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Drought Stress Effects on Wheat and the Mitigating Effect of CO2
Manderscheid, R. and Weigel, H.-J. 2007. Drought stress effects on wheat are mitigated by atmospheric CO2 enrichment. Agronomy for Sustainable Development 27: 79-87.

What was done
The authors grew spring wheat (Triticum aestivum cv. Minaret) in open-top chambers on an experimental field of the Federal Agricultural Research Center in Braunschweig, Germany, in two different growing seasons at either current or future (current + 280 ppm) atmospheric CO2 concentrations and under sufficient-water-supply (WET) or drought-stress (DRY) conditions, the latter of which was imposed just after the crop first-node stage was reached (approximately 35 days after emergence) by halving the subsequent water supplied to the plants.

What was learned
Manderscheid and Weigel found that, "in both years, biomass and grain yield were decreased by drought and increased by CO2 enrichment," with the positive CO2 effect being greater under drought conditions. "Averaged over both years," as they describe it, "CO2 enrichment increased biomass and grain yield under WET conditions by <=10% and under DRY conditions by >=44%." In addition, they likewise determined that the CO2-induced increase in crop water-use efficiency was 20% in the sufficient-water-supply treatment and 43% in the drought-stress treatment.

What it means
Based on their findings, the two German researchers concluded that "negative effects on wheat yield resulting from increased water shortage, as predicted from global climate models for the future [our italics], may be mitigated by the higher CO2 concentration and yield may be decreased to a lesser extent if all other environmental conditions remain the same."

But the truth is even better than that. As recently noted by Wentz et al. (2007), for example, one of the most important environmental conditions pertaining to this conclusion is clearly not "remaining the same," for as stated in the mini-abstract of the latter researchers' paper in Science's table of contents, "humidity and precipitation unexpectedly increased at the same rate in response to global warming during the past 20 years, yielding more rainfall than predicted by models [our italics]." As a result, we can realistically expect future wheat yields to be significantly enhanced by the ongoing rise in the air's CO2 content throughout the coming decades of continued fossil fuel utilization.

Wentz, F.J., Ricciardulli, L., Hilburn, K. and Mears, C. 2007. How much more rain will global warming bring? Science 317: 233-235.

Reviewed 10 October 2007