Is the Biogeochemical Effect of Increased CO2 on the Climate System a First-Order Climate Forcing? Is it at Least as Important as the Radiative Forcing of CO2 in Influencing Climate Change?

Biogeochemical forcing involves changes in vegetation biomass and soils ( which result in changes in the climate.

The answer to the first question is a definitive YES on both the regional and global scales. The second question also appears to be YES, although further investigation is required to confirm.

The evidence for this conclusion on the global scale comes from published papers (e.g., Cox, P. M., R. A. Betts, C. D. Jones, S. A. Spall, and I. J. Totterdell, 2000: Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model. Nature, 408, 184-187; Friedlingstein P., L. Bopp, P. Ciais, J.-L Dufresne, L. Fairhead, H. LeTreut, P. Monfray, and J. Orr, 2001: Positive feedback between future climate change and the carbon cycle. Geophys. Res. Lett., 28, 1543-1546. ). This conclusion, based on model process studies (see my weblog of July 15 entitled What Are Climate Models? What Do They Do?) is clearly articulated in the 2005 National Research Council report . These experiments tell us that this is an important climate forcing but, of course, the published papers should not be interpreted as predictions.

On the regional scale, we found (Eastman, J.L., M.B. Coughenour, and R.A. Pielke, 2001: The effects of CO2 and landscape change using a coupled plant and meteorological model. Global Change Biology, 7, 797-815) that the radiative effect of doubling CO2 was swamped by the combined biophysical (changes in the fluxes of trace gases and heat between vegetation, soils, and the atmosphere), and biogeochemical effects of doubling CO2. Significant climate influences included that maximum surface air temperatures were decreased and minimum surface air temperatures increased during this growing season simulation of the central Great Plains.

The reason for this large effect was that the increased atmospheric concentration of CO2 permitted individual stoma on the leaves to be more water efficient, which resulted in more plant growth than occurred in the model study when the current atmospheric concentrations of CO2 were prescribed. This effect was immediate, whereas, the radiative effect of CO2 requires a feedback with a warming ocean so that the more important greenhouse gas, H2O, increases in the atmosphere. The doubled biophysical/ biogeochemical CO2 experiment, however, resulted in greater atmospheric concentrations of H2O without any ocean feedback, since the larger amount of vegetation transpired more water vapor within the atmosphere, than otherwise would have occurred. This added water vapor produced a greenhouse effect, which resulted in warmer nights. The greater amount of incident solar insolation that went into latent heat flux during the day, however, resulted in cooler daytime maximum temperatures.

This conclusion, of course, is only from a process study for just for one landscape type. It does strongly support, however, a first-order effect on the climate system due the biogeochemical effect of increased atmospheric CO2. Clearly more work is needed to understand the multi-faceted effect of biogeochemical climate forcings, but we already know that this additional first order effect further complicates our ability to provide skillful climate predictions to policymakers.

Readers of the weblog are invited to add additional papers which provide (or refute, if available), published studies on the role of the biogeochemical role of CO2 as a first-order climate forcing.

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