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Climate change: the ocean is running out of oxygen as global temperatures rise.

Rising global temperature is a main cause of ocean “deoxygenation”, affecting how much oxygen water can absorb and the circulation patterns that carry oxygen-rich water to the deeper sea waters.

Global warming, acidification and pollution are increasingly affecting the Earth's marine ecosystems - but as scientists now point out, there is another urgent problem in Poseidon's realm: oxygen levels are dwindling, associated with shifts in the biological, chemical and physical balance of the oceans.

As the scientists explain, global warming is again playing a problematic role in the global loss of oxygen in the oceans. The "elixir of life" in the oceans has long been the focus of an international team of researchers, including researchers from GEOMAR Helmholtz Centre for Ocean Research Kiel.

In recent years, they have used extensive sampling and analysis to investigate the oxygen content in many of the world's water bodies, to clarify recent developments and to identify the causes of the decline in the O2 content.

A report of the International Union for Conservation of Nature (IUCN) now presents its results. Since a large part of the oxygen loss is caused by global warming, the report should have been yet another wake-up call to the participants of the 2019 UN Climate Conference in Madrid.

Why does warm water have less oxygen?

The researchers report that the global oxygen content has decreased by more than two percent on average over the past 50 years. According to them, increased water temperatures in the wake of climate change are an important factor: the warmer surface water is, the less oxygen it can absorb.

The warm water also makes the stratification of ocean water more stable, reducing mixing and circulation and making it more difficult to ventilate the interior of the seas. The report also highlights the very different regional levels of oxygen deficiency. Regions in which other factors contribute to oxygen scarcity are particularly hard hit.

Especially in coastal waters, unnaturally intensive nutrient inputs from fertilisers and fish farms promote algae growth, which increases oxygen consumption. According to one of the researchers, considering that coastal areas such as Peru and West Africa are responsible for a large proportion of global fishing yields, this can already have a direct impact on humans.

How does global warming affect the oceans?

The data collected by researchers in recent years show that in the last half century the amount of water in the open ocean, which lacks oxygen, has more than quadrupled. In coastal waters, including estuaries and marginal seas, low oxygen sites have increased more than tenfold since 1950. Scientists also expect oxygen concentrations outside these areas to drop further as the earth continues to warm.

Oceanic oxygen minimum zones (dissolved oxygen values at 200m depth). Global warming is causing the oceans to absorb less oxygen.

As the researchers emphasize, however, further research in this field is now needed to enable more accurate assessments. For example, computer models still show less oxygen loss than is actually measured in the ocean. This means that researchers have not yet understood all the related processes. The researchers note that they are therefore pleased that the IUCN is taking up this topic with the new report and showing new ways for future research.