Comment on "Climate and Management Contributions to Recent Trends in U.S. Agricultural Yields"

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Abstract
The first concern relates to selective use of data in (1). To estimate climatic and nonclimatic contributions to recent trends in U.S. corn and soybean production, Lobell and Asner used only data that exhibited significantly negative correlations between summer (June to August) temperature anomalies and corn and soybean yield anomalies (1). By doing so, they guaranteed that there would be a temperature effect on the yield trends of these two crops. To demonstrate this, I adopted several key assumptions made by Lobell and Asner (1) to randomly construct an artificial data set of crop yields and air temperatures for 1500 hypothetical locations over a 17-year period (2) and analyzed this artificial data set using the same method. I found that if data from all locations were included in the analysis, there was no statistical relationship between yield trends and temperature trends (Fig. 1A). However, if the locations were ranked based on correlation coefficients between temperature anomalies and yield anomalies and if only those locations with positive correlations between the anomalies of the two variables were included, yield trends increased with temperature trends (Fig. 1B). Conversely, if only locations with negatively correlated temperature and yield anomalies were used, yield trends decreased with temperature trends (Fig. 1C). Clearly, this negative correlation between yield trends and temperature trends was the result of selective use of my artificial data set. Lobell and Asner (1) used this method of analysis, which, by the same token, could have led to data with biased temperature–yield relationships, although the authors stated that their selected subsets were “representative of total national production” [supporting note S2 in (1)].