A lot of projections about the effects of global warming come from easily confirmed and long-known facts about the physical properties of water. Remember what you learned about the water cycle in school? All of that is dependent on temperature, and when things get warmer, the patterns change. Another property of water is its ability to “hold” other chemicals, like oxygen. For all water itself includes oxygen as a major component, that’s not what fish are taking in when they “breathe” through their gills. Instead, they’re absorbing dissolved oxygen, that’s not bound up in any water molecules. Think of it like the air we breathe. About 70% of that is nitrogen that’s sort of neutral to us. What we’re breathing for, is the 21% of O2 molecules within that mix. For fish, water takes the role of “nitrogen” in this comparison, and dissolved oxygen is the O2 that they’re able to absorb and use. The problem is that warmer water holds less dissolved oxygen. It’s just a fact about how water and dissolved oxygen work.
That means that we’ve known from the beginning that as temperatures rise, and as oceans absorb most of that extra heat, oxygen content in the water is going to drop. Just as we can predict that, we can predict how it will affect oceanic ecosystems. While there are dead zones that don’t have enough oxygen to support any vertebrate life, most of the decrease has been far more subtle, mainly causing problems for those fish, like marlin, that are large, have active lifestyles, and so require a lot of oxygen. When it comes to those fish, the drop in oxygen has been affecting them for well over a decade, but those areas that are “dead zones” for them generally still support other species. It might be better to say that they’re suffering from hypoxia – lower oxygen levels than usual, and than needed by some species.
Ocean currents, agricultural runoff, landscape, and a variety of other local and regional conditions mean that our oceans have just as diverse an array of habitats as dry land, if not more. Consequently, while we know that, overall oceanic oxygen levels are decreasing, that doesn’t actually tell us how that is progressing in different ecosystems. For that, we actually need to pay people to go check. Fortunately, while I think we don’t spend enough on that, we do spend quite a bit on it, as a species, and so we have some new information about how the ocean’s deoxygenation is progressing in what may be the single most “charismatic” set of oceanic ecosystems – coral reefs.
An international research effort led by the Scripps Institute of Oceanography took data from a selection of reef sites around the globe – the most comprehensive oxygenation study focused on reefs to date – and found that hypoxia due to global warming is already affecting many of them:
The study, published March 16 in the journal Nature Climate Change, is the first to document oxygen conditions on coral reef ecosystems at this scale.
“This study is unique because our lab worked with a number of collaborators to compile this global oxygen dataset especially focused on coral reefs—no one has really done that on a global scale before with this number of datasets,” said marine scientist Ariel Pezner, now a postdoctoral fellow at the Smithsonian Marine Station in Florida. “We were surprised to find that a lot of coral reefs are already experiencing what we would define as hypoxia today under current conditions.”
The authors found that low oxygen levels are already happening in some reef habitats now, and are expected to get worse if ocean temperatures continue to warm due to climate change. They also used models of four different climate change scenarios to show that projected ocean warming and deoxygenation will substantially increase the duration, intensity, and severity of hypoxia on coral reefs by the year 2100.
And, of course, beyond. If you’ll indulge a slight tangent here, I’m starting to get frustrated with the convention of saying “by 2100”. I get why it’s so common, but I’m starting to worry that it gives some people the mistaken impression that that’s when global warming will be “done” or something. The reality, in case anyone reading is unclear on this, is that we’ve started a process on this planet that has historically lasted anywhere from tens of thousands of years, to millions of years. There’s no reason to think that, absent drastic changes by us (one of which could well be extinction), the warming event we’ve started won’t last longer than our species has existed so far. Ok, tangent done. Back to the reefs.
As I mentioned before, the diversity in oceanic ecosystems means that there are going to be a variety of things that affect oxygen levels. In the crushing depths of the midnight zone, life seems to be pretty slow, and pretty uniform. There’s no night or day, just eternal, cold darkness and slowly falling sediment, punctuated by the occasional dead whale. Nearer to the surface, photosynthesis plays a major role, causing oxygen levels to fluctuate over the course of the day. When things are starting out a bit too low to begin with, the results can be unpleasant:
Historically, hypoxia has been defined by a very specific concentration cutoff of oxygen in the water—less than two milligrams of oxygen per liter—a threshold that was determined in the 1950s. The researchers note that one universal threshold may not be applicable for all environments or all reefs or all ecosystems, and they explored the possibility of four different hypoxia thresholds: weak (5 mg/L), mild (4 mg/L), moderate (3 mg/L), and severe hypoxia (2 mg/L).
Based on these thresholds, they found that more than 84 percent of the reefs in this study experienced “weak to moderate” hypoxia and 13 percent experienced “severe” hypoxia at some point during the data collection period,
As the researchers expected, oxygen was lowest in the early morning at all locations and highest in the mid-afternoon as a result of nighttime respiration and daytime photosynthesis, respectively. During the day when primary producers on the reef have sunlight, they photosynthesize and produce oxygen, said Pezner. But at night, when there is no sunlight, there is no oxygen production and everything on the reef is respiring—breathing in oxygen and breathing out carbon dioxide—resulting in a less oxygenated environment, and sometimes a dip into hypoxia.
This is a normal process, said Andersson, the study’s senior author, but as ocean temperature increases, the seawater can hold less oxygen while the biological demand for oxygen will increase, exacerbating this nighttime hypoxia.
“Imagine that you’re a person who is used to sea-level conditions, and then every night you have to go to sleep somewhere in the Rocky Mountains, where the air has less oxygen. This is similar to what these corals are experiencing at nighttime and in the early morning when they experience hypoxia,” said Andersson. “And in the future, if the duration and intensity of these hypoxic events gets worse, then it might be like sleeping on Mount Everest every night.”
The only real encounters I’ve had with high altitude were during my 2006 semester abroad in Tanzania, when I climbed Mt. Meru and Mt. Kilimanjaro. It was an interesting experience, as I’d spent the summer before working as a ridgerunner on the Connecticut section of the Appalachian Trail. I was used to hiking, and while Meru was a pretty nasty climb (though not a technical one), I actually didn’t have much trouble with the trail I took up Kilimanjaro. Well, not much trouble until the very last bit of the hike. I reached Barafu camp at around 10am, instead of the scheduled late afternoon, so we decided to head for the summit to catch the beginning of sunset, rather than getting up at midnight to hike up in the dark for sunrise. I was definitely feeling the altitude by then – getting out of breath much more easily – but the last climb up was rough. It didn’t help that it was a steep scrabble up a slope of volcanic sand, but it felt like I had to take a break every two steps, and I have to confess that if the snows of Kilimanjaro weren’t melting, there would be a nice sample of my upper gut bacteria preserved near the summit, because I absolutely lost my lunch up there. I’m sure some of that was low pressure, but a lot of it was intense exertion in low oxygen.
All of this is to say that while some people could probably adjust somewhat to that kind of change in oxygen across each day, it would honestly just be terrible, and I have a hard time believing that human society as we recognize it could do very well under those conditions. The same goes for fish society. Unfortunately, this is a trend that’s just going to continue for as long as the temperature keeps rising. Like I said at the beginning, we knew this was coming because it’s all about the basic physical properties of water as a substance. The only real question is how ecosystems will respond to the change. In general, it will probably mean fewer, smaller, and less active fish, and more stuff like sea jellies and other goopy critters that require less oxygen to function, but that’s an educated guess, and life has a way of developing strange and unexpected ways to thrive. Hell, for all we know this will end up creating new species of oceanic lungfish that periodically surface to top up their O2 levels. The only way we’ll know, though, is by actually checking.
This research was funded by the NSF – an institution that has funded a lot of good work over the years, including some of the science education research that has paid my father’s salary for most of my life. I guess that’s me stating a conflict of interest, but the reality is that publicly funded research is vital to humanity’s future, and the same bloodthirsty capitalists who are driving this climate crisis have also been working tirelessly to smother public research funding, focusing on projects they think they can sell as “frivolous” to a public that doesn’t know much about the topic. They’re not just attacking the habitability of our planet, they’re also attacking our ability to measure what’s happening. I don’t talk much about this aspect of conservative politics, because things like their genocidal hatred of trans people are much more urgent. But they’re hurting us in other ways. That’s the problem with having a capitalist class – they have unlimited money to pay people to further their interests in so many ways that it’s difficult to keep up.
I guess I can’t help myself. I start writing a science brief, and end by ranting about politics. It almost feels cartoonish to say, but the ruling class really does seem committed to ruining life as much as possible, for as many people as possible. I’m willing to believe that that’s not what they think they’re doing, but extreme wealth seems to absolutely melt the human brain, and most of them clearly live in a fantasy world maintained by a swarm of parasitic yes-men, and a total detachment from 99% of humanity. They want to blind us to what’s happening in the world, and so we must fight that battle as well, in case you needed more reasons why nobody should be that rich and powerful.
I’ve been snorkeling a couple times, and seen reefs in the Caribbean Sea and the Indian Ocean. They were absolutely beautiful, and the experience was worth the sunburn (though if I get a chance to go again, I’ll be dipping my whole self in sunscreen at regular intervals). What’s happening to ecosystems around the world is depressing to see, but it gets to me more when it’s a place I’ve seen with my own eyes. I wouldn’t say I have a deep emotional tie to coral reefs, but they’re a small part of the experiences that made me who I am today, and it is beyond unacceptable that that’s being taken away.
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