Volume 10, Number 46: 14 November 2007
Nearly a quarter-century ago, in a paper entitled "rising atmospheric carbon dioxide concentrations may increase streamflow," Idso and Brazel (1984) suggested there was evidence (from their study of twelve drainage basins in Arizona, USA) that such was already occurring, as a consequence of CO2-induced reductions in plant transpirational water losses. Twenty-two years later, Gedney et al. (2006) confirmed that such was also true for the globe as a whole; and, now, Betts et al. (2007) have demonstrated that the globally-observed phenomenon may be reproduced by an ensemble of "experiments" with a global climate model that "includes a vegetation component to assess the contribution of physiological forcing to future changes in continental runoff." More specifically, they found that "the physiological effect of doubled carbon dioxide concentrations on plant transpiration increases simulated global mean runoff by 6 percent relative to pre-industrial levels; an increase that is comparable to that simulated in response to radiatively forced climate change (11 ± 6 per cent)."
What are the implications of these several observations?
Betts et al. say their results suggest that "freshwater resources may be less limited than previously assumed under scenarios of future global warming, although there is still an increased risk of drought." The first of these conclusions is indeed correct. The latter, however, is debatable, for recently analyzed data suggest that precipitation has been increasing over the world at a much greater rate (between 2.3 and 7 times larger, in fact) than that anticipated by climate models (see Wentz et al., 2007). On the other hand, Betts et al. note that "risks of rain and river flooding may increase more than has previously been anticipated, because intense precipitation events would be more likely to occur over saturated ground," and such is indeed a possibility.
So how can the potential benefits (more freshwater resources) and possible risks (more river flooding) of the transpiration-reducing effect of the ongoing rise in the air's CO2 content be rationally weighed against each other?
We feel that the key to answering this question resides in the serious concern of many knowledgeable scientists that we will need to divert essentially all usable non-saline water on the face of the planet to the agricultural enterprises that will be required to meet the food and fiber needs of our growing numbers by the fast-approaching year 2050 (Wallace, 2000; Tilman et al., 2001). Although increased river-flooding could be devastating when and where it might occur, the prospect of taking nearly all of what remains of earth's freshwater resources from the plant and animal species that comprise what some have called "wild nature" would be unconscionable in the highest degree, as it would lead to the extinction of huge numbers of them.
Clearly, the water-conserving benefits of allowing the air's CO2 content to continue to rise far outweigh the adverse consequences that might be experienced with even a huge increase in river flooding, especially when it is realized that in many parts of the world major rivers are already having their lifeblood sucked out of them by ever-increasing human withdrawals of water.
Sherwood, Keith and Craig Idso
References
Betts, R.A., Boucher, O., Collins, M., Cox, P.M., Falloon, P.D., Gedney, N., Hemming, D.L., Huntingford, C., Jones, C.D., Sexton, D.M.H. and Webb, M.J. 2007. Projected increase in continental runoff due to plant responses to increasing carbon dioxide. Nature 448: 1037-1041.
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.
Idso, S.B. and Brazel, A.J. 1984. Rising atmospheric carbon dioxide concentrations may increase streamflow. Nature 312: 51-53.
Tilman, D., Fargione, J., Wolff, B., D'Antonio, C., Dobson, A., Howarth, R., Schindler, D., Schlesinger, W.H., Simberloff, D. and Swackhamer, D. 2001. Forecasting agriculturally driven global environmental change. Science 292: 281-284.
Wallace, J.S. 2000. Increasing agricultural water use efficiency to meet future food production. Agriculture, Ecosystems & Environment 82: 105-119.
Wentz, F.J., Ricciardulli, L., Hilburn, K. and Mears, C. 2007. How much more rain will global warming bring? Science 317: 233-235.