Scientists quantify the state of global oceanic de-oxygenation

by    DiveSSI    1st March 2017
Above all, large fish like this marlin are dependent on sufficient oxygen supply. Previous studies have already shown that their habitat is becoming smaller due to expanding oxygen minima. (c) Bill Boyce
A crane water cremator is launched by the research vessel "METEOR". For the current study, the authors have evaluated hundreds of thousands of historical and current oxygen measurements. (c) Martin Visbeck, GEOMAR
The authors of the current study: Dr. Lothar Stramma, Dr. Sunke Schmidtko and Professor Martin Visbeck. (c) Jan Steffen, GEOMAR

Global change leads to rising ocean temperatures and less oxygen

The current global change leads to rising ocean temperatures and
changes in the ocean circulation. Hence, less oxygen is dissolved in
surface waters and less oxygen is being transported to the depths of
the sea. This drop in oceanic oxygen supply has serious consequences
for organisms in the ocean.

Oxygen is necessary for all living organisms, both on land and in the
ocean. The oxygen supply in the oceans is being threatened by global
warming in two ways: Warmer ocean surfaces absorb less oxygen than
colder waters. Secondly, warmer water stablises the stratification of
the ocean, and this weakens the ocean circulation that connects the
surface with the depths of the ocean, thus leading to less oxygen being
transported to the ocean depths.

Many models predict a drop in global
oceanic oxygen inventory of the oceans as a result of global warming.
This trend appears to be confirmed by the first global evaluation of
millions of oxygen measurements and points to the initial impact of
global change.

Oceanographers Dr Sunke Schmidtko, Dr Lothar Stramma and Prof Dr Martin
Visbeck from GEOMAR Helmholtz Centre for Ocean Research have conducted
the most comprehensive study into the global oxygen content in the
world's oceans to date. The results of this study have recently been
published in the Nature journal. It shows that the oxygen content has
decreased by more than two percent over the last 50 years.

large fishes in particular avoid or do not survive in areas with low
oxygen content, these changes can have far-reaching biological
consequences,” said lead author Dr Schmidtko.

For the study, the researchers utilised all the historic oxygen data
available worldwide, supplementing it with current measurements and
refining the interpolation procedures to reconstruct the development of
the oxygen budget over the past 50 years. In fact, some areas have
already experiencing a decrease in oxygen.

According to Dr Schmidtko, “To
quantify trends for the entire ocean, however, was more difficult since
oxygen data from remote regions and the deep ocean is sparse. We were
able to document the oxygen distribution and its changes for the entire
ocean for the first time. These numbers are an essential prerequisite
for improving forecasts for the ocean of the future.”

The study shows that, with the exception of several regions, the oxygen
content has dropped throughout the ocean during the time of the

“While the slight decrease
of oxygen in the atmosphere is currently considered non-critical, the
oxygen losses in the ocean can have far-reaching consequences because
of the uneven distribution. For fisheries and coastal economies, this
process may have detrimental consequences,” said co-author Dr Stramma.

“However, with measurements alone, we
cannot explain all the causes. Natural processes occurring on time
scales of a few decades may also have contributed to the observed
decrease,” added Prof Visbeck.

Nevertheless, the findings are
consistent with most model calculations that predict more oxygen
decline in the oceans as a result of higher carbon dixoide content in
the atmosphere, and consequently higher global temperatures.

Link to the study

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1st March 2017
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