Increasing CO2 levels fertilize C4 grass production.

Rising atmospheric CO2 concentrations are impacting the global terrestrial biosphere through indirect climate effects and direct effects on plant performance1-3. In tropical forests, long-term monitoring indicates a substantial CO2-driven carbon sink4. C4-grass-dominated tropical and subtropical savannas contribute approximately 30% of terrestrial net primary production5, and yet equivalent long-term analyses of CO2 responses are lacking. Here we show a clear and consistent result across a meta-analysis of 70 CO2-addition experiments and 32 years of in situ field observations from southern Africa: CO2 fertilization of wild C4 grasses is widespread in dry conditions. In experiments, grasses reduced stomatal conductance under higher levels of CO2, limiting water loss while increasing carbon gain. In the field, improved water use efficiency translated into increased C4 grass biomass production across three decades of observations. Finally, simulations via the Community Land Model6 suggest that CO2 fertilization of C4 grass aboveground productivity may continue to increase under future conditions. Together, these results challenge the view that C4 grasses are unresponsive to increasing levels of CO2, demonstrating instead that annual aboveground production of grasses in the field in southern Africa has increased by 28% over three decades (a CO2-driven increase of 75.1 g m-2 (95% confidence interval of 74.5-75.8 g m-2) or 0.37 tons C ha-1 of annual production). Although the fate of this carbon is uncertain (depending on feedbacks with fire, herbivory and woody vegetation), effects on the global carbon cycle may be profound.