Ocean
Oxygen : Suffocating the Seas
From
the shallows down to its yawning depths, the ocean gets its oxygen from the
surface, supplied either by the atmosphere or from the release of oxygen in
phytoplankton through photosynthesis.
Scientists
have long known that one expected consequence from a warming climate is
a gradual drop in the amount of oxygen dissolved into ocean waters as they absorb extra heat trapped
in the atmosphere by increasing concentrations of greenhouse gas.
Warming
surface waters absorb less heat. The oxygen that does take into
the surface waters has a harder time circulating down into the deeper
water due to the expansion of the warmer upper layers.
This expansion makes the
surface water lighter than the water below it, less likely to sink and
circulate.
On
the other hand, an unusually warm period stifles oxygen uptake and circulation,
which can lead to “dead zones” where fish and marine life cannot survive.
“Oxygen
varies naturally in the ocean quite substantially,” said Matthew Long,
a scientist at the National Center for Atmospheric Research (NCAR)
and lead author of new research into the impact of a warming climate
impacts oxygen levels in the ocean.
“Without any human-driven climate change we could expect oxygen levels at a particular location to go up and down in such a way that low levels may be persistent for a number of years, followed by a period of high levels,” Long said in a recent press release.
Historically,
teasing out this natural variability in ocean oxygen from warming-driven
loss has been difficult, Long explains. “Loss of oxygen in the ocean is
one of the serious side effects of a warming atmosphere, and a major threat to
marine life,” he said.
“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 impact from climate change to overwhelm the natural variability.”
Research raises new concerns for ocean
health
Distinguishing
ocean deoxygenation caused by natural variability from climate change is the
focus of Long’s research, published this week in Global Biogeochemical
Cycles, a journal of the American Geophysical Union.
In
a study titled “Finding forced trends in oceanic
oxygen” Long and his colleagues found that ocean
deoxygenation from climate change can already be detected in the southern
Indian Ocean and parts of the eastern tropical Pacific and Atlantic basins. The
research also determined that a more widespread loss of oxygen from climate
change will likely be seen between 2030 and 2040.
Long’s
team used the output from more than two dozen model runs of NCAR’s Community
Earth System Model for the years 1920 to 2100 , with
each subsequent run starting with tiny variations in air temperature.
As each model run progressed, these small differences grew and expanded,
affording a set of simulations useful for studying questions about change and
variability.
Using
these simulations to study dissolved oxygen, the simulations
guided the researchers on past oxygen concentration variability. With
this foundation they could then determine when ocean deoxygenation is
likely to become more intense than at any point in the modeled past.
With
this same dataset, the research team mapped ocean oxygen levels, visually
representing where waters are oxygen rich at the same time that others are
oxygen starved.
From this mapping researchers could determine distinct
patterns between natural variability and climate change in ocean
oxygenation. What’s more, the maps will serve as a useful resource for
deciding where to place oxygen monitoring equipment for ongoing research,
crucial for sharpening the picture of ocean health and change.
“We need comprehensive and sustained observations of what’s going on in the ocean to compare with what we’re learning from our models and to understand the full impact of a changing climate,” Long said.
One more assault on ocean health
This
new study adds yet one more offense to the health of the world’s oceans.
Increased uptake of CO2 from the atmosphere causes ocean acidification, rapidly warming waters imperil coral reefs across
the globe, highlighted by recent news that 93 percent of the Great Barrier
Reef has been impacted by the most severe coral bleaching
event on record.
Add
to all this the persistent pollution and plastic waste in the oceans.
As
oxygen levels become more pronounced, the resulting dead zones could have
serious affects on marine ecosystems and sustainable fisheries.
The
ocean itself is the living, breathing source of life on earth. We ignore
its future health and vitality at our own peril.
Graph
courtesy of AGU