Global warming is deoxygenating the world’s oceans on a much bigger scale than current models can capture, warn marine biochemists and oceanographers from the Helmholtz Centre for Ocean Research (GEOMAR) in a recent guest post for Carbon Brief.
“While the oxygen dissolved in seawater only amounts to around 0.6% of what the atmosphere contains,” write marine biogeochemist Prof. Andreas Oschlies and physical oceanographers Prof. Peter Brandt, Dr. Lothar Stramma, and Dr. Sunke Schmidke, “it is nevertheless essential for all higher forms of marine life.” They cite several dynamics that put that essential access in jeopardy.
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Anthropogenic global warming means there is less and less oxygen available for all marine life because, “among other things, it influences the solubility of oxygen in the water. The warmer the water, the less gas that can dissolve in it.”
“In addition,” they write, “warming alters patterns of global ocean circulation, which affects the mixing of oxygen-rich surface waters with deeper oxygen-poor water. It also changes how quickly organisms metabolize and respire, which affects consumption of marine oxygen.”
Just what lower oxygen oceans might portend is revealed in the world’s naturally occurring “oxygen minimum zones,” such as those off the coasts of Namibia and Peru, where oxygen levels “are low enough to be lethal to most marine life.”
These natural death zones appear to be expanding, the scientists write, a shift which in itself “could have ‘dramatic’ biological, ecological, economic, and climatic consequences.”
But “the risk of deoxygenation is not just limited to these specific zones,” they warn. Rather, “reducing oxygen levels is occurring at all oxygen concentrations, in all ocean basins, and affects a growing number of coastal regions.”
The findings point to a need for better analytic tools, the GEOMAR team concludes. “While the decreased solubility of oxygen in warming seawater is generally well represented in models,” the researchers write, “improvements are needed to better capture the other impacts of rising temperatures,” especially at depths below 1,200 metres, and “particularly in the Southern Ocean, the Arctic, and the tropical Pacific.”
Current models also fail to account for the “biogeochemical feedbacks that can lead to accelerated oxygen loss, such as the release of phosphorus and iron from sediments beneath oxygen-free waters”—a failure which “may explain part of why they underestimate the deoxygenation to date.”
New modelling methods are also needed to “describe the complex system of surface and deep currents that supplies oxygen to the deeper ocean.” And looming over all else is the fact that “deoxygenation feedbacks on climate via the production of potent greenhouse gases such as NO2 and methane under low-oxygen conditions become more likely in a warmer climate.”