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 trait acclimations remain to be tested. Here we use Earth system model experiments to quantify the influence of one plant trait acclimation to elevated carbon dioxide - a one third increase in leaf mass per area - on physical warming and carbon uptake on land.
We show that leaf trait acclimation in response to elevated carbon dioxide significantly impacts climate and carbon cycling. Global net primary productivity decreases (-6 PgC/yr), representing a flux of carbon dioxide to the atmosphere of similar magnitude to current annual fossil fuel emissions (8 PgC/yr). Additional anomalous warming over land (+0.3°C globally), especially of the northern extratropics (+0.4°C), 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 greater leaf mass per area increases 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.