The Silent Threat of Slowing Deep Ocean Currents

Approx. 3 min read

There is a growing threat in our oceans, out of sight and, largely, out of mind. 

Ocean overturning circulations are critical for the cycling of nutrients, heat, oxygen and carbon from the top of the ocean to the bottom of the ocean and vice versa. Since water is practically incompressible, when one section of ocean moves, water from elsewhere needs to fill what would otherwise be a void. In other words, if you have two bodies of water pulling apart from each other, water from below needs to flow upwards – if it didn’t you would create a giant parting in the middle of the ocean, separated by two towering walls of water.  

This is a fundamental property of water and is part of the reason why we not only have surface ocean currents but also deepwater currents. The formation of deepwater in the Southern Ocean, which encircles Antarctica, occurs when dense, saline (salty) and oxygen-rich water sinks downwards, in a process known as downwelling. Elsewhere, nutrient-rich, cold water flows up from the depths to the surface, in a process known as upwelling. The combination of downwelling and upwelling is what creates the overturning circulations that are essential for keeping the deep ocean supplied with oxygen, modulating the temperature and carbon content of the ocean, and keeping the surface supplied with the nutrients that marine life thrives off.  

However, climate change is now posing a threat to the engine room of this circulation. The melting freshwater ice of Antarctica is starting to affect the formation of this deepwater. The influx of freshwater is decreasing the salinity (saltiness) of the seawater and is slowing the downwelling, or sinking, of water off the Antarctic coast, since the water is less saline and therefore less dense. If the rate of oxygen-rich water that downwells slows, then the oxygen concentration of the deepwater will slowly and steadily decrease. This is in effect like slowly suffocating the deepwater of the ocean.  

Fig. 1 – Diagram showing the formation of Antarctic Bottom Water and how climate change is affecting the rate at which it sinks and provides oxygen to the deep ocean. 

Reproduced from: Eric Taylor, Woods Hole Oceanographic Institution, Jan 25, 2017. Available at: https://www.whoi.edu/press-room/news-release/antarctic-bottom-waters-warming-freshening/  

Decreasing oxygen concentrations in deepwater is a serious problem for a multitude of reasons. For marine life that lives in the deep ocean and on the ocean floor, the creation and expansion of an anoxic zone (an area with no oxygen) is fatal. Imagine what would happen to you if someone switched off your oxygen supply. Previous mass extinction events have, in part, been driven by widespread anoxic zones forming in the ocean.  

A mass extinction event is a term for when the majority of species in existence (at that time) die off and go extinct at a much quicker rate than the rate at which new species emerge. They are usually specifically defined as losing 75% or more of the world's species over a short geological timescale: shorter than 3 million years.

Fig. 2 – Diagram depicting the timing of various extinction events and associated ocean anoxia, from 280 to 50 million years ago. Grey bars denote periods of ocean anoxia and green stars show extinction events. There is a clear association between ocean anoxia and extinction events.

Reproduced from: Percival, L.M., Jenkyns, H.C., Mather, T.A., Dickson, A.J., Batenburg, S.J., Ruhl, M., Hesselbo, S.P., Barclay, R., Jarvis, I., Robinson, S.A. and Woelders, L., 2018. Does large igneous province volcanism always perturb the mercury cycle? Comparing the records of Oceanic Anoxic Event 2 and the end-Cretaceous to other Mesozoic events. American Journal of Science, 318(8), pp.799-860.  Available at: https://www.ajsonline.org/content/318/8/799  

And anoxia in the world’s deepwater won’t just affect ocean biology.  

Anoxia alters the chemistry of the water. If the oxygen concentration of the water decreases, then it will alter the oxidation states of metals that sit on the bottom of the ocean. On a timescale of centuries, this could lead to the dissolving of valuable metal nodules, such as manganese, that are at the heart of the current fierce debate over the ethics and efficacy of deep-sea mining.

So, if climate change carries on unabated then the very benthic organisms and the pristine habitat in which they live will eventually, over several centuries, cease to exist altogether. We’re running out of time in a race where the rate at which the stopwatch itself ticks is getting faster and faster.  

It is important to note that whilst a timescale of centuries sounds like a long time, on a geological timescale it is the equivalent of a mere blink of the eye. 

This is why we need to dramatically upscale effective, responsible and sustainable carbon removal solutions. If we do not, then it will not just be the flora and fauna that we can see that will suffer. Sadly, those who are out of sight and out of mind will also slowly and silently suffer too.