The ocean is becoming more stable – here’s why that might not be a good thing
If you’ve ever been seasick, “stable” may be the last word you associate with the ocean. But as global temperatures rise, the world’s oceans are technically becoming more stable.
When scientists talk about ocean stability, they refer to how much the different layers of the sea mix with each other. A recent study analysed over a million samples and found that, over the past five decades, the stability of the ocean increased at a rate that was six times faster than scientists were anticipating.
Ocean stability is an important regulator of the global climate and the productivity of marine ecosystems which feed a substantial portion of the world’s people. It controls how heat, carbon, nutrients and dissolved gases are exchanged between the upper and lower layers of the ocean.
So while a more stable ocean might sound idyllic, the reality is less comforting. It could mean the upper layer trapping more heat, and containing less nutrients, with a big impact on ocean life and the climate.
How the oceans circulate heat
Sea surface temperatures get colder the further you travel from the equator towards the poles. It’s a simple point, but it has enormous implications. Because temperature, along with salinity and pressure, controls the density of seawater, this means that the ocean surface also becomes denser as you move away from the tropics.
Seawater density increases with depth too, because the sunlight that warms the ocean is absorbed at the surface, whereas the deep ocean is full of cold water. The change in density with depth is referred to by oceanographers as stability. The faster density increases with depth, the more stable the ocean is said to be.
It helps to think of the ocean as divided into two layers, each with different levels of stability.
The surface mixed layer occupies the upper (roughly) 100 metres of the ocean and is where heat, freshwater, carbon and dissolved gases are exchanged with the atmosphere. Turbulence whipped up by the wind and waves at the sea surface mixes all the water together.
The lowest layer is called the abyss, which extends from a few hundred metres depth to the seafloor. It’s cold and dark, with weak currents slowly circulating water around the planet that remains isolated from the surface for decades or even centuries.
Dividing the abyss and the surface mixed layer is something called the pycnocline. We can think of it like a layer of cling film (or Saran Wrap). It’s invisible and flexible, but it stops water moving through it. When the film is ripped into shreds, which happens in the ocean when turbulence effectively pulls the pycnocline apart, water can leak through in both directions. But as global temperatures rise and the ocean’s surface layer absorbs more heat, the pycnocline is becoming more stable, making it harder for water at the ocean’s surface and in the abyss to mix.