https://www.washingtonpost.com/news/energy-environment/wp/2017/02/15/its-official-the-oceans-are-losing-oxygen-posing-growing-threats-to-marine-life/
[video in on-line article]
Scientists have just detected a major change to the Earth’s oceans
linked to a warming climate
By Chris Mooney February 15
A large research synthesis, published in one of the world’s most
influential scientific journals, has detected a decline in the amount of
dissolved oxygen in oceans around the world — a long-predicted result of
climate change that could have severe consequences for marine organisms
if it continues.
The paper, published Wednesday in the journal Nature by oceanographer
Sunke Schmidtko and two colleagues from the GEOMAR Helmholtz Centre for
Ocean Research in Kiel, Germany, found a decline of more than 2 percent
in ocean oxygen content worldwide between 1960 and 2010. The loss,
however, showed up in some ocean basins more than others. The largest
overall volume of oxygen was lost in the largest ocean — the Pacific —
but as a percentage, the decline was sharpest in the Arctic Ocean, a
region facing Earth’s most stark climate change.
The loss of ocean oxygen “has been assumed from models, and there have
been lots of regional analysis that have shown local decline, but it has
never been shown on the global scale, and never for the deep ocean,”
said Schmidtko, who conducted the research with Lothar Stramma and
Martin Visbeck, also of GEOMAR.
Ocean oxygen is vital to marine organisms, but also very delicate —
unlike in the atmosphere, where gases mix together thoroughly, in the
ocean that is far harder to accomplish, Schmidtko explained. Moreover,
he added, just 1 percent of all the Earth’s available oxygen mixes into
the ocean; the vast majority remains in the air.
Climate change models predict the oceans will lose oxygen because of
several factors. Most obvious is simply that warmer water holds less
dissolved gases, including oxygen. “It’s the same reason we keep our
sparkling drinks pretty cold,” Schmidtko said.
But another factor is the growing stratification of ocean waters. Oxygen
enters the ocean at its surface, from the atmosphere and from the
photosynthetic activity of marine microorganisms. But as that upper
layer warms up, the oxygen-rich waters are less likely to mix down into
cooler layers of the ocean because the warm waters are less dense and do
not sink as readily.
“When the upper ocean warms, less water gets down deep, and so
therefore, the oxygen supply to the deep ocean is shut down or
significantly reduced,” Schmidtko said.
The new study represents a synthesis of literally “millions” of separate
ocean measurements over time, according to GEOMAR. The authors then used
interpolation techniques for areas of the ocean where they lacked
measurements.
The resulting study attributes less than 15 percent of the total oxygen
loss to sheer warmer temperatures, which create less solubility. The
rest was attributed to other factors, such as a lack of mixing.
Matthew Long, an oceanographer from the National Center for Atmospheric
Research who has published on ocean oxygen loss, said he considers the
new results “robust” and a “major advance in synthesizing observations
to examine oxygen trends on a global scale.”
Long was not involved in the current work, but his research had
previously demonstrated that ocean oxygen loss was expected to occur and
that it should soon be possible to demonstrate that in the real world
through measurements, despite the complexities involved in studying the
global ocean and deducing trends about it.
That’s just what the new study has done.
“Natural variations have obscured our ability to definitively detect
this signal in observations,” Long said in an email. “In this study,
however, Schmidtko et al. synthesize all available observations to show
a global-scale decline in oxygen that conforms to the patterns we expect
from human-driven climate warming. They do not make a definitive
attribution statement, but the data are consistent with and strongly
suggestive of human-driven warming as a root cause of the oxygen decline.
“It is alarming to see this signal begin to emerge clearly in the
observational data,” he added.
“Schmidtko and colleagues’ findings should ring yet more alarm bells
about the consequences of global warming,” added Denis Gilbert, a
researcher with the Maurice Lamontagne Institute at Fisheries and Oceans
Canada in Quebec, in an accompanying commentary on the study also
published in Nature.
Because oxygen in the global ocean is not evenly distributed, the 2
percent overall decline means there is a much larger decline in some
areas of the ocean than others.
Moreover, the ocean already contains so-called oxygen minimum zones,
generally found in the middle depths. The great fear is that their
expansion upward, into habitats where fish and other organism thrive,
will reduce the available habitat for marine organisms.
In shallower waters, meanwhile, the development of ocean “hypoxic”
areas, or so-called “dead zones,” may also be influenced in part by
declining oxygen content overall.
On top of all of that, declining ocean oxygen can also worsen global
warming in a feedback loop. In or near low oxygen areas of the oceans,
microorganisms tend to produce nitrous oxide, a greenhouse gas, Gilbert
writes. Thus the new study “implies that production rates and efflux to
the atmosphere of nitrous oxide … will probably have increased.”
The new study underscores once again that some of the most profound
consequences of climate change are occurring in the oceans, rather than
on land. In recent years, incursions of warm ocean water have caused
large die-offs of coral reefs, and in some cases, kelp forests as well.
Meanwhile, warmer oceans have also begun to destabilize glaciers in
Greenland and Antarctica, and as they melt, these glaciers freshen the
ocean waters and potentially change the nature of their circulation.
When it comes to ocean deoxygenation, as climate change continues, this
trend should also increase — studies suggest a loss of up to 7 percent
of the ocean’s oxygen by 2100. At the end of the current paper, the
researchers are blunt about the consequences of a continuing loss of
oceanic oxygen.
“Far-reaching implications for marine ecosystems and fisheries can be
expected,” they write.