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The Impact of Soil Nitrogen Availability on Global Warming
Volume 12, Number 42: 21 October 2009

Earth's land plants grow best when supplied with optimum levels of soil nitrogen; and when nitrogen is present in soils in concentrations that are less than optimal, terrestrial vegetation grows less vigorously and removes less CO2 from the atmosphere. As a result, climate alarmists contend that less-than-optimal soil nitrogen concentrations will sooner or later lead to a significant reduction in the strength of the growth stimulation that is provided by the aerial fertilization effect of the ongoing rise in the air's CO2 content, due to their contention that the limited amount of nitrogen in the soil simply cannot supply enough of this essential nutrient to maintain the increase in growth that could otherwise be supported by the atmosphere's rising CO2 concentration, which string of suppositions has come to be known as the progressive nitrogen limitation hypothesis (Hungate et al., 2003; Luo et al., 2004).

The incorporation of this concept into the modeling study of Thornton et al. (2009) has provided powerful political fodder for the cap-and-trade crowd's claim that there will be even greater global warming in the future than the huge amount that has already been predicted by state-of-the-art climate models, due to the supposedly smaller amount of CO2 being removed from the air by the supposedly less vigorously growing vegetation, due to the supposedly gradual weakening of CO2's aerial fertilization effect on plants growing in nitrogen-limited soils.

As for why we should believe this model-based projection, the ten authors of the study state that their conclusion "is supported by previous studies: for stand-alone ecosystem models (McGuire et al., 2001), [an] offline land component of a coupled climate model (Thornton et al., 2007), [a] coupled model of intermediate complexity (Sokolov et al., 2008), and now here for the case of a fully-coupled climate system model," where they additionally state that "each of these studies is based on either the TEM or the CLM-CN model [italics added]."

Do you get the impression that Thornton et al.'s work may depend just a little too heavily on models, as compared to real-world experimental data? If you answer "yes," congratulations, you are correct, as you may verify for yourself by visiting the Grasslands, Loblolly Pine and Miscellaneous sub-headings of Nitrogen (Progressive Limitation Hypothesis) in our Subject Index, where we briefly describe the findings of fully twenty-six different experimental studies that provide absolutely no evidence for the progressive nitrogen limitation hypothesis, even in plants growing in soils of extremely low nitrogen content, where evidence for it would surely be expected to be found if the hypothesis was correct.

Historically, far too many falsehoods have been foisted upon the people of the world by mere hypotheses and models to justify accepting the outcome of Thornton et al.'s 2009 study, especially when it is described by Thornton's employer - the U.S. Department of Energy's Oak Ridge National Laboratory - as a breakthrough (which seems a bit overly hyped) and the first time (which does not promote great confidence) that the terrestrial nitrogen cycle has been incorporated "into global simulations for climate change." Much closer to the truth is the institution's news release statement that the study represents but "one more step toward a more realistic prediction for the future of the earth's climate."

The problem, however, is that the journey has only just begun; and there are many more steps that must be taken before it will be anywhere near complete, and before the answer to its quest will be anywhere near correct. Already, in fact, the numerous results of the many real-world experiments we have reviewed on our website clearly indicate that Thornton et al.'s current conclusion is patently false.

Sherwood, Keith and Craig Idso

Hungate, B.A., Dukes, J.S., Shaw, M.R., Luo, Y. and Field, C.B. 2003. Nitrogen and climate change. Science 302: 1512-1513.

Luo, Y., Su, B., Currie, W.S., Dukes, J.S., Finzi, A., Hartwig, U., Hungate, B., McMurtrie, R.E., Oren, R., Parton, W.J., Pataki, D.E., Shaw, M.R., Zak, D.R. and Field, C.B. 2004. Progressive nitrogen limitation of ecosystem responses to rising atmospheric carbon dioxide. BioScience 54: 731-739.

McGuire, A.D., Sitch, S., Clein, J.S., Dargaville, R., Esser, G., Foley, J., Heimann, M., Joos, F., Kaplan, J., Kicklighter, D.W., Meier, R.A., Melillo, J.M., Moore III, B., Prentice, I.C., Ramankutty, N., Reichenau, T., Schloss, A., Tian, H., Williams, L.J. and Wittenberg, U. 2001. Carbon balance of the terrestrial biosphere in the twentieth century: Analyses of CO2, climate and land use effects with four process-based ecosystem models. Global Biogeochemical Cycles 15: 183-206.

Sokolov, A.P., Kicklighter, D.W., Melillo, J.M., Felzer, B.S., Schlosser, C.A. and Cronin, T.W. 2008. Consequences of considering carbon-nitrogen interactions on the feedbacks between climate and the terrestrial carbon cycle. Journal of Climate 21: 3776-3796.

Thornton, P.E., Doney, S.C., Lindsay, K., Moore, J.K., Mahowald, N., Randerson, J.T., Fung, I., Lamarque, J.-F., Feddema, J.J. and Lee, Y.-H. 2009. Carbon-nitrogen interactions regulate climate-carbon cycle feedbacks: results from an atmosphere-ocean general circulation model. Biogeosciences 6: 2120-2120.

Thornton, P.E., Lamarque, J.-F., Rosenbloom, N.A. and Mahowald, N.M. 2007. Influence of carbon-nitrogen cycle coupling on land model response to CO2 fertilization and climate variability. Global Biogeochemical Cycles 21: 10.1029/2006GB002868.