The amount of dissolved oxygen in the water of the world’s oceans — an important marker of overall oceanic biological health/livability — has been declining at a notable rate for more than 2 decades now, according to a new analysis from the Georgia Institute of Technology.
Map of the linear trend of dissolved oxygen at the depth of 100 meters. Credit: Georgia Tech
The new analysis, which looked at historic data going back more than 5 decades, found that oceanic oxygen levels started dropping notably in the 1980s, just as ocean temperatures began to increase at a relatively fast rate.
“The oxygen in oceans has dynamic properties, and its concentration can change with natural climate variability,” commented Taka Ito, an associate professor in Georgia Tech’s School of Earth and Atmospheric Sciences who led the research. “The important aspect of our result is that the rate of global oxygen loss appears to be exceeding the level of nature’s random variability.”
With falling oxygen levels in ocean water, habitability for larger forms of marine life becomes harder and large-scale hypoxic events (dead zones) became more likely.
While it’s long been known that rising ocean temperatures would result in less oxygen being present in the waters, oxygen levels have been falling much more rapidly than was expected.
“The trend of oxygen falling is about 2 to 3 times faster than what we predicted from the decrease of solubility associated with the ocean warming,” Ito commented. “This is most likely due to the changes in ocean circulation and mixing associated with the heating of the near-surface waters and melting of polar ice.”
The press release provides more: “The majority of the oxygen in the ocean is absorbed from the atmosphere at the surface or created by photosynthesizing phytoplankton. Ocean currents then mix that more highly oxygenated water with subsurface water. But rising ocean water temperatures near the surface have made it more buoyant and harder for the warmer surface waters to mix downward with the cooler subsurface waters. Melting polar ice has added more freshwater to the ocean surface — another factor that hampers the natural mixing and leads to increased ocean stratification.”
“After the mid-2000s, this trend became apparent, consistent, and statistically significant — beyond the envelope of year-to-year fluctuations,” Ito continued. “The trends are particularly strong in the tropics, eastern margins of each basin and the subpolar North Pacific.”
Interestingly, earlier research from Ito and his team found that air pollution originating in East Asia was responsible for the decrease of oceanic oxygen levels thousands of miles away in the tropical Pacific Ocean. The exact cause? Iron and nitrogen pollution was/is causing plankton blooms that deplete oxygen.
It’s worth bearing that reality in mind when discussing proposed geoengineering plans involving the “seeding” of the oceans with massive amounts of iron and/or other nutrients.
The new findings are detailed in a paper published in the journal Geophysical Research Letters.
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