced evapotranspiration and enhanced absorption of solar radiation at the surface. Leaf trait acclimation drives declines in productivity and evapotranspiration by reducing leaf area growth in response to elevated carbon dioxide, as a one third increase in leaf mass per area raises the cost of building leaf area and productivity fails to fully compensate. Our results suggest that plant trait acclimations, such as changing leaf mass per area, should be considered in climate projections and provide additional motivation for ecological and physiological experiments that determine plant responses to environment.