Projected land photosynthesis constrained by changes in the seasonal cycle of atmospheric CO2.
Uncertainties in the response of vegetation to rising atmospheric CO2concentrations contribute to the large spread in projections of future climate change. Climate-carbon cycle models generally agree that elevated atmospheric CO2concentrations will enhance terrestrial gross primary productivity (GPP). However, the magnitude of this CO2fertilization effect varies from a 20 per cent to a 60 per cent increase in GPP for a doubling of atmospheric CO2concentrations in model studies. Here we demonstrate emergent constraints on large-scale CO2fertilization using observed changes in the amplitude of the atmospheric CO2seasonal cycle that are thought to be the result of increasing terrestrial GPP. Our comparison of atmospheric CO2measurements 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 CO2seasonal cycle at both measurement sites is consistent with increasing annual mean GPP, driven in part by climate warming, but with differences in CO2fertilization controlling the spread among the model trends. As a result, the relationship between the amplitude of the CO2seasonal cycle and the magnitude of CO2fertilization of GPP is almost linear across the entire ensemble of models. When combined with the observed trends in the seasonal CO2amplitude, these relationships lead to consistent emergent constraints on the CO2fertilization 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 CO2concentrations on the basis of the Point Barrow and Cape Kumukahi records, respectively.
Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany.
Sabrina
S
Wenzel
College of Engineering, Mathematics &Physical Sciences, University of Exeter, Exeter EX4 4QE, UK.
Peter M
PM
Cox
Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany.
Veronika
V
Eyring
College of Engineering, Mathematics &Physical Sciences, University of Exeter, Exeter EX4 4QE, UK.
Pierre
P
Friedlingstein
D000413:Alaska; D001272:Atmosphere / Q000737:chemistry*; D057486:Carbon Cycle; D002245:Carbon Dioxide / Q000032:analysis* / Q000378:metabolism*; D057231:Climate Change; D017753:Ecosystem; D006254:Hawaii; D008962:Models, Theoretical; D010788:Photosynthesis; D012621:Seasons; D035501:Uncertainty
D002245:Carbon Dioxide