Comment on "Climate and management contributions to recent trends in U.S. agricultural yields".

Management Contributions to Recent Trends in U.S. Agricultural Yields” According to Lobell and Asner (1), the atypical summer cooling trend from 1982 to 1998 increased U.S. yields of corn and soybeans during the same period. However, three potential problems with their analysis bring their results and conclusion into question. 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)]. A second concern lies in the treatment of effects of precipitation and solar radiation trends on yield trends. Lobell and Asner (1) found that yield trends were not significantly influenced by precipitation or solar radiation trends, but they drew that conclusion by analyzing the selected subset of the data thathad previously been filtered according to correlations between yield anomalies and temperature anomalies. Had the authors treated precipitation and solar radiation in the same manner they treated temperature, their results might have been different. In other words, if they had selected their data based on correlations between yield anomalies and precipitation or solar radiation anomalies, they might have found that yield trends were indeed correlated with precipitation or solar radiation trends. In addition, with this differently filtered data set, they might not have found that temperature trends contributed to yield trends. The third concern—perhaps minor compared with the first two—is the application of a linear model to calculate contributions of climate effects versus nonclimate effects (for example, management, technology, increased atmospheric CO2 concentrations) to observed trends in corn and soybean yields (1). Using a linear model assumes that the nonclimate contribution is constant over time. If the nonclimate contribution were to increase or decrease gradually, however, this second-order trend would be miscalculated as a temperature contribution to the yield trend. Because of the problems identified above, the results obtained by Lobell and Asner (1) are not substantiated by their method of analysis, and their conclusion regarding the effects of climate on food production should be reevaluated.