Climate Change Depleting Ocean Oxygen
April 28, 2016
New study profiles decreasing oxygen levels in oceans.
Above, scientists tell us about what’s happened in Earth’s deep past when the process went to extremes.
The oceans are getting warmer — they are, after all, where 90 percent of global warming actually ends up. And when they warm up they expand, because that’s what warm water does. This raises our sea levels, but it also has another effect — it reduces the amount of oxygen dissolved in the water. That’s simply physics: Warmer water contains less oxygen.
But it’s worse: If surface water is warmer, it doesn’t mix down as much into the ocean depths any longer. It’s less dense, and so less capable of doing that. That means that oxygen that enters the ocean in its upper layers — either through exchange with the atmosphere, or because it is generated by tiny photosynthesizing microorganisms, called phytoplankton, that hang out up there — won’t mix down into the deep as often.
“What’s happening is, there’s a physical mechanism that impedes the delivery of surface waters into the interior,” said Matthew Long, an oceanographer with the National Center for Atmospheric Research who is lead author of a troubling new study on what scientists call the “deoxygenation” of the oceans. The work appeared in Global Biogeochemical Cycles, co-authored with Curtis Deutsch of the University of Washington and Taka Ito of the Georgia Institute of Technology.
The problem is that marine life needs oxygen. If there’s less of it, that could expand the number of areas sometimes called “oxygen minimum zones” where plants, fish, and other organisms would struggle to survive.
Now, in the new study, Long and his colleagues have found that some parts of the ocean are already likely showing an oxygen deficiency, due to the effects of global warming. And by around the year 2030, their model suggests, the human role in driving widespread ocean oxygen loss will be even more apparent if greenhouse gas emissions continue unchecked.
“Its fairly widespread detection….is basically evident in the 2030s to 2040s decade,” Long said.
What they found is sobering. Deoxygenation due to human-made global warming is already detectable in the southern Indian Ocean, and in some regions in the eastern tropical Pacific and Atlantic. By 2030 to 2040—two decades from now—they expect to see more and more widespread deoxygenation over the globe. By the year 2100 (which is how far into the future they ran the models) a significant fraction of the global oceans will see some deoxygenation due to human activity.
This is, obviously, bad. The amount of deoxygenation may not be very much, just a drop of a few percent. But as we learned with increasing carbon dioxide levels in the atmosphere, and rising temperatures, it doesn’t take much change to destabilize a system.
The worst effects will come from areas already low in oxygen, called hypoxic zones, where the levels can be as much as 70–90 percent lower than average, and in suboxic zones, where it’s even lower. In those regions, a few percent drop can mean the difference between life barely holding on, and death.
Even when the change isn’t so dramatic, it can be devastating. You might think of the ocean as one big fish tank, but it’s actually incredibly diverse, depending on water temperatures, currents, pressure, and more. Changes in oxygen levels in the water reduce marine life habitats, stressing the inhabitants there. Changes in regional oxygen levels have caused migrations of fish, and even massive die-offs. Besides the effect on the life there, this has an impact on human activity including fishing, on which many countries depend.
Mind you, about half the oxygen we breathe comes from ocean phytoplankton. Messing with their habitat is like setting fire to your own house. Which is pretty much what we’re doing.