Experiments under controlled conditions have unambiguously demonstrated that photosynthesis is stimulated when atmospheric CO2 concentrations are increased. Although these studies help to quantify the strength of this CO2 fertilisation effect, extrapolating such estimates to the global scale is extremely difficult, especially since other nutrients such as nitrogen may be the locally limiting factor in nature. CO2 fertilisation is a very important process in the global carbon cycle as it acts as a negative feedback for anthropogenic CO2 emissions. Increased atmospheric CO2 stimulates photosynthesis so that some fraction of these emissions is taken up from the atmosphere by vegetation.
Researchers in the Palaeoenvironmental Change group have derived a probabilistic estimate of the globally averaged strength of CO2 fertilisation that is independent of experimental evidence. Instead, we simply considered the changes in the global carbon budget since preindustrial times. Historical changes in atmospheric CO2 concentrations are very accurately known. Historical fossil fuel emissions are relatively well known. Emissions from deforestation can be quantified, albeit with significant uncertainty. These historical emissions are partitioned between the atmosphere, the ocean and the terrestrial biosphere. With careful attention to the considerable uncertainties (evaluated by performing many hundreds of simulations of the Earth system model GENIE), the strength of CO2 fertilisation can be treated as the unknown quantity that balances the carbon budget.
This figure summarizes the results of the calculation. Observed CO2 concentrations are plotted as the red line. The green line plots the modeled prediction of CO2 when no knowledge is assumed about the strength of CO2 fertilisation, besides some plausible range of possible strengths. The red line plots the CO2 that is simulated after the calibration, and brings the present day simulated CO2 much closer to observations. This calibrated model is likely to provide more reliable projections of future CO2 concentrations and climate change.
We estimate that the increase in gross primary productivity in response to a doubling of CO2 is very likely (90% confidence) to exceed 20%, with a most likely value of 40-60%. The most important caveat to this estimate is that GENIE does not represent all of the possible contributing mechanisms to the land sink. If these missing processes comprise a net sink, then our calibration of the strength of CO2 fertilisation represents an upper estimate.
Read more about the calibration of CO2 fertilization in our scientific paper: Holden PB, Edwards NR, Gerten D and Schaphoff S: A model-based constraint on CO2 fertilisation, Biogeosciences, 10, 339, 2013. www.biogeosciences.net/10/339/2013/
Posted by Philip Holden