Why do crop models diverge substantially in climate impact projections? A comprehensive analysis based on eight barley crop models

Robust projections of climate impact on crop growth and productivity by crop models are key to designingeffective adaptations to cope with future climate risk. However, current crop models diverge strongly in theirclimate impact projections. Previous studies tried to compare or improve crop models regarding the impact ofone single climate variable. However, this approach is insufficient, considering that crop growth and yield areaffected by the interactive impacts of multiple climate change factors and multiple interrelated biophysicalprocesses. Here, a new comprehensive analysis was conducted to look holistically at the reasons why cropmodels diverge substantially in climate impact projections and to investigate which biophysical processes andknowledge gaps are key factors affecting this uncertainty and should be given the highest priorities for im-provement. First, eight barley models and eight climate projections for the 2050s were applied to investigate theuncertainty from crop model structure in climate impact projections for barley growth and yield at two sites:Jokioinen, Finland (Boreal) and Lleida, Spain (Mediterranean). Sensitivity analyses were then conducted on theresponses of major crop processes to major climatic variables including temperature, precipitation, irradiation,and CO2, as well as their interactions, for each of the eight crop models. The results showed that the temperatureand CO2relationships in the models were the major sources of the large discrepancies among the models inclimate impact projections. In particular, the impacts of increases in temperature and CO2on leaf area devel-opment were identified as the major causes for the large uncertainty in simulating changes in evapotranspira-tion, above-ground biomass, and grain yield. Ourfindings highlight that advancements in understanding thebasic processes and thresholds by which climate warming and CO2increases will affect leaf area development,crop evapotranspiration, photosynthesis, and grain formation in contrasting environments are needed formodeling their impacts.