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Soil Moisture Trends: IPCC Simulations vs. Real-World Observations
Li, H., Robock, A. and Wild, M. 2007. Evaluation of Intergovernmental Panel on Climate Change Fourth Assessment soil moisture simulations for the second half of the twentieth century. Journal of Geophysical Research 112: 10.1029/2006JD007455.

Because "soil moisture trends, particularly during the growing season, are an important possible consequence of global warming," in the words of the authors, "climate model simulations of future soil moisture changes should be made with models that can produce reliable simulations of soil moisture for past climate changes," which is a proposition with which almost everyone would have to agree.

What was done
Li et al. compared soil moisture simulations derived from the IPCC's Fourth Assessment climate models (which were driven by observed climate forcings) for the period 1958-1999 with actual measurements of soil moisture made at over 140 stations or districts in the mid-latitudes of the Northern Hemisphere, which were averaged in such a way as to yield six regional results: one each for the Ukraine, Russia, Mongolia, Northern China, Central China and Illinois (USA).

What was learned
The three researchers report that the models showed realistic seasonal cycles for the Ukraine, Russia and Illinois but "generally poor seasonal cycles for Mongolia and China." In addition, they say that the Ukraine and Russia experienced soil moisture increases in summer "that were larger than most trends in the model simulations." In fact, they report that "only two out of 25 model realizations show trends comparable to those observations," and they note that the two realistic model-derived trends were "due to internal model variability rather than a result of external forcing," which means that the two reasonable matches were actually accidental.

Noting further that "changes in precipitation and temperature cannot fully explain soil moisture increases for [the] Ukraine and Russia," Li et al. write that "other factors might have played a dominant role on the observed patterns for soil moisture." In this regard they mention solar dimming, as well as the fact that in response to elevated atmospheric CO2 concentrations, "many plant species reduce their stomatal openings, leading to a reduction in evaporation to the atmosphere," so that "more water is likely to be stored in the soil or [diverted to] runoff," correctly reporting that this phenomenon was recently detected in continental river runoff data by Gedney et al. (2006).

What it means
The climate models employed in the IPCC's Fourth Assessment are clearly deficient in their ability to correctly simulate soil moisture trends, even when applied to the past and when driven by observed climate forcings. In other words, they fail the most basic type of test imaginable; and in the words of Li et al., this finding suggests that "global climate models should better integrate the biological, chemical, and physical components of the earth system."

Gedney, N., Cox, P.M., Betts, R.A., Boucher, O., Huntingford, C. and Stott, P.A. 2006. Detection of a direct carbon dioxide effect in continental river runoff records. Nature 439: 835-838.

Reviewed 13 June 2007