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

Uncertainties in the response of vegetation to rising atmospheric CO concentrations contribute to the large spread in projections of future climate change. Climate-carbon cycle models generally agree that elevated atmospheric CO concentrations will enhance terrestrial gross primary productivity (GPP). However, the magnitude of this CO fertilization effect varies from a 20 per cent to a 60 per cent increase in GPP for a doubling of atmospheric CO concentrations in model studies. Here we demonstrate emergent constraints on large-scale CO fertilization using observed changes in the amplitude of the atmospheric CO seasonal cycle that are thought to be the result of increasing terrestrial GPP. Our comparison of atmospheric CO 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 CO seasonal cycle at both measurement sites is consistent with increasing annual mean GPP, driven in part by climate warming, but with differences in CO fertilization controlling the spread among the model trends. As a result, the relationship between the amplitude of the CO seasonal cycle and the magnitude of CO fertilization of GPP is almost linear across the entire ensemble of models. When combined with the observed trends in the seasonal CO amplitude, these relationships lead to consistent emergent constraints on the CO 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 CO concentrations on the basis of the Point Barrow and Cape Kumukahi records, respectively.