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CMIP5 Earth System Models: Trying to Model Soil Carbon Stocks
Todd-Brown, K.E.O., Randerson, J.T., Post, W.M., Hoffman, F.M., Tarnocai, C., Schuur, E.A.G. and Allison, S.D. 2013. Causes of variation in soil carbon simulations from CMIP5 Earth system models and comparison with observations. Biogeosciences 10: 1717-1736.

The authors write that "because future climate projections depend on the carbon cycle, ESMs [Earth System Models] must be capable of accurately representing the pools and fluxes of carbon in the biosphere, particularly in soils that store a large fraction of terrestrial organic carbon," but they note that "there have been few quantitative assessments of ESM skill in predicting soil carbon stocks, contributing to uncertainty in model simulations."

What was done
"To help reduce this uncertainty," as Todd-Brown et al. describe it, they say they "analyzed simulated soil carbon from ESMs participating in the Fifth Climate Model Intercomparison Project (CMIP5)," comparing the results from 11 different model centers to empirical data they obtained from the Harmonized World Soil Database (HWSD) and the Northern Circumpolar Soil Carbon Database (NCSCD).

What was learned
The seven scientists report that some ESMs "simulated soil carbon stocks consistent with empirical estimates at the global and biome scales." However, they state that all of the models "had difficulty representing soil carbon at the 1° scale," indicating that "despite similar overall structures, the models do not agree well among themselves or with empirical data on the global distribution of soil carbon."

What it means
As for the significance of these results, Todd-Brown et al. conclude that "all model structures may have serious shortcomings, since net primary productivity and temperature strongly influenced soil carbon stocks in ESMs but not in observational data [italics added]." And they thus go on to outline a number of things that may need to be done in order to resolve this failure of ESMs to adequately replicate the real world, including "better prediction of soil carbon drivers, more accurate model parameterization, and more comprehensive representation of critical biological and geochemical mechanisms in soil carbon sub-models."

Clearly, we are not yet where we need to be in order to be confident about the degree to which climate-change impacts on soil carbon stocks may either reinforce or reduce the global warming initiated by anthropogenic CO2 emissions.

Reviewed 11 September 2013