Signal 0.5 to 3 percent of ocean oxygen lost in five decades

Summary

Beyond ocean acidification, the report is weighing ocean warming and deoxygenation as possible additions to the planetary boundaries framework. Over the past fifty years about 0.5 to 3 percent of the dissolved oxygen in the global ocean has been lost. Deoxygenation enlarges the volume of water that is very low in oxygen or entirely without it, widening oxygen minimum zones. Under low oxygen, sinking organic matter breaks down through slower anaerobic pathways, so more carbon ends up buried in seafloor sediments, which can affect atmospheric carbon dioxide and climate over long timescales. Oxygen shortage also drives microbial processes such as denitrification and anammox that turn usable nitrogen into inert nitrogen gas, with a smaller share becoming the potent greenhouse gas nitrous oxide. The report thus sees deoxygenation as affecting Earth system stability through the carbon and nitrogen cycles.