Further drivers of ocean deoxygenation identified

June 11, 2018, Helmholtz Association of German Research Centres
Credit: CC0 Public Domain

Measurements as well as model calculations equally show that the oxygen inventory of the oceans is decreasing. However, the models underestimate this decrease significantly making projections into the future problematic. In a study published today in the international journal Nature Geoscience, four GEOMAR researchers reveal the gaps in the models and identify previously underestimated drivers for the deoxygenation.

The oceans are losing . Numerous studies at local, regional and global level confirm this trend. For example, a comprehensive data analysis published by Kiel oceanographers at the beginning of 2017 has shown that the oceans have lost two percent of their oxygen content worldwide in the past 50 years. Computer models of the oceans and the Earth system also show this trend and predict an even faster decrease in the future. But the models have a problem. "They are not able to reproduce the recent oxygen decline exactly. Instead, they significantly underestimate the observed oxygen loss," says Prof. Dr. Andreas Oschlies from the GEOMAR Helmholtz Centre for Ocean Research Kiel.

This mismatch makes projections into the future more problematic. Today, Professor Oschlies together with his colleagues Prof. Dr. Peter Brandt, Dr. Lothar Stramma and Dr. Sunke Schmidtko, all from GEOMAR, have published a study in the international journal Nature Gesoscience, which shows deficiencies of the models and also identifies drivers of deoxygenation that have been underestimated so far. "The comparison with our observation data reveals various inadequacies of the models and gives us indications in which direction we must concentrate our research efforts," says co-author Peter Brandt.

It is certain that global warming is the main cause of marine oxygen loss. But warming affects the in several ways. Among other things, it influences the solubility of oxygen in the water. The warmer the water, the less gas it can take up. "This process mainly affects the uppermost layers of water, which are in direct contact with the atmosphere," explains Dr. Schmidtko. This effect can explain up to 20 percent of the deoxygenation so far, and is well represented in the models.

But warming also changes patterns of global ocean circulation. Since the complex system of surface and deep currents supplies oxygen to the deeper ocean, these changes can affect the oxygen content throughout the ocean. "Many models have problems describing this effect realistically, because transport processes are often not resolved well enough or reproduced incorrectly," says co-author Dr. Lothar Stramma.

The extremely complicated interactions between biological, chemical and physical processes in the ocean are also insufficiently represented in current models. "We often lack the data or the knowledge about many processes that interact in the ocean's response to global warming," says Andreas Oschlies, who specializes in the modeling of biogeochemical processes. "Our study shows that previous models significantly underestimate the effects of this interaction, at least on the oxygen distribution."

To close these gaps, the authors argue for more intensive and internationally coordinated ocean observation. "We need multidisciplinary process studies to better understand the delicate balance of oxygenation and in the ocean," says Andreas Oschlies, "therefore, international initiatives such as the Global Ocean Oxygen Network are helpful."

An improvement of the models in terms of the oceans' oxygen budget would also have another advantage: "Oxygen is ideal for calibrating models that calculate the uptake of carbon dioxide by the ocean. So, at the same time, we would improve our knowledge of the carbon cycle," concludes Oschlies.

Explore further: Global ocean de-oxygenation quantified

More information: Andreas Oschlies et al, Drivers and mechanisms of ocean deoxygenation, Nature Geoscience (2018). DOI: 10.1038/s41561-018-0152-2

Related Stories

Global ocean de-oxygenation quantified

February 15, 2017

Oxygen is an essential necessity of life on land. The same applies for almost all organisms in the ocean. However, the oxygen supply in the oceans is threatened by global warming in two ways: Warmer surface waters take up ...

Trade winds ventilate the tropical oceans

December 19, 2014

Long-term observations indicate that the oxygen minimum zones in the tropical oceans have expanded in recent decades. The reason is still unknown. Now scientists at the GEOMAR Helmholtz Centre for Ocean Research Kiel and ...

The Baltic Sea as a time machine

May 9, 2018

Warming, acidification, eutrophication, and the loss of oxygen are examples of major changes being observed or expected for the future in coastal zones around the world. These processes are occurring in the Baltic Sea at ...

Recommended for you

Evidence of earliest life on Earth disputed

October 17, 2018

When Australian scientists presented evidence in 2016 of life on Earth 3.7 billon years ago—pushing the record back 220 million years—it was a big deal, influencing even the search for life on Mars.

Arctic greening thaws permafrost, boosts runoff

October 17, 2018

A new collaborative study has investigated Arctic shrub-snow interactions to obtain a better understanding of the far north's tundra and vast permafrost system. Incorporating extensive in situ observations, Los Alamos National ...

Arctic ice sets speed limit for major ocean current

October 17, 2018

The Beaufort Gyre is an enormous, 600-mile-wide pool of swirling cold, fresh water in the Arctic Ocean, just north of Alaska and Canada. In the winter, this current is covered by a thick cap of ice. Each summer, as the ice ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.