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 less oxygen than colder waters. In addition, warmer water stabilizes the stratification of the ocean, weakening the circulation connecting the surface with the deep ocean and less oxygen is transported into the deep sea. The first global evaluation of millions of oxygen measurements seems to confirm this trend.
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.
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.
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 researchers used all historic oxygen data available around the world for their work, supplemented it with current measurements and refined the interpolation procedures to more accurately reconstruct the development of the oxygen budget over the past 50 years. In some areas previous research had already shown a decrease in oxygen.
"To quantify trends for the entire ocean, however, was more difficult since oxygen data from remote regions and the deep ocean is sparse," explains Schmidtko, "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 also shows that, with the exception of a few regions, the oxygen content decreased throughout the entire ocean during the period investigated. The greatest loss was found in the North Pacific. "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," emphasizes the co-author Dr. Lothar Stramma.
"However, with measurements alone, we cannot explain all the causes," adds Professor Martin Visbeck, "natural processes occurring on time scales of a few decades may also have contributed to the observed decrease." 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.
The new study is an important result for the ongoing work in the Collaborative Research Center (SFB) 754 funded by the German Research Foundation (DFG) at the Kiel University and GEOMAR. The SFB 754's aim is to better understand the interaction between climate and biogeochemistry of the tropical ocean. "From the beginning of March onwards, four expeditions aboard the German research vessel METEOR will investigate the tropical oxygen minimum zone in the eastern Pacific off Peru. We hope to obtain further data on regional development which will also help us to better understand the global trends," emphasizes Dr. Stramma, the expedition coordinator for the SFB.
Over the past few years, incursions of warm ocean water have caused large die-offs of coral reefs, and in some areas, 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. 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.