Societal decisions about climate mitigation will have dramatic impacts on eutrophication in the 21st century
Excessive nitrogen loading leads to degraded water quality that harms human and ecosystem health. Here, we examine the impact of changes in land use and land management for six combinations of socioeconomic pathways (SSPs) and climate outcomes and find that future societal choices will substantially impact riverine nitrogen loading (+54% to -7%) for the continental United States by the end-of-the-century. Regional impacts will be even larger. The increase in nitrogen loading is projected both for high emission pathways with increased food demand and for societal pathways that achieve high climate mitigation through increased biofuel production. Conversely, projected decreases in nitrogen loading are associated with future pathways with low food demand and no climate mitigation efforts. Thus, climate mitigation may have unintended consequences in the form of increased eutrophication, although some pathways suggests that limiting climate change and eutrophication need not be mutually exclusive. We find that changes in cropland, developed land, and fertilizer application rates are all important drivers in explaining the projected changes in nitrogen loading. Climate change–induced precipitation changes will further amplify the projected increase in nitrogen loading or offset the improvement in loading resulting in a net change of +1% to +68% by the end-of-the-century. Globally, vast portions of South, East, and Southeast Asia will also likely experience increased eutrophication by the end-of-the-century due to increased cropland area and agricultural intensification. These findings highlight the need to account for the impact of both societal and climate trends in designing strategies for managing inland and coastal water quality.