Decades
of data on world's oceans reveal a troubling oxygen decline
Date: May 4, 2017
Source: Georgia
Institute of Technology
Summary: The amount of dissolved oxygen contained in
the water -- an important measure of ocean health -- has been declining for
more than 20 years, reveals a new analysis of decades of data on oceans across
the globe.
FULL STORY
Global map of the linear trend of dissolved oxygen at the depth
of 100 meters.
Credit: Georgia Tech
A new analysis of decades of data on oceans across the globe has
revealed that the amount of dissolved oxygen contained in the water -- an
important measure of ocean health -- has been declining for more than 20 years.
Researchers at Georgia Institute of Technology looked at a
historic dataset of ocean information stretching back more than 50 years and
searched for long term trends and patterns. They found that oxygen levels
started dropping in the 1980s as ocean temperatures began to climb.
"The oxygen in oceans has dynamic properties, and its
concentration can change with natural climate variability," said 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."
The study, which was published April in Geophysical Research Letters, was
sponsored by the National Science Foundation and the National Oceanic and
Atmospheric Administration. The team included researchers from the National
Center for Atmospheric Research, the University of Washington-Seattle, and
Hokkaido University in Japan.
Falling oxygen levels in water have the potential to impact the habitat
of marine organisms worldwide and in recent years led to more frequent
"hypoxic events" that killed or displaced populations of fish, crabs
and many other organisms.
Researchers have for years anticipated that rising water
temperatures would affect the amount of oxygen in the oceans, since warmer
water is capable of holding less dissolved gas than colder water. But the data
showed that ocean oxygen was falling more rapidly than the corresponding rise
in water temperature.
"The trend of oxygen falling is about two to three times
faster than what we predicted from the decrease of solubility associated with
the ocean warming," Ito said. "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 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 said. "The trends are particularly strong in the
tropics, eastern margins of each basin and the subpolar North Pacific."
In an earlier study, Ito and other researchers explored why
oxygen depletion was more pronounced in tropical waters in the Pacific Ocean.
They found that air pollution drifting from East Asia out over the world's
largest ocean contributed to oxygen levels falling in tropical waters thousands
of miles away.
Once ocean currents carried the iron and nitrogen pollution to
the tropics, photosynthesizing phytoplankton went into overdrive consuming the
excess nutrients. But rather than increasing oxygen, the net result of the
chain reaction was the depletion oxygen in subsurface water.
That, too, is likely a contributing factor in waters across the
globe, Ito said.
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