Evidence points to widespread loss of ocean oxygen by 2030s
April 27, 2016 - 11:55 AM
Climate change has caused a drop in the amount of oxygen dissolved in
the oceans in some parts of the world, and those effects should become
evident across large parts of the ocean between 2030 and 2040, according
to a new study led by researchers at the National Center for
Atmospheric Research (NCAR) in Boulder, Colorado.
Scientists expected a warming climate to sap oceans of oxygen,
leaving fish, crabs, squid, sea stars, and other marine life struggling
to breathe. But they had encountered difficulties in determining whether
this anticipated oxygen drain was already having a noticeable effect.
"Loss of oxygen in the oceans is one of the serious side effects of a
warming atmosphere, and a major threat to marine life," said NCAR
scientist Matthew Long, lead author of the study. “Since oxygen
concentrations in the ocean naturally vary depending on variations in
winds and temperature at the surface, it's been challenging to attribute
any deoxygenation to climate change. This new study tells us when we
can expect the effect from climate change to overwhelm the natural
The study is published in the American Geophysical Union journal Global Biogeochemical Cycles. The research was funded by the National Science Foundation (NSF).
Cutting through the natural variability
The entire ocean -- from the depths to the shallows -- gets its
oxygen supply from the surface, either from the atmosphere or from
phytoplankton, which release oxygen into the water through
Warming surface waters, however, absorb less oxygen. And, in a double
whammy, the absorbed oxygen has a more difficult time traveling deeper
into the ocean. That's because as water heats up, it expands, becoming
lighter than the water below it and less likely to sink.
Thanks to natural warming and cooling, oxygen concentrations at the
sea's surface change constantly -- and deeper in the ocean, those
changes can linger for years or decades.
For example, an exceptionally cold winter in the North Pacific would
allow the ocean surface to soak up a large amount of oxygen. Thanks to
the natural circulation pattern, that oxygen would then be carried
deeper into the ocean, where it might still be detectable years later as
it travels along its flow path.
On the flip side, unusually hot weather could lead to "dead zones" in
the ocean, where fish and other marine life cannot survive.
To cut through this natural variability and investigate the impact of
climate change, the research team -- including Curtis Deutsch of the
University of Washington and Taka Ito of Georgia Tech -- relied on the
NCAR-based Community Earth System Model, which is funded by NSF and the
U.S. Department of Energy.
“This study shows how far comprehensive Earth System Models have come
in the effort to quantify, along with relatively sparse observations,
large-scale changes in oxygen in the oceans due to both natural
variability and climate change,” said Eric Itsweire, program director in
NSF's Division of Ocean Sciences.
The scientists used output from a project that ran the model more
than two dozen times for the years 1920 to 2100. Each individual run
started with miniscule variations in air temperature. As the model runs
progressed, those tiny differences grew and expanded, producing a set of
climate simulations useful for studying questions about variability and
Using the simulations to study dissolved oxygen gave the researchers
guidance on the degree to which concentrations may have varied naturally
in the past. With this information, they could determine when ocean
deoxygenation due to climate change is likely to become more severe than
at any point in the modeled historic range.
The researchers found they could already detect deoxygenation caused
by climate change in the southern Indian Ocean and parts of the eastern
tropical Pacific and Atlantic basins.
They also determined that more widespread detection of deoxygenation
caused by climate change would be possible between 2030 and 2040.
However, in some parts of the ocean, including areas off the east
coasts of Africa, Australia, and Southeast Asia, deoxygenation caused by
climate change would not become evident even by 2100.
Detecting a global pattern
The researchers also created a visual way to distinguish between
deoxygenation caused by natural processes and deoxygenation caused by
Using the same model dataset, the scientists created maps of oxygen
levels in the ocean, showing which waters were oxygen-rich and which
were oxygen-poor. They found they could distinguish between oxygenation
patterns caused by natural weather phenomena and the pattern caused by
The climate change pattern also became evident in the model runs
around 2030, adding confidence to the conclusion that widespread
deoxygenation due to climate change will become detectable around that
The maps could also be useful resources for deciding where to place
instruments to monitor ocean oxygen levels in the future to get the best
picture of climate change effects. Currently, ocean oxygen measurements
are relatively sparse.
"We need comprehensive and sustained observations of what's going on
in the oceans to compare with what we're learning from our models, and
to understand the full effect of a changing climate," Long said.
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