Projected land photosynthesis constrained by changes in the seasonal cycle of atmospheric CO2

Abstract

Uncertainties in the response of vegetation to rising atmospheric CO2 concentrations1,2 contribute to the large spread in projections of future climate change3,4. Climate–carbon cycle models generally agree that elevated atmospheric CO2 concentrations will enhance terrestrial gross primary productivity (GPP). However, the magnitude of this CO2 fertilization effect varies from a 20 per cent to a 60 per cent increase in GPP for a doubling of atmospheric CO2 concentrations in model studies5–7. Here we demonstrate emergent constraints8–11 on large-scale CO2 fertilization using observed changes in the amplitude of the atmospheric CO2 seasonal cycle that are thought to be the result of increasing terrestrial GPP12–14.