Modeling the impacts of climate change on wheat yields in Northwestern Turkey

This study investigated the impacts of elevated atmospheric CO2 concentrations and associated changes in climate on winter wheat yields in northwestern Turkey. The analysis was based on climate change predictions of four global circulation models (GCMs) for three greenhouse gas emission scenarios during three time periods in the 21st century. Climate change predictions by most GCMs used in this study suggested a consistent pattern of increase in the mean growing season air temperature and this increase is more pronounced under the aggressive emission scenario. Growing season precipitation experienced various levels of reduction, although with more variability than air temperature, depending on the model used and the scenario considered. Using these inputs, a mechanistic wheat crop model that successfully simulated contemporary wheat yields as well as the timing of critical growth stages indicated that increased atmospheric CO2 concentrations in the absence of changing climatic conditions had a slightly positive effect on yields. This was due to both the stimulating function of CO2 for photosynthesis and the regulatory function of CO2 that increases water use efficiency. However, these positive effects failed to counteract the significant decline in yields when temperature and precipitation were allowed to vary with increased atmospheric CO2 concentrations. Under these conditions, winter wheat yields were predicted to decline between 5 and 35 percent, depending on the GCM input used. When multi-model ensemble GCM inputs were considered to reduce inter-model variation, wheat yields were predicted to decline in excess of 20 percent across all emission scenarios and time periods. Further investigation of crop model outputs suggested that the main drivers of reduced yields were a shorter grain filling period caused by rapid plant development and increased water loss through transpiration, exacerbated by a significant decline in precipitation. The combined effects of reduced precipitation and enhanced transpiration in a future climate are particularly important in this semi-arid region, which is already at the limit of rain-fed winter wheat production. This study also points to the urgent need for controlled field- and laboratory-based experiments for temperature, water, and greenhouse gas effects on cereal yields as well as development of cultivars with longer grain filling periods in Turkish environments.