July 30th, 2024
Key Findings
- By projecting 21st-century river nitrogen loads under future socioeconomic climate pathways, the authors find that fertilizer usage is the primary determinant of future river nitrogen loads.
- Changing precipitation and warming have limited impacts.
- CO2 fertilization-induced vegetation growth enhancement leads to modest load reductions.
- Fertilizer applications to produce bioenergy in climate change mitigation scenarios cause larger load increases than in the highest emission scenario.
- Loads generally increase in low-income regions.
- Loads remain stable or decrease in high-income regions where agricultural advances, low food and feed crop production and waste, and/or well-enforced air pollution policies balance biofuel-associated fertilizer burdens.
Minjin Lee, Charles A. Stock, Elena Shevliakova, Sergey Malyshev, Maureen Beaudor, Nicolas Vuichard. Nature Communications . DOI: 10.1038/s41467-024-49866-x
Nitrogen fertilizer usage and cultivation-induced biological nitrogen fixation help feed nearly half the global population. These practices, in addition to fossil fuel burning for energy production, have increased reactive nitrogen losses to the environment, causing a cascade of negative impacts on the ecosystem and human health. Nitrogen emissions to the atmosphere have contributed to acid rain, air pollution, stratospheric ozone depletion, and the radiative forcing underlying climate change. Nitrogen fluxes from lands have also impaired freshwater quality and contributed to coastal eutrophication, hypoxia, and harmful algal blooms, putting aquatic resources and the communities that depend upon them at risk.
Future socio-economic climate pathways have regional water-quality consequences whose severity and equity have not yet been fully understood across geographic and economic spectra. The authors used a process-based, terrestrial-freshwater ecosystem model to project 21st-century river nitrogen loads under these pathways.
Global river nitrogen loads to the coastal ocean are projected to rise in the 21st century under all three scenarios used by the authors. Fertilizer usage is found to be the primary determinant of future river nitrogen loads; changing precipitation and warming have limited impacts; and CO2 fertilization-induced vegetation growth enhancement leads to modest load reductions.
Fertilizer applications to produce bioenergy in climate mitigation scenarios cause larger load increases than in the highest emission scenario. Loads generally increase in low-income regions, yet remain stable or decrease in high-income regions where agricultural advances, low food and feed production and waste, and/or well-enforced air pollution policies balance biofuel-associated fertilizer burdens. Consideration of biofuel production options with low fertilizer demand and rapid transfer of agricultural advances from high- to low-income regions may help avoid inequitable water-quality outcomes from climate mitigation.
By using the GFDL’s process-based, terrestrial-freshwater ecosystem model LM3-TAN to project spatially explicit, global river nitrogen loads over the 21st century, the authors analyzed the combined and interactive effects of projected future changes in land use, nitrogen inputs, atmospheric CO2, and climate (e.g., temperature and precipitation) on water nitrogen pollution under three Coupled Model Intercomparison Project, Phase 6 (CMIP6) Integrated Assessment Model marker scenarios: (i) SSP1-2.6 (Sustainability – Taking the Green Road), (ii) SSP2-4.5 (Middle of the Road), and (iii) SSP5-8.5 (Fossil-fueled Development – Taking the Highway).
