Oceans will become oxygen depleted if fossil-fuel use remains unchecked
Researchers in Denmark have shown that unless human-induced greenhouse-gas emissions are substantially reduced, large areas of the world's oceans will become oxygen depleted and hostile to fish and shellfish. The study is published in the journal Nature Geoscience. The scientists used a computer model developed at the Danish Centre for Earth System Science (DCESS) that analyses global changes extending 100,000 years into the future. The DCESS model includes atmosphere, ocean, ocean-sediment, land-biosphere and lithosphere modules. According to the study, it reproduces 'observed evolutions since 1765 of key climate metrics including atmosphere and ocean warming, atmospheric gas contents and ocean and land-biosphere CO2 uptakes'. Two emissions scenarios of the Intergovernmental Panel on Climate Change (IPCC) were evaluated: one with a moderate (3°C) temperature increase and one with a high (4.8°C) temperature rise. In both simulations, there was oxygen loss in the upper 500m of the ocean, largely in response to surface-layer warming. Importantly, overturning circulation in the deep ocean, which pulls oxygenated surface waters down to the depths of the ocean, decreased. The high-emissions scenario in particular predicted 'severe, long-term ocean oxygen depletion,' and it was clear that the suboxic regions of the ocean, which are void of fish and other larger creatures, would expand in both cases. 'We find significant, long-term oxygen depletion and expansions of ocean oxygen-minimum zones even for the relatively modest [..] emission scenario with constant ocean exchange,' the study reads. 'Our projected oxygen depletions would have wide-reaching, negative effects on the marine environment. Fish stocks would avoid expanding hypoxic regions and mortality events would become more frequent on the continental shelf and slope.' Observations in the oceans already show that suboxic areas are expanding as the atmosphere and ocean warm. In line with this and other supporting observations, the model projects a three- to seven-fold expansion in suboxic zones. The authors explain that as suboxic zones expand, different microbes and plankton take over. This forces a shift towards nitrogen fixers, which the researchers say would probably force large, unpredictable changes in ocean ecosystem structure and productivity, with serious consequences. The study's conclusions are simple: 'Reduced fossil-fuel emissions would be needed to limit ongoing oxygen depletion and its long-term adverse effects.' Extreme oceanic oxygen depletion events are thought to be behind some of the large extinction events in the Earth's history, including the largest such event 250 million years ago.
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