Ongoing greenhouse gas emissions can modify climate processes and induce shifts in ocean temperature, pH, oxygenconcentration, and productivity, which in turn could alter biological and social systems. Here, we provide a synoptic globalassessment of the simultaneous changes in future ocean biogeochemical variables over marine biota and their broaderimplications for people. We analyzed modern Earth System Models forced by greenhouse gas concentration pathways until2100 and showed that the entire world’s ocean surface will be simultaneously impacted by varying intensities of oceanwarming, acidification, oxygen depletion, or shortfalls in productivity. In contrast, only a small fraction of the world’s oceansurface, mostly in polar regions, will experience increased oxygenation and productivity, while almost nowhere will there beocean cooling or pH elevation. We compiled the global distribution of 32 marine habitats and biodiversity hotspots andfound that they would all experience simultaneous exposure to changes in multiple biogeochemical variables. Thissuperposition highlights the high risk for synergistic ecosystem responses, the suite of physiological adaptations needed tocope with future climate change, and the potential for reorganization of global biodiversity patterns. If co-occurringbiogeochemical changes influence the delivery of ocean goods and services, then they could also have a considerable effecton human welfare. Approximately 470 to 870 million of the poorest people in the world rely heavily on the ocean for food,jobs, and revenues and live in countries that will be most affected by simultaneous changes in ocean biogeochemistry.These results highlight the high risk of degradation of marine ecosystems and associated human hardship expected in afuture following current trends in anthropogenic greenhouse gas emissions.
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