INCREASE OF CO2 AND CLIMATE-CHANGE EFFECTS ON IOWA SOYBEAN YIELD, SIMULATED USING GLYCIM

Increases in atmospheric CO2 concentration are likely to have a signif icant impact on the climate and to affect the growth and development o f crops. The effect of climate change on crops has to be assessed for large areas to provide information for global estimates and regional s trategy development. In this study, computer simulations were done usi ng the soybean [Glycine max (L.) Merr.] crop model GLYCIM to assess th e effects of these changes on soybean yields in the state of Iowa. The ability of the model to accurately predict the effect of changes in a tmospheric CO2 concentration was tested by comparing model results wit h a curve fit of CO2 concentration yield response derived from measure d data. To simulate the effects of climate change, we used projected m onthly weather variables for ambient and increased CO2 from three gene ral circulation models (GCMs): Goddard Institute for Space Studies (GI SS), Geophysical Fluid Dynamics Laboratory (GFDL), and United Kingdom Meteorological Office (UKMO). The monthly weather variables were conve rted to the daily weather to simulate soybean crop yields in nine crop reporting districts (CRDs) of Iowa, with and without increases in atm ospheric CO2. When climate change was simulated with increasing levels of CO2 (baseline 350, 450, 550, and 650 mu L L-1), yields increased. Without increases in atmospheric CO2, yields remained essentially cons tant in the GFDL and GISS scenarios, while decreasing by 2 to 10% in t he UKMO scenario. The variability in yields among nine CRDs increased for all scenarios as climate changed and levels of CO2 increased. Over all, projected climate changes did not substantially alter simulated s oybean yields. The increase in CO2 concentration in the atmosphere was the primary determinant of yield response.