Breathing water in a warming world: Oceanic dissolved oxygen update

While the general public – even in the United States – is ever more convinced of the reality and dangers of climate change, most of the focus is, understandably, on large events with a human death toll and a big price tag. While these big changes are important to track, to respond to, and to prepare for, it’s useful to check up on the ones that aren’t as easy to see, so that we can, at least in theory, plan for when they become more obvious, and more dangerous.

One of the big “hidden changes” is dissolved oxygen content in the world’s oceans. This is what most ocean life relies on to exist. For air-breathing critters like us, our lungs can’t actually use all of the oxygen that we breathe in. CO2, CO, and other compounds contain oxygen, we can’t break those molecules and just use the oxygen in them. The energy required would be much greater than it’s worth.

Likewise, oxygen-breathing organisms in the ocean don’t get their oxygen from H2O molecules, but from “non-compound” oxygen that’s dissolved into the water. How much oxygen is in the water depends on a number of factors, but the upper limits are always determined by temperature and pressure. Higher temperature and higher pressure both mean less “room” for dissolved oxygen, along with other things like CO2.

So – higher temperatures means less oxygen in the water, and less oxygen in the water means fewer organisms can survive. Too little oxygen, and you get what’s called a “dead zone“, where most fish simply can’t survive. The most famous of these, at least in the U.S., is the one at the mouth of the Mississippi river. That one is the result of farm runoff triggering eutrophication. In the open ocean a major natural cause seems to be giant eddies that temporarily isolate a large amount of water, and prevents the mixing that generally replenishes oxygen levels.

This is a natural part of how the world works, but then, so are hurricanes and wildfires. Heat is energy, and we’re trapping a lot of energy here, and pretty much everything in the global climate system is getting amped up as a result. Heat waves are hotter, and last longer. Floods are worse, and more frequent. Air pollution is more toxic.

Oceans hold less oxygen. 

In the open ocean, global warming, which is primarily caused by increased greenhouse gas emissions, is considered the primary cause of ongoing deoxygenation. Numerical models project further oxygen declines during the 21st century, even with ambitious emission reductions. Rising global temperatures decrease oxygen solubility in water, increase the rate of oxygen consumption via respiration, and are predicted to reduce the introduction of oxygen from the atmosphere and surface waters into the ocean interior by increasing stratification and weakening ocean overturning circulation.

This is a slow-moving problem, because while the scale of our climate screw-up is huge, so are the oceans. The primary threat to the oceanic ecosystem remains our unsustainable habits of resource extraction. Overfishing, destruction of the sea floor by trawling nets, leaky oil wells, industrial pollution, and so on. Fish aren’t going to run out of oxygen to breathe in the oceans any time soon, but it’s something to pay attention to, because even in the best-case scenario, it’s going to come up again, and we should be ready.

There’s already too much going on for any one person to keep track of, and that’s only going to get worse. It’s going to get easier to let things slip through the cracks, now that we’re entering an era of permanent “recovery” from natural disasters. As always I’ll end with a reminder that we have the technology and resources to weather the coming storms, and build a more just, resilient, and sustainable civilization. We’re just not doing it right now.

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  1. StevoR says

    Good article on an issue I must admit i wasn’t aware of and has gone well beneath the radar – or sonar here as the case may be. Thanks.

  2. says

    Thanks for this.

    I’m curious as to how much depletion is necessary before animalia can’t live in an area. If you deplete the oxygen 50% relative to some typical value for a region, can the creatures of that region survive that much change? What about 80%? And how much change on ocean oxygenation is imposed by 1 degree C ocean warming?

    Obviously the planet has warmed a great deal over the last 20ky, and up til 250 years ago ocean productivity was booming. I assume that some of that biological tolerance had to do with the ability of metazoans to move, and since the warming is greatest at the poles there were always animals relatively more equatorial used to somewhat less oxygen who could move poleward.

    But then, there are these local depletion conditions that are obviously causing far more depletion than a mere 1 degree warming could impose. Should we be primarily concerned about dead zones, with most locations in tropical and temperate oceans retaining sufficient oxygen even with a 3-5 degree warming? Or would waters warming 3-5 degrees (at least in the top 100-200m) have deadly effects everywhere?

    Obviously your summary above has just made me hungry for more information about this. Deadzones near shores could create military conflicts (your fisheries are fucked while a neighboring country’s fisheries are still going gangbusters) much more quickly while general loss of ocean productivity can’t kick start a local war over 5-10% productivity loss very easily, but sustained over time can contribute substantially to global hunger, and that – while it may take longer to arrive – would be even worse.

    I really wish we could figure out a way to unfuck ourselves now, please.

  3. says

    I think step one is to look to the warmer waters of today. I would expect tropical waters to be more or less barren, even without the issue of falling pH.

    Farther north, I think we’ll still see fish and the like, assuming we don’t just kill and eat all of them. The polar oceans could end up giving refuge to a lot of sea life that currently just visits on occasion.

    On solutions, I’ll say that while I can’t see a way through for us, that has more to do with human behavior than technology. Specifically, if we got access to the hoards of wealth and resources that are currently being kept from good use by the super-rich, I think we have all the technical know-how to move most human activity indoors, so to speak, in a way that could allow for more wilderness, and for ecosystems to sort themselves out without humans coming along and tearing them apart every now and then.

    I don’t think going to pre-industrial technology is a path to survival, even if it WAS a realistic option. All of that lifestyle relies on predictable annual weather patterns, and that’s not an option anymore, and won’t be for centuries.

    The only path forward I can see is just that – forward. We’ve reached the point where we are one of the largest and most active forces working on the surface of the plant. Like it or not, we’re controlling the climate now as much as anything short of the sun can do. If we can embrace that, and include that in how we structure ourselves, we can thrive.

    Probably need to get the doomsday cultists like Pence away from the halls of power, first.

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