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River Runoff: The Effect of Atmospheric CO2 Enrichment
Volume 9, Number 8: 22 February 2006

In a paper published in the 16 February issue of Nature, Gedney et al. (2006) claim to have detected "a direct carbon dioxide effect in continental river runoff records." This was done, they said, using "the formal detection and attribution techniques developed to isolate the causes of twentieth-century temperature change," whereby a model is used to define the spatial and temporal responses in river runoff associated with known changes in individual forcing factors, after which the "spatio-temporal patterns of response act as 'fingerprints' that allow the observed change to be separated into contributions from each factor."

Application of this technique with respect to 20th-century changes in climate, aerosols, land use and atmospheric CO2 concentration revealed that only climate and the direct anti-transpirant effect of CO2, i.e., "suppression of plant transpiration due to CO2-induced stomatal closure," were detectable in the world river runoff data at the 5% significance level. In an interview with New Scientist's Fred Pearce (2006), Gedney further reports 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 phenomena coming close to the 3-4% flow increase actually observed.

In commenting on their findings, the six UK scientists note that although field and laboratory studies had long shown that "stomatal openings of many plant species reduce under elevated CO2, it was unclear whether this reduction would have any significance for the global water cycle, in which real ecosystems are typically limited by water and nutrient availability." Well over two decades ago, however, one of us and a colleague reported in the very same journal (Nature) that our analysis of just such real-world ecosystems (12 drainage basins in Arizona, USA) indicated that a doubling of the air's CO2 content could increase the flow rates of the streams they fed by between 40 and 60% (Idso and Brazel, 1984).

For the increase in CO2 experienced over the 20th-century, these results imply streamflow increases in the range of 9-13%. Although these increases are higher than the observed increase in global river-runoff over this period, such is only to be expected, since climate change effects over the same period work against the CO2 effect, since human withdrawal of water from the world's rivers increased dramatically over the 20th-century, and since the CO2-induced decrease in plant transpiration is partially countered by the concomitant expansion of vegetative cover that is promoted by CO2-induced increases in plant water use efficiency, which lead to plant water extraction from previously unvegetated soils.

What are the ramifications of these new - and old - findings? The most important outcome is undoubtedly increasing freshwater availability, which is highlighted in the Gedney et al. Nature paper, the Editor's Summary of their paper, and Matthews' (2006) News & Views item about it. We concur in this conclusion, noting that the importance of increasing freshwater availability can hardly be emphasized enough, as indicated by the study of Wallace (2000).

It seems that good news cannot be much tolerated these days, however, unless there are some negative consequences attached to it ... and so it is in this case. In the 15 February report of New Scientist's Fred Pearce, it says that the CO2-induced increases in river flow are "raising the risks of flooding," while in the 16 February report of Agence France-Presse's Richard Ingham it says essentially the same thing, i.e., that "the extra runoff will magnify the risk of flooding," as is also stated in the Editor's Summary of Nature. If one peruses the many items we have archived under Runoff and Streamflow (Recent Trends) in our Subject Index, however, it should be clear to most everyone that there is little justification for this claim in terms of real-world observations; and to their credit, neither Gedney et al. nor Matthews openly raise this specter in their reports.

What Gedney et al. do say, however, is that "as the direct CO2 effect reduces surface energy loss due to evaporation, it is likely to add to surface warming," as is also stated in the Editor's Summary in Nature and as is implied by Matthews. But to the degree that such a warming does in fact occur, that amount of warming is simply that much less that can be attributed to the augmented greenhouse effect of CO2 over the 20th century. And if the greenhouse effect of CO2 must therefore be reduced, without any reduction in the amount of CO2 added to the air, the reduced warming must be the result of a smaller planetary climate sensitivity than what has heretofore been assumed to be correct by climate modelers. Hence, it can be appreciated that the extra surface warming provided by the anti-transpirant effect of atmospheric CO2 enrichment does nothing to advance the cause of climate alarmism. In fact, it may actually work against it.

The major take-home message of the findings of Gedney et al. would therefore seem to be that the biological impact of 20th-century atmospheric CO2 enrichment has prevailed over the impact of what climate alarmists describe as a global warming that is without precedent over the past two millennia, with respect to altering the planet's hydrologic cycle. And if there is a secondary message, it is that their findings do nothing to promote the case for CO2-induced global warming of the magnitude they typically predict.

Sherwood, Keith and Craig Idso

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.

Ingham, R. 2006. Greenhouse gas driving river flow by making plants less thirsty. Agence France-Presse.

Matthews, D. 2006. The water cycle freshens up. Nature 439: 793-794.

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

Wallace, J.S. 2000. Increasing agricultural water use efficiency to meet future food production. Agriculture, Ecosystems & Environment 82: 105-119.