Grassland conversion into croplands in the Prairie Pothole Region of the United States is a persistent hurdle toward mitigating climate change. Several carbon offset markets have been designed to reward landowners for keeping lands in their native state when incentives to convert are high. We explore the role of a critical determinant in such programs: the additionality threshold. This factor, if appropriately selected and applied, reduces the participation of landowners that would choose to enroll in the program but would not have converted their land under business-as-usual conditions. Using a simple model relating land quality and land use to economic rents, we simulate potential avoided grassland conversion offset market participation across a range of cropland over pasture rent difference threshold (RDT) values. We find mitigation potential and simulated program costs are widely variable depending on this parameter and assume carbon prices: across the five states studied, the full range is 0.41 tCO₂e • yr⁻¹ (0.2 RDT, $10 •·t⁻¹ carbon price) to 4.6 million tCO₂e • yr⁻¹ (1.2 RDT, $40 tCO₂e • t⁻¹ carbon price), assuming average land use change emissions values for pastureland in the region. Total program costs for these offsets also exhibit a wide range, spanning $2−$120 million • yr⁻¹ depending on parameterization. Results across the full range of RDTs (0.2−2) demonstrate a tendency toward higher RDTs for achieving high levels of avoided emissions, with cost efficiency being maximized in the 1.4−1.8 range for RDTs. A state-level breakdown of results demonstrate the importance of modeling economic trends in land use and setting region-specific additionality thresholds for avoided grassland conversion offsets. Although our study is specific to grassland conversion in one region of the United States, similar offset markets exist elsewhere, where additionality concerns are paramount. We believe our framework can be useful in improving protocol design.

在美国大草原坑洼地区，草地转化为农田是缓解气候变化的持续障碍。设计了几个碳补偿市场，以奖励在转化动机很高时将土地保持在其本国的土地所有者。我们探索了关键因素在此类程序中的作用：附加性阈值。如果适当地选择和应用此因素，将会减少土地所有者的参与，土地所有者会选择加入该计划，但不会像往常一样转换土地。使用将土地质量和土地使用与经济租金相关联的简单模型，我们模拟了在牧场租金差异阈值（RDT）值之上的一系列耕地中可能避免的草地转化抵消了市场参与。假设平均土地利用变化，则₂ e•年^-1（0.2 RDT，$ 10•·t ^-1碳价格）至460万吨tCO ₂ e•年^-1（1.2 RDT，$ 40 tCO ₂ e•t ^-1碳价格）该地区牧场的排放值。这些补偿的总计划成本也表现出很大的范围，范围在2到1.2亿美元之间•年^-1取决于参数设置。整个RDT（0.2-2）范围内的结果表明，为了实现高水平的避免排放量，RDD趋向于更高，在RDT的1.4-1.8范围内成本效益最大化。州一级的结果明细表明，对土地使用中的经济趋势进行建模并设定特定于区域的额外性阈值对于避免草地退化补偿的重要性。尽管我们的研究仅针对美国一个地区的草地转换，但在其他地方，对额外性的关注最为重要的地方也存在类似的补偿市场。我们认为我们的框架对于改善协议设计很有用。