Upper ocean O2 trends

A new analysis of data on ocean oxygen levels has revealed that the amount of dissolved oxygen contained in the water – an important measure of ocean health – has been declining for more than 20 years. Researchers looked at a historic dataset of information stretching back more than 50 years and searched for long term trends and patterns. They found that oxygen levels started dropping in the 1980s as ocean temperatures began to climb. 

The study, “Upper Ocean O2 trends: 1958-2015” was published in the April 2017 Geophysical Research Letters, and was sponsored by the National Science Foundation and the National Oceanic and Atmospheric Administration. The team included researchers from the National Center for Atmospheric Research, the University of Washington-Seattle, and Hokkaido University in Japan. (Takamitsu Ito, Shoshiro Minobe, Matthew C. Long and Curtis Deutsch).

Falling oxygen levels in water have the potential to impact the habitat of marine organisms worldwide and in recent years has led to more frequent “hypoxic events” that killed or displaced populations of fish, crabs and many other organisms. Researchers have surmised that rising water temperatures would affect the oxygen in the oceans because warmer water is capable of holding less dissolved gas than colder water. But in fact, the data showed that ocean oxygen was falling more rapidly than the corresponding rise in water temperature. “The trend of oxygen falling is about two to three times faster than what we predicted from the decrease of solubility associated with the ocean warming” Ito said. “This is most likely due to the changes in ocean circulation and mixing associated with the heating of the near-surface waters and melting of polar ice.

The majority of the oxygen in the ocean is absorbed from the atmosphere at the surface or created by photosynthesizing phytoplankton. Ocean currents then mix that more highly oxygenated water with subsurface water. But rising ocean water temperatures near the surface have made it more buoyant and harder for the warmer surface waters to mix downward with the cooler subsurface waters. Melting polar ice has added more freshwater to the ocean surface too- yet another factor that hampers the natural mixing and leads to increased ocean stratification.