So is this climate-alarmist claim just another one of those "scare stories" that certain people feel justified in feeding to the public to achieve ends they consider lofty enough to justify such less-than-noble means? Let's take a look at the pertinent scientific literature and see what real-world data have to say about the subject.

In a turn-of-the century evaluation of how climate modelers had progressed in their efforts to improve their simulations of soil moisture content over the prior few years, Srinivasan et al. (2000) examined "the impacts of model revisions, particularly the land surface representations, on soil moisture simulations." This they did by comparing the simulations to actual soil moisture observations. And in summarizing their findings, they stated that "the revised models do not show any systematic improvement in their ability to simulate observed seasonal variations of soil moisture over the regions studied." And if those words were not clear enough, they also said "there are no indications of conceptually more realistic land surface representations producing better soil moisture simulations in the revised climate models." In addition, they reported there was a "tendency toward unrealistic summer drying in several models," which they noted was "particularly relevant in view of the summer desiccation projected by GCMs considered in future assessments of climate change."

Although Srinivasan et al. noted that "simpler land-surface parameterization schemes were being replaced by conceptually realistic treatments" as the climate modeling enterprise moved ever forward, they stated that the improvements gained by these changes were "not very apparent."

This fact was truly astounding, even to us. We would have thought that in an avowed attempt to improve this particular aspect of climate modeling, there would have been at least some type of improvement; and we surely believe that it will yet occur. But these were the conclusions of those who had studied the subject in depth as of the publication date of their journal article (16 November 2000); and in view of their findings, we are forced to conclude that at that time there had indeed been no real progress, only attempted progress.

More evidence for the validity of this conclusion was supplied in the very same year by Robock et al. (2000), who developed a massive collection of soil moisture data for over 600 stations from a wide variety of climatic regimes found within the former Soviet Union, China, Mongolia, India and the United States. In describing these datasets they also stated an important ground rule. Sometimes, they said, "the word 'data' is used to describe output from theoretical model calculations, or values derived from theoretical analysis of radiances from remote sensing." However, as they put it, "we prefer to reserve this word for actual physical observations," noting that "all the data in our data bank are actual in situ observations."

This distinction is important, for one of the illuminating analyses Robock et al. performed with their data was to check summer soil moisture trends simulated by the Geophysical Fluid Dynamics Laboratory's general circulation model of the atmosphere as forced by transient CO2 and tropospheric sulfate aerosols for specific periods and regions for which they had actual soil moisture data. What they learned from this exercise, in their words, was that "although this model predicts summer desiccation in the next century, it does not in general reproduce the observed upward trends in soil moisture very well," which is a huge understatement, indeed, considering that the predictions and observations go in opposite directions!

Unfortunately, the predictions of sophisticated global climate models are sometimes treated with a reverence as great as -- or even greater than -- actual real-world data. This study is a classic in demonstrating the dangers inherent in such behavior, which has actually led to the creation of an international treaty of wrenching economic and social implications based on faulty premises. In the case considered here, for example, Robock et al. wrote that "in contrast to predictions of summer desiccation with increasing temperatures, for the stations with the longest records, summer soil moisture in the top 1 m has increased while temperatures have risen."

The moral of this story is that when model predictions and actual measurements fail to coincide, or actually diverge, as in this study, the data must rule! Indeed, it was Robock et al.'s hope that the real-world data they had assembled in their data bank might help "improve simulations of the recent past so we may have more confidence in predictions for the next century," which was our hope also.

Skipping ahead five years, we find another important report on the subject from Robock et al. (2005), who noted that "most global climate model simulations of the future, when forced with increasing greenhouse gases and anthropogenic aerosols, predict summer desiccation in the mid-latitudes of the Northern Hemisphere (e.g., Gregory et al., 1997; Wetherald and Manabe, 1999; Cubasch et al., 2001)," and who said that "this predicted soil moisture reduction, the product of increased evaporative demand with higher temperatures overwhelming any increased precipitation, is one of the gravest threats of global warming, potentially having large impacts on our food supply."

With the explicit purpose "to evaluate these model simulations," the three American and two Ukrainian scientists thus went on to present "the longest data set of observed soil moisture available in the world, 45 years of gravimetrically-observed plant available soil moisture for the top 1 m of soil, observed every 10 days for April-October for 141 stations from fields with either winter or spring cereals from the Ukraine for 1958-2002." And as they described it, these observations showed "a positive soil moisture trend for the entire period of observation, with the trend leveling off in the last two decades," while they further noted that "even though for the entire period there is a small upward trend in temperature and a downward trend in summer precipitation, the soil moisture still has an upward trend for both winter and summer cereals."

As a result of these real-world observations, Robock et al. noted that "although models of global warming predict summer desiccation in a greenhouse-warmed world, there is no evidence for this in the observations yet, even though the region has been warming for the entire period." In attempting to explain this dichotomy, they opined that the real-world increase in soil moisture content possibly may have been driven by a downward trend in evaporation caused by the controversial "global dimming" hypothesis (Liepert et al., 2004). Alternatively, it may have been driven by the well-known anti-transpiration effect of atmospheric CO2 enrichment, which tends to conserve water in the soils beneath crops and thereby leads to enhanced soil moisture contents, as has been demonstrated in a host of experiments conducted in real-world field situations.

One especially outstanding study, in this regard, was that of Zaveleta et al. (2003), who tested the hypothesis that soil moisture contents may decline in a CO2-enriched and warmer world in a two-year study of an annual-dominated California grassland at the Jasper Ridge Biological Preserve, Stanford, California, USA, where they delivered extra heating to a number of FACE plots (enriched with an extra 300 ppm of CO2) via IR heat lamps suspended over the plots that warmed the surface of the soil beneath them by 0.8-1.0°C.

The individual effects of atmospheric CO2 enrichment and soil warming were of similar magnitude; and acting together they enhanced mean spring soil moisture content by about 15% over that of the control treatment. The effect of the extra CO2 was produced primarily as a consequence of its ability to cause partial stomatal closure and thereby reduce season-long plant water loss via transpiration; while in the case of warming, there was an acceleration of canopy senescence that further increased soil moisture by reducing the period of time over which transpiration losses occur, all without any decrease in total plant production.

Zaveleta et al. went on to note that their findings "illustrate the potential for organism-environment interactions to modify the direction as well as the magnitude of global change effects on ecosystem functioning." Indeed, whereas for the past many years we have been bombarded with climate-alarmist predictions of vast reaches of agricultural land drying up and being lost to profitable production in a CO2-enriched world of the future, this study suggested that just the opposite could well occur. As the six researchers described it, "we suggest that in at least some ecosystems, declines in plant transpiration mediated by changes in phenology can offset direct increases in evaporative water losses under future warming."

In light of these several real-world observations, it would appear that essentially all climate models employed to date have greatly erred with respect to what Robock et al. (2005) describe as "one of the gravest threats of global warming." Not only has the model-predicted decline in Northern Hemispheric midlatitude soil moisture contents failed to materialize under the combined influence of many decades of rising atmospheric CO2 concentrations and temperatures, it has actually become less of a threat, possibly as a direct consequence of biological impacts of the ongoing rise in the air's CO2 content.

One year later, Guo and Dirmeyer (2006) compared soil moisture simulations made by eleven different models within the context of the Second Global Soil Wetness Project (a multi-institutional modeling research activity intended to produce a complete multi-model set of land surface state variables and fluxes by using current state-of-the-art land surface models driven by the 10-year period of data provided by the International Satellite Land Surface Climatology Project Initiative II) against real-world observations made on the top meter of grassland and agricultural soils located within parts of the former Soviet Union, the United States (Illinois), China and Mongolia, which are archived in the Global Soil Moisture Data Bank.

In doing so, the two researchers found that "simulating the actual values of observed soil moisture is still a challenging task [our italics] for all models [our italics]," noting that "both the root mean square of errors (RMSE) and the spread of RMSE across models are large [our italics]," and that "the absolute values of soil moisture are poorly simulated [our italics] by most models [our italics]." In addition, they found that "within regions there can be tremendous variations [our italics] of any model [our italics] to simulate the time series of soil moisture at different stations."

So just how serious are these large errors and tremendous variations? It would appear that they are very serious, based on a number of explanatory statements made by Guo and Dirmeyer. First of all, the two researchers say "the land surface plays a vital role [our italics] in the global climate system through interactions with the atmosphere." Second, they state that "accurate simulation of land surface states is critical [our italics] to the skill of weather and climate forecasts." Third, they write that soil moisture "is the definitive [our italics] land surface state variable; key [our italics] for model initial conditions from which the global weather and climate forecasts begin integrations, and a vital factor [our italics] affecting surface heat fluxes and land surface temperature."

Therefore, in consequence of what "those in the know" thus describe as large errors and tremendous variations in what they readily characterize as vital, critical, definitive and key elements of state-of-the-art land surface model simulations of soil wetness (which is a pretty basic parameter), it would appear that little faith should be placed in what they portend about the future.

With the passing of another year, Li et al. (2007) had a paper published wherein they wrote that because "soil moisture trends, particularly during the growing season, are an important possible consequence of global warming," 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.

So what did they find when they 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)?

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 Ukraine and Russia," Li et al. wrote that "other factors might have played a dominant role in the observed patterns for soil moisture." And in this regard they mention solar dimming, as well as the fact that in response to elevated atmospheric CO 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 had recently been detected in continental river runoff data by Gedney et al. (2006).

In conclusion, and in light of a wealth of real-world data, the climate models employed in the IPCC's Fourth Assessment were 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 failed the most basic type of test imaginable; and in the words of Li et al., this fact suggests that "global climate models should better integrate the biological, chemical, and physical components of the earth system."

References
Cubasch, U., Meehl, G.A., Boer, G.J., Stouffer, R.J., Dix, M., Noda, A., Senior, C.A., Raper, S. and Yap, K.S. 2001. Projections of future climate change. In: Houghton, J.T. et al. (Eds.), Climate Change 2001: The Scientific Basis: Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, New York, USA, pp. 525-582.

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.

Gleick, P.H. 1989. Climate change, hydrology and water resources. Reviews of Geophysics 27: 329-344.

Gregory, J.M., Mitchell, J.F.B. and Brady, A.J. 1997. Summer drought in northern midlatitudes in a time-dependent CO2 climate experiment. Journal of Climate 10: 662-686.

Guo, Z. and Dirmeyer, P.A. 2006. Evaluation of the Second Global Soil Wetness Project soil moisture simulations: 1. Inter-model comparison. Journal of Geophysical Research 111: 10.1029/2006JD007233.

Komescu, A.U., Eikan, A. and Oz, S. 1998. Possible impacts of climate change on soil moisture availability in the Southeast Anatolia Development Project Region (GAP): An analysis from an agricultural drought perspective. Climatic Change 40: 519-545.

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.

Liepert, B.G., Feichter, J., Lohmann, U. and Roeckner, E. 2004. Can aerosols spin down the water cycle in a warmer and moister world? Geophysical Research Letters 31: 10.1029/2003GL019060.

Manabe, S. and Wetherald, R.T. 1986. Reduction in summer soil wetness induced by an increase in atmospheric carbon dioxide. Science 232: 626-628.

Rind, D. 1988. The doubled CO2 climate and the sensitivity of the modeled hydrologic cycle. Journal of Geophysical Research 93: 5385-5412.

Robock, A., Mu, M., Vinnikov, K., Trofimova, I.V. and Adamenko, T.I. 2005. Forty-five years of observed soil moisture in the Ukraine: No summer desiccation (yet). Geophysical Research Letters 32: 10.1029/2004GL021914.

Robock, A., Vinnikov, K.Y., Srinivasan, G., Entin, J.K., Hollinger, S.E., Speranskaya, N.A., Liu, S. and Namkhai, A. 2000. The global soil moisture data bank. Bulletin of the American Meteorological Society 81: 1281-1299.

Srinivasan, G., Robock, A., Entin, J.K., Luo, L., Vinnikov, K.Y., Viterbo, P. and Participating AMIP Modeling Groups. 2000. Soil moisture simulations in revised AMIP models. Journal of Geophysical Research 105: 26,635-26,644.

Vlades, J.B., Seoane, R.S. and North, G.R. 1994. A methodology for the evaluation of global warming impact on soil moisture and runoff. Journal of Hydrology 161: 389-413.

Wetherald, R.T. and Manabe, S. 1999. Detectability of summer dryness caused by greenhouse warming. Climatic Change 43: 495-511.

Zavaleta, E.S., Thomas, B.D., Chiariello, N.R., Asner, G.P., Shaw, M.R. and Field, C.B. 2003. Plants reverse warming effect on ecosystem water balance. Proceedings of the National Academy of Science USA 100: 9892-9893.