The ocean is losing oxygen, and global warming is largely to blame. As water temperatures rise, oxygen solubility decreases, and ocean stratification intensifies, limiting both the amount of dissolved oxygen in the water and the supply of the gas mixed into deeper layers from the surface. What’s less clear is whether these trends will hold over millennial timescales. Some studies suggest that this deoxygenation could reverse after the end of the century, but these studies have been based on low-complexity Earth system models.
Here Frölicher et al. report results from a comprehensive Earth system model that includes representations of land, ocean, atmosphere, and sea ice to project both deep- and upper-ocean oxygen concentrations under a 4,500-year scenario in which atmospheric carbon dioxide doubles from preindustrial levels (287 parts per million) and then remains constant.
The model, developed by NOAA’s Geophysical Fluid Dynamics Laboratory, involved three phases of response. In the 70-year first phase, as atmospheric carbon dioxide rose to 574 parts per million, ocean oxygen levels fell. In the second phase, lasting from year 71 to 720, atmospheric carbon dioxide levels were held constant, and oxygen levels continued to decline. In the third phase after 721 years, ocean oxygen levels gradually recovered.
By the end of the simulation, overall ocean oxygen levels had climbed even higher than they were under the initial modeled conditions, even though ocean temperatures and heat content had risen. The researchers found that although oxygen solubility in the ocean decreased throughout the simulation, this change was offset by increasing ventilation of surface water to the deep ocean and by reduced biological oxygen consumption. The authors note that the model did not include melting land ice, which could influence ocean circulation and hinder or delay the oxygen recovery, and they suggest that future studies should account for ice sheet–ocean interactions.
Still, the study demonstrates that large changes in oceanic oxygen levels could occur centuries or even millennia after atmospheric carbon dioxide levels stabilize, which, the authors note, means that marine life could face significant effects well after humans get a handle on carbon emissions. (Global Biogeochemical Cycles, https://doi.org/10.1029/2020GB006601, 2020)
—Kate Wheeling, Science Writer