Loss of ocean oxygen to become widespread by 2030s: study

A reduction in the amount of oxygen dissolved in the oceans due to climate change is already noticeable in some parts of the world and should be evident across large regions of the oceans between 2030 and 2040, according to a new study.
Scientists know that a warming climate can be expected to gradually sap the ocean of oxygen, leaving fish, crabs, squid, sea stars, and other marine life struggling to breathe.
But it has been difficult to determine whether this anticipated oxygen drain is already having a noticeable impact, researchers said.
"Loss of oxygen in the ocean is one of the serious side effects of a warming atmosphere, and a major threat to marine life," said Matthew Long from National Centre for Atmospheric Research (NCAR).

"Since oxygen concentrations in the ocean naturally vary depending on variations in winds and temperature at the surface, it has been challenging to attribute any deoxygenation to climate change," said Long.
The entire ocean - from the depths to the shallows - gets its oxygen supply from the surface, either directly from the atmosphere or from phytoplankton, which release oxygen into the water through photosynthesis, researchers said.

Warming surface waters, however, absorb less oxygen. And the oxygen that is absorbed has a more difficult time travelling deeper into the ocean, researchers said.
That is because as water heats up, it expands, becoming lighter than the water below it and less likely to sink, they said.

To study the impact of climate change, researchers used the NCAR-based Community Earth System Model.
They used output from a project that ran the model more than two dozen times for the years 1920 to 2100 on the Yellowstone supercomputer, which is operated by NCAR.
Each individual run was started with miniscule variations in air temperature, researchers said.
As the model runs progressed, those tiny differences grew and expanded, producing a set of climate simulations useful for studying questions about variability and change.
Using the simulations to study dissolved oxygen gave the researchers guidance on how much concentrations may have varied naturally in the past.

With this information, they could determine when ocean deoxygenation due to climate change is likely to become more severe than at any point in the modelled historic range.
Researchers found that deoxygenation caused by climate change could already be detected in the southern Indian Ocean and parts of the eastern tropical Pacific and Atlantic basins.
They also determined that more widespread detection of deoxygenation caused by climate change would be possible between 2030 and 2040.
However, in some parts of the ocean, including areas off the east coasts of Africa, Australia, and Southeast Asia, deoxygenation caused by climate change was not evident even by 2100, researchers said.

The findings were published in the journal Global Biogeochemical Cycles.