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

Uncertainties in the response of vegetation to rising atmospheric CO₂ concentrations contribute to the large spread in projections of future climate change. 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 studies. Here we demonstrate emergent constraints 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 GPP. Our comparison of atmospheric CO₂ measurements from Point Barrow in Alaska and Cape Kumukahi in Hawaii with historical simulations of the latest climate-carbon cycle models demonstrates that the increase in the amplitude of the CO₂ seasonal cycle at both measurement sites is consistent with increasing annual mean GPP, driven in part by climate warming, but with differences in CO₂ fertilization controlling the spread among the model trends. As a result, the relationship between the amplitude of the CO₂ seasonal cycle and the magnitude of CO₂ fertilization of GPP is almost linear across the entire ensemble of models. When combined with the observed trends in the seasonal CO₂ amplitude, these relationships lead to consistent emergent constraints on the CO₂ fertilization of GPP. Overall, we estimate a GPP increase of 37 ± 9 per cent for high-latitude ecosystems and 32 ± 9 per cent for extratropical ecosystems under a doubling of atmospheric CO₂ concentrations on the basis of the Point Barrow and Cape Kumukahi records, respectively.