A pair of recent studies examining purported cropland expansion in the Midwest are based on a flawed methodology that “suffers from accuracy and certainty issues,” according to a new review of the studies by researchers at Southern Illinois University Edwardsville. In reviewing studies by Zhang et al. and Lark et al., the SIUE authors found that the inherent defects in their methodology “severely hinder its use for estimating land use change over time.”
In their paper, Joshua Pritsolas and Randall Pearson of SIUE’s GeoSpatial Mapping, Applications, and Research Center pointed out that both studies relied on the U.S. Department of Agriculture’s Cropland Data Layer (CDL) tool to estimate the conversion of grassland to cropland, a use for which the tool was not intended and is poorly suited. As the USDA itself has noted, “Unfortunately, the pasture and grass-related land cover categories have traditionally had very low classification accuracy in the CDL,” meaning grassland is often confused with cropland in the CDL dataset.
The reliance of Zhang et al. and Lark et al. on USDA’s CDL tool renders the results of both studies highly questionable. “Given these issues, policy makers should exercise caution in referencing studies that have performed or integrated land cover/use change analysis that relies on the CDL,” according to Pritsolas and Pearson.
According to the SIUE analysis, it is likely that Zhang et al. and Lark et al. grossly overstated the amount of cropland expansion between 2008 and 2016 because the CDL tool frequently misclassified cropland as grassland in the early part of this time period. “The cropland expansion claimed by Lark et al. (2020) and adopted by Zhang et al. (2021) has a high potential of being false change due to poor classification certainty in the earlier CDL,” the authors found.
Meanwhile, researchers from Purdue University, the University of Illinois—Chicago, and the Department of Energy’s Argonne National Laboratory earlier this month responded to unfounded criticism from a British consulting group about the land use change modeling framework developed by Purdue and the DOE. In a point-by-point rebuttal, the Purdue, UIC, and DOE authors corrected the record regarding their methodology for estimating land use change emissions. “The existing literature has reached the conclusion that early research in this area significantly overstated the land use implications of biofuels,” they wrote. “As the conversation continues, it is important for the community to remain focused on the big picture regarding agriculture’s role as a very effective GHG mitigation tool that can shape the new policies to govern production and consumption of biofuels.”
Renewable Fuels Association President and CEO Geoff Cooper commented on the importance of the new reports from both SIUE and Purdue, UIC, and DOE.
“As part of the process to propose Renewable Fuel Standard volumes for 2023 and beyond, the Environmental Protection Agency is currently analyzing the environmental impacts of the RFS to date,” Cooper said. “At the same time, the National Academy of Sciences is examining the state of the science regarding lifecycle analysis of low-carbon transportation fuels like ethanol. Therefore, it is crucial that the scientific and regulatory communities have access to current, reliable data and robust methodologies for assessing the climate impacts of a broad array of transportation fuel options. Important decisions regarding the future of the RFS should be based on sound science—not political science. We applaud the experts at SIUE, Purdue, UIC, and DOE for defending their good work and scrutinizing questionable studies that misrepresent the lifecycle impacts of biofuels.”
RFA Chief Economist Scott Richman testified at a NAS hearing Tuesday and stressed the fact that historical predictions about land use/cover change have turned out to be greatly exaggerated. “There has not been a significant increase in U.S. cropland since the Renewable Fuel Standard was expanded in 2007,” Richman said. “Given the clarity of statistics on this fact, opponents have turned to contorting satellite-based imagery to try to find land cover and land use change.”