What was done
The authors "integrated a number of selected algorithms into the soil-crop model HERMES to test their suitability to describe CO2 impacts on crop growth against data of a Free Air Carbon Enrichment (FACE) experiment (Weigel and Dammgen, 2000)." This experiment covered two cycles of a three-year crop rotation (consisting of winter barley, sugar beet and winter wheat) that was grown under optimum soil water and nutrient conditions at atmospheric CO2 concentrations of either about 375 ppm (ambient) or 550 ppm (elevated). The algorithms that best described the experimental results were then used in combination with downscaled climate change scenarios to realistically simulate crop productivity at different sites in Germany under the SRES-A1B scenario with and without the inclusion of CO2 effects.

What was learned
Kersebaum et al. report that the FACE experiment "showed two important results: increased CO2 (i) enhanced crop growth for all investigated species and (ii) decreased evapotranspiration rate of the canopies resulting in higher soil moisture content (Weigel et al., 2006)." As a result, they found that "without consideration of the CO2 effect, mostly negative impacts on crop yields were simulated," but that "considering the CO2 effect compensated the negative trend in most cases and turned yield effects to a positive impact."

What it means
If the temperature increases and precipitation decreases predicted for Germany in the 2031-2050 time period ever occur, the increase in the atmosphere's CO2 concentration that is predicted to accompany them appears capable of leading to a concurrent increase in the productivity of "a typical agricultural crop rotation in Germany."

References
Weigel, H.J. and Dammgen, U. 2000. The Braunschweig Carbon Project: atmospheric flux monitoring and free air carbon dioxide enrichment (FACE). Journal of Applied Botany 74: 55-60.

Weigel, H.J., Manderscheid, R., Burkart, S., Pacholski, A., Waloszczyk, C.K., Fruhauf, O. and Heinemeyer, O. 2006. Responses of an arable crop rotation system to elevated CO2. In Nosberger et al. (Eds.), Managed Ecosystems and CO2 Case Studies, Processes and Perspectives. Ecological Studies 187: 121-137.