A reduction in the amount of oxygen dissolved in the oceans due to climate change is already discernible in some parts of the world and should be evident across large regions of the oceans between 2030 and 2040. — The National Center for Atmospheric Research in a press release on April 27th.
Loss of oxygen in the world’s oceans. It’s one of those really, really bad effects of a human-forced warming of our Earth. One of the those climate monsters in the closet that Steve Pacala talks about. The kind of thing we really don’t want to set loose.
Deoxygenated Oceans as Major Killing Mechanism During Hothouse Extinctions
The damage caused by ocean oxygen loss is multi-variant and wide-ranging. The most obvious harm comes in the form of generating environments in which oxygen-dependent life in the oceans can no longer breathe. Any living creature that filters oxygen out of the water for respiration falls under threat due to lowered ocean oxygen levels. A group that includes pretty much all the advanced, multi-cellular life in the seas.
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.
(Hydrogen sulfide producing bacteria blooms off the coast of Namibia during 2007. Hydrogen sulfide is a highly toxic gas. One that is produced by microbes that live in waters containing little or no oxygen. Image source: Earth Observatory.)
But a second, less immediately obvious hit comes in the form of generating expanding anoxic environments that favor the proliferation of toxin-producing microbes. Called dead zones, these oxygen-poor regions not only provide a suffocation threat to sea life, but they also form areas of water in which environmental toxins can build up. The result is a long-lasting negative impact to the health of life in the ocean and, in the most extreme cases, on land and in the airs as well.
The worst of these toxin-generating microbes are the hydrogen-sulfide producing bacteria. An ancient organism that is incompatible with oxygen-dependent life. A horror out of deep time that has tended to crop up again and again on the list of usual suspects of major hothouse extinction killers. A likely perpetrator of the big ocean and land die offs during pretty much all global warming based extinctions. An organism that dominated the world’s seas and likely vented its deadly gasses into the airs of the world of the Permian — during the worst die-off Earth has ever seen.
In short, hydrogen sulfide is deadly to almost all forms of life that currently dominate the world’s oceans, lands, and airs. And the bacteria that produces hydrogen sulfide requires oxygen-poor environments in which to grow and thrive. A world ocean high in oxygen keeps these little killers hidden away in the deep, dark corners of our Earth. But heat the world ocean up. Deprive it of oxygen. And they start to come out and become a threat (see more in Awakening the Horrors of the Ancient Hothouse).
Oxygen Loss to Become Widespread by the 2030s
Already today we see regions of the world ocean that are experiencing oxygen loss. Some of this oxygen loss is due to a process called eutrophication. In eutrophication, nutrients overload the ecosystems of water-based environments. As nutrient content rises, large bacterial blooms emerge. Eventually, these blooms overpopulate the waters and devour all the food sources. When the microbes then die en masse, their decay robs the surrounding waters of oxygen — generating a dead zone.
Eutrophication has been sapping the world’s oceans of oxygen over wider and wider regions due to both agricultural run-off (fertilizers and top soils flushed into rivers, lakes and oceans that feed large microbial blooms and related dead zones) and due to nitrogen fall out from fossil fuel burning. But human forced global warming also plays a key roll in the loss of oxygen to the world ocean system.
(According to a new study from NCAR, ocean oxygen levels are already starting to fall in some regions due to global warming. If warming continues, NCAR finds that most of the world’s oceans will experience some level of oxygen loss due to this warming and due to a related increased stratification of surface waters. Image source: NCAR.)
The new NCAR study provides an excellent description of how warming the world’s surface waters can reduce ocean oxygen levels:
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. Warming surface waters, however, absorb less oxygen. And in a double whammy, the oxygen that is absorbed has a more difficult time traveling deeper into the ocean. That’s because as water heats up, it expands, becoming lighter than the water below it and less likely to sink.
Waters that are less likely to sink are less likely to mix. And waters that are less likely to mix transfer less of the atmosphere’s oxygen to the global ocean. It’s a process called ocean stratification. A set of circumstances triggered by warming that can sap the world’s waters of their ability to support life even as it enhances their ability to generate environments favorable to toxin-producing microbes. And in the absolute worst cases, a stratified, oxygen-deprived ocean can transition into a dead, life-on-Earth-threatening Canfield Ocean.
(Mobile ocean dead zones, like this one seen off the West African Coast during 2015, may grow more widespread as the world’s surface waters are depleted of oxygen due to a fossil fuel emission based warming. A new study from NCAR both explains how warming waters can hold less oxygen and notes that loss of oxygen to ocean surface waters becomes very widespread by the 2030s. Image source: Biogeosciences.)
In the NCAR study, which is well worth reading in full, scientists used model runs to determine when and where climate change would start to deprive the world ocean system of oxygen. The study found that regions off the coast of West Africa, regions west of South America, an area to the west of Australia, and a section of the Beaufort Sea were already experiencing lower levels of ocean oxygen due to global warming. West African seas were the first and hardest hit by warming in the models. This is interesting due to the fact that Namibia on the West Coast of Africa is one of the only regions of the world now observed to experience blooms of hydrogen sulfide producing bacteria that extend into the surface waters. West African waters have also generated a number of mobile, low-oxygen dead zones that have spiraled on off into the North Atlantic.
The fact that the NCAR study indicates that global warming has already reduced ocean oxygen levels in a region that is producing both dead zones and, in the case of Nambia, periods during which hydrogen sulfide producing bacteria appear at the surface, is cause for some concern. For by the 2030s, the NCAR model study indicates that global warming will be actively reducing ocean oxygen levels across the vast majority of the North Pacific, a majority of the South Pacific, most of the South Atlantic, and pretty much all of the Indian Ocean region covered in the new research. This raises the risk that open water dead zones like the ones seen off Africa and even hydrogen sulfide producing hot spots like Nambia may begin to creep into other regions of the world ocean — generating further threats to sea life, to fishing industry, and to human beings who depend on healthy oceans for livelihood and for life.
Hat Tip to Colorado Bob
Hat Tip to June