What was done
In an effort to better understand what conditions lead to the occurrence of particularly severe European heat waves, the authors conducted regional climate simulations with and without land-atmosphere coupling for the major summer heat waves of 1976, 1994, 2003 and 2005.

What was learned
Because of the fact that depletion of soil moisture (which has long been predicted to accompany CO2-induced global warming) results in reduced latent cooling, Fischer et al. found that during all simulated heat wave events, "soil moisture-temperature interactions increase the heat wave duration and account for typically 50-80% of the number of hot summer days," noting that "the largest impact is found for daily maximum temperatures," which were amplified by as much as 2-3°C in response to observed soil moisture deficits in their study.

So what does all of this have to do with atmospheric CO2? Quite a lot, it would appear, as explained in the following section.

What it means
In the words of Robock et al. (2000) - who developed a massive collection of soil moisture data from over 600 stations spread across a variety of climatic regimes (including the former Soviet Union, China, Mongolia, India and the United States) - "for the stations with the longest records, summer soil moisture in the top 1 m has increased [our italics] while temperatures have risen." This counter-intuitive finding was confirmed by Robock et al. (2005) and Li et al. (2007), the latter of whom note that when exposed to elevated concentrations of atmospheric CO2, "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," which latter phenomenon has in turn been confirmed by the work of Gedney et al. (2006), the senior author of whom was quoted by Pearce (2006) as saying that "climate change on its own would have slightly reduced runoff, whereas the carbon dioxide effect on plants would have increased global runoff by about 5%," with the combined effect of the two competing phenomena leading to the 3-4% flow increase actually observed.

In light of these complementary global soil moisture and river runoff observations, it would appear that the anti-transpiration effect of the historical rise in the air's CO2 content has more than compensated for the soil-drying effect of concomitant global warming; and this observation brings us to the ultimate point of our Journal Review. Based upon (1) the findings of Fischer et al. (2007) that soil moisture depletion greatly augments both the intensity and duration of summer heat waves, plus (2) the findings of Robock et al. (2000, 2005) and Li et al. (2007) that global soil moisture has actually increased over the past half century, likely as a result of the anti-transpiration effect of atmospheric CO2 enrichment - as Gedney et al. (2006) have also found to be the case with closely associated river runoff - it directly follows that the increase in soil moisture caused by rising atmospheric CO2 concentrations will tend to decrease both the intensity and duration of summer heat waves as time progresses.

References
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.

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.

Pearce, F. 2006. Increased CO2 may cause plant life to raise rivers. NewScientist.com.

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.