Ocean oxygen levels dropped 2 percent in last 50 years, study says

Ocean oxygen levels dropped 2 percent in last 50 years, study says

But further decrease in these levels could spell disaster for marine life forms which rely on oxygen to survive.

Climate models have long predicted such a decline. Oxygen enters the ocean at its surface, from the atmosphere and from the photosynthetic activity of marine microorganisms.

The new paper, published by climate and marine scientists at the GEOMAR Helmholtz Center for Ocean Research in Germany, concluded that the ocean's dissolved oxygen content has declined 2 percent globally, with the Pacific and Arctic Oceans experiencing the sharpest drops.

A new study has found the world's oceans have lost two per cent of their oxygen in the past 50 years, mostly because of climate change.

None of this is news to climate scientists. Less mixing means the deep ocean gets supplied with less oxygen from the surface. However, the results of the research are consistent with most model calculations that predict a further decrease in oxygen in the oceans due to higher atmospheric carbon dioxide concentrations and consequently higher global temperatures.

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To fill in some blank spots between measurements, the team used interpolation techniques, connecting the dots between existing data points. 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. As warmer air warms the ocean surface, the seas become less able to absorb and keep oxygen. The drop in this region accounted for about 40 percent of the total 2 percent decrease in oxygen levels.

But, he warned, "the implications of this for marine ecosystems could be severe in parts of the ocean where oxygen is already low". These oceanic "dead zones" tend to be areas where microorganisms that produce greenhouse gasses like nitrous oxide thrive. Oxygen in the ocean is dissolved in water, and when water warms, its ability to hold onto trapped gasses decreases.

Global warming has had an indelible impact on the oxygen levels in the oceanic waters. 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. In recent years, incursions of warm ocean water have caused large die-offs of coral reefs, and in some cases, kelp forests as well.

"While the slight decrease of oxygen in the atmosphere is now considered non-critical, the oxygen losses in the ocean can have far-reaching consequences because of the uneven distribution", said researcher Lothar Stramma.