Projecting global marine eutrophication under climate change: An absolute environmental sustainability assessment approach

Driven by excessive anthropogenic nitrogen emissions, marine eutrophication poses a major threat to coastal ecosystems worldwide. Marine eutrophication and its most severe consequence, hypoxia, are expected to worsen due to both rising nitrogen emissions and climate change. This study employs an absolute environmental sustainability assessment (AESA) approach to project the future extent and severity of marine eutrophication under various climate scenarios. Parameterized under the five Shared Socioeconomic Pathways (SSPs), a characterization model for waterborne nitrogen emissions was developed to estimate the effects of climate change on the marine eutrophication impact pathway. Using state-of-the-art nitrogen projections, the model was then used to project future marine eutrophication impacts at a global scale. Our results indicate that marine eutrophication will intensify globally, placing more coastal waters at risk, with tropical regions being particularly affected. This worsening trend is largely influenced by shifts in the inland nitrogen cycle, which alter nutrient transport and exacerbate coastal eutrophication. Our findings highlight the urgent need for substantial nitrogen emission reductions to achieve environmental sustainability, even under low-carbon development pathways. Importantly, this study stresses that future climate change impacts must be explicitly considered when setting nitrogen reduction targets, as mitigation strategies based solely on current conditions may underestimate future risks. By addressing the compounded effects of nitrogen emissions and climate change, this study highlights the relevance of combining AESA methods with scenario analyses and provides valuable insights to policymakers in designing adequate nitrogen emission reduction targets.