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The Terrestrial Carbon Cycle in Eight CMIP5 Earth System Models

Paper Reviewed
Shao, P., Zeng, X., Sakaguchi, K., Monson, R.K. and Zeng, X. 2013. Terrestrial carbon cycle: Climate relations in eight CMIP5 Earth system models. Journal of Climate 26: 8744-8764.

In a study published in the American Meteorological Society's Journal of Climate, Shao et al. (2013) write that "the terrestrial carbon sink is affected by temperature, precipitation, soil moisture, water vapor, CO2 concentration, and solar visible radiation and, hence, will be altered by climate change," citing Long et al. (2004), Boer and Arora (2010), Rutishauser et al. (2012) and Wolkovich et al. (2012). And so it was only to be expected that some folks - such as them - would someday evaluate the reliability of Earth System Models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) by focusing on both the net carbon dioxide flux into Earth's land masses and its several components, along with their relationships to the climatic variables of temperature, precipitation and soil moisture, which they themselves thus proceeded to do for the historical period of 1850-2005.

At the conclusion of their analysis, the five researchers reported, sadly, that (1-3) "the CMIP5 historical simulations tend to overestimate global GPP [gross primary production], Ra [autotrophic respiration], and LAI [leaf area index] against reference data," that (4-7) "large intermodal disagreements exist in NPP [net primary production], Rh [heterotrophic respiration], D [disturbances] and the cumulative NBP [net biome production] over the historical period." In addition, they state that (8) "even the most complex CMIP5 ESMs [Earth system models] do not incorporate all the diversity of the biogeochemical processes," that (9, 10) "precipitation (Pr) and soil moisture (SM) trends are model dependent," that (11) "over the three regions (Amazonia, western United States, and eastern Siberia), model results differ significantly in all carbon cycle and climatic variables."

In the concluding remarks of their paper, Shao et al. also state that (12) "even the most complex CMIP5 ESMs do not incorporate all the diversity of the biogeochemical processes" that are part of the Earth's terrestrial carbon cycle, noting that (13, 14) "phosphorus limitation (Zhang et al., 2011) and mycorrhizal contributions to soil respiration (Heinemeyer et al., 2007) are not yet represented or less well represented than other processes." In addition, they say that methane production - another important biogeochemical process that is loosely coupled to the carbon cycle - (15) "is not commonly represented in the ESMs." Last of all, they end their paper with the revelation that the very important nitrogen cycle, which is closely coupled to the carbon cycle, [16] "is not included" in six of the eight ESMs they evaluated.

Based on these many observations, it is clear that the eight most up-to-date ESMs are not up to the task of adequately representing the many biological and climatological phenomena that are required to correctly assess the current status of Earth's terrestrial carbon cycle or, much less, its future trajectory, as the atmosphere's CO2 concentration continues to rise.

References
Boer, G.J. and Arora, V.K. 2010. Geographic aspects of temperature and concentration feedbacks in the carbon budget. Journal of Climate 23: 775-784.

Heinemeyer, A., Hartley, I.P., Evans, S.P., Carrera, J.A. and Ineson, P. 2007. Forest soil CO2 flux: Uncovering the contribution and environmental responses of ectomycorrhizas. Global Change Biology 13: 1786-1797.

Long, S.P., Ainsworth, E.A., Rogers, A. and Ort, D.R. 2004. Rising atmospheric carbon dioxide: Plants FACE the future. Annual Review of Plant Biology 55: 591-628.

Rutishauser, T., Stockli, R., Harte, J. and Kueppers, L. 2012. Climate change: Flowering in the greenhouse. Nature 485: 448-49.

Wolkovich,E.M., Cook, B.I., Allen, J.M., Crimmins, T.M., Betancourt, J.L., Travers, S.E., Pau, S., Regetz, J., Davies, T.J., Kraft, N.J.B., Ault, T.R., Bolmgren, K., Mazer, S.J., McCabe, G.J., McGill, B.J., Parmesan, C., Salamin, N., Schwartz, M.D. and Cleland, E.E. 2012. Warming experiments under-predict plant phenological responses to climate change. Nature 485: 494-497.

Zhang, Q., Wang, Y.P., Pitman, A.J. and Dai, Y.J. 2011. Limitations of nitrogen and phosphorous on the terrestrial carbon uptake in the 20th century. Geophysical Research Letters 38: 10.1029/2011GL049244.

Posted 5 November 2014