When scientists talk about the stability of the ocean, they refer to how much the different layers of the ocean mix with each other. A recent study analyzed over a million samples and found that, over the past five decades, the stability of the ocean increased at a rate six times faster than expected.
Ocean stability is an important regulator of the global climate and the productivity of marine ecosystems which feed a large portion of the world’s people. It controls how heat, carbon, dissolved gasses and nutrients are exchanged between the upper and lower layers of the ocean.
A more stable ocean could mean the upper layer trapping more heat, and containing less nutrients, with a big impact on ocean life and the climate.
The ocean is divided into two layers, each with different levels of stability.
The surface mixed layer occupies the upper (roughly) 100 meters of the ocean and is where fresh water, heat, carbon and dissolved gasses are exchanged with the atmosphere. Wind turbulence and waves at the sea surface mixes all the water together.
The lowest layer is referred to as the abyss, which extends from a few hundred meters depth to the seafloor. It’s dark and cold, with weak currents slowly circulating water around the planet that remains isolated from the surface for decades or even centuries.
Dividing the surface mixed layer and the abyss is something called the pycnocline. 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 the abyss to mix.
The problem is that there is an invisible conveyor belt of seawater which moves warm water from the equator to the poles, where it is cooled and becomes more dense and so sinks, returning back to the equator at depth. During this journey, the heat absorbed at the ocean’s surface is moved to the abyss, helping redistribute the ocean’s heat burden, accumulated from an atmosphere that is rapidly warming due to greenhouse gas emissions.
If a stabler pycnocline traps more heat in the ocean surface, it could disrupt how effectively the ocean absorbs excess heat.