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.
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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