So is this climate-alarmist claim just another of those "scary 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, by comparing the simulations to actual soil moisture observations." In summarizing their findings, they stated that "the revised models do not show any [our italics] systematic improvement in their ability to simulate observed seasonal variations of soil moisture over the regions studied." And if those words are not clear enough, they also said "there are no [our italics] 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. note that "simpler land-surface parameterization schemes are being replaced by conceptually realistic treatments," as the climate modeling enterprise moves ever forward, they state that "improvements gained by such changes are ... not very apparent," or as we might describe the situation, things that do not exist, i.e., true model improvements, are very hard to see.

These findings are 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 improvement; and we surely believe 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 mammoth understatement, 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 this case, for example, Robock et al. note 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 is our hope also.

Skipping ahead five years, we find another important report on the subject from Robock et al. (2005), who note that "most global climate model simulations of the future, when forced with increasing greenhouse gases and anthropogenic aerosols, predict summer desiccation in the midlatitudes of the Northern Hemisphere (e.g., Gregory et al., 1997; Wetherald and Manabe, 1999; Cubasch et al., 2001)," and say 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 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." As they describe it, "the observations show a positive soil moisture trend for the entire period of observation, with the trend leveling off in the last two decades," noting 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. note 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 say 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-transpirant effect of atmospheric CO2 enrichment (see Transpiration in our Subject Index), 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 (see Water Status of Soil (Field Studies) in our Subject Index).

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 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. 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. 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 15 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 suggests that just the opposite could well occur. As the six researchers describe 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 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.

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.

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.

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.

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.