Background
CO2-induced global warming has long been predicted to increase evapotranspiration, causing, in the words of the authors, "soil moisture declines that may offset modest increases in continental precipitation and lead to greater aridity in water-limited systems around the world (Manabe and Wetherald, 1986; Rind, 1988; Gleick, 1989; Vlades et al., 1994; Gregory et al., 1997; Komescu et al., 1998)."

What was done
The authors tested this hypothesis 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.

What was learned
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 (the length of the growing season) over which transpiration losses occur, all without any decrease in total plant production.

What it means
The authors 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 authors 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."

For more on the CO2 aspect of this topic, see Soil (Water Status - Field Studies and Growth Chamber Studies) in our Subject Index.  Also, we call your attention to the study of Robock et al. (2000), who developed a collection of real-world soil moisture data for over 600 stations from a wide variety of climatic regimes and found 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."

Finally, we note that if biological feedbacks can have such dramatic impacts on some of the most basic predictions of today's state-of-the-art climate models, even to the point of altering their qualitative predictions, it is not unreasonable to believe they could produce significant alterations in their basic warming predictions as well [see, for example, Feedback Factors (Biophysical) in our Subject Index].

References
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.

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., 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.

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.