evidence: The effects of CO₂ pollution on the ocean can be grouped into two large categories: thermal stress and chemical stress. [...] All this excess heat going into the ocean is literally unraveling the fabric of the system. Warmer ocean water holds less oxygen; already, there has been about a 2% average decrease in dissolved oxygen throughout the ocean. Warmer upper layers of the ocean inhibit mixing with the middle layer of the ocean, which is a primary exchange system that brings nutrients into the global food web. These warmer waters expand, and that expansion is causing a significant portion of the sea-level rise that coastal ecosystems and communities have been experiencing. Warmer waters also lead to marine heat waves that decimate coral reefs; we have already lost more than half of the Earth’s tropical coral reefs primarily due to heating and bleaching. In addition, warmer waters are driving species that can migrate to do so; their move toward cooler water is leading to large-scale migrations of fish stocks poleward. And warmer waters mean less Arctic sea ice, which has functioned like a planetary air conditioning system that we all depend on. [...] These two systemwide threats to the ocean—thermal and chemical stress caused by CO₂ pollution—outweigh, in terms of their ultimate risk, anything else that we have done or are doing to the ocean. The threats are also growing and will continue to do so as atmospheric concentrations of CO₂ and other greenhouse gases increase.