The domestic and international implications of future climate for U.S. agriculture in GCAM

Price changes are a primary economic mechanism through which international yield changes are transmitted to U.S. agricultural producers. Because agricultural commodities are traded across the globe, the prices of these commodities are affected by events that occur both in the U.S. and internationally. An event that affects U.S. yields will have consequences for U.S. production but also for prices in the rest of the world and will therefore affect production throughout the world agricultural system. The reverse also holds. Because the U.S. is a significant but not majority con- tributor to the global agricultural market for most crops, the magnitude of U.S. price changes in the Domestic is consistently smaller than in the Full scenario, across all climate-crop impact com- binations for all GCAM commodities, illustrated in Figs 2 and 3 and S3 Fig. This is regardless of whether prices have increased or decreased relative to the baseline scenario, consistent with many of the results reported in [10,16]. Further, applying impacts in the Full case versus in the Domestic results in differing price changes from reference, even for commodities such as Corn that display very similar values in the Full and Domestic cases for physical variables such as area, production, and endogenous yield. It is possible that the relationship between the Full impacts price change from reference and the Domestic impacts price change from reference may break down as the structure of the system being modeled fundamentally changes. For example, in a more restrictive

trade scenario, U.S. producers would be more restricted in their options to respond to future cli- mate and prices would be increasingly dictated solely by the direct impacts on U.S. productivity. For commodities such as Corn and OilCrop, for which the U.S. is a significant producer and exporter, a major shock only to U.S. production (Domestic) is closer in magnitude to a shock across the entire world (Full) than it would be for other commodities. For these com- modities, the Domestic and Full changes from reference are therefore most similar. Indeed for Corn in particular, the inclusion of even spatially heterogenous, direction varying impacts across the entire world (Full) versus only in the U.S. (Domestic) makes very little difference in the physical output variables of GCAM considered: area, production, and endogenous yield. It is primarily in price (and by extension, revenue) that a difference between Domestic and Full is detectable for a given climate-crop impact combination. For commodities such as Wheat and Rice, shown in Fig 3, for which production is more spatially distributed globally, a shock to U.S. production is a smaller change in the scope of the full global system. Therefore for commodities such as these, the inclusion of impacts globally can lead to a reversal in the direction of changes relative to the Domestic case, particularly for production and/or revenue. For Wheat, reversals in the direction of revenue change occur in three of ten spatially heterogenous agricultural impacts scenarios driven by structurally differ-

ent crop models, across a range of GCM drivers; for Rice, in five of ten. This suggests that the reversal in the direction of change when impacts are applied globally is emergent from the international dynamics themselves and not an artifact of the scenarios considered. These find- ings are again consistent with other models in the literature, and adds an additional regionally resolved, global-scale multi-sector economic model’s results to confirm the importance of examining global systems holistically, as conclusions may fundamentally change for several commodities when only domestic impacts are considered. Figs 2 and 3 highlight that different changes from reference in the area allocated to individ- ual commodities occur when impacts are applied in the Domestic versus Full scenario. For a more aggregated investigation of area allocation, Table 1 summarizes the change in landcover for total cropland, as well as changes from reference in the GCAM ‘other arable land’ type, for- est, and grassland cover. Recall that impacts were not applied to forest or grassland in any of the scenarios under consideration, and so these area changes are strictly emergent from the changes in cropland as areas move into or out of agricultural crop production. The changes in forest and grassland are generally small, as GCAM features an explicit other arable land type in the crop competition logit nest (see Fig 1 and [20] for details). In the system modeled by PLOS ONE The domestic and international implications of future climate for U.S. agriculture in GCAM

PLOS ONE | https://doi.org/10.1371/journal.pone.0237918 August 28, 2020 7 / 11