We’re all used to the green landscapes and rugged mountain views that define the Earth we live on, but from space, it’s clear that we live on a blue planet. Despite its extreme saltiness, its unpredictable nature, and its unprecedented impact on our weather, this amazing body of water is vital to all life on Earth.
Helen Chersky, a British oceanographer and broadcaster, has spent her career studying the ocean. In her new book, The Blue Machine: How the Ocean Works (W.W. Norton & Company, 2023), she explores how the vast mass of water surrounding our planet created the world we live in today, and why some places are abundant with life. While others are not, how civilizations formed around ocean currents, and why they act as the Earth’s battery, powering its weather systems.
In this interview, she talks to us about what drives this ocean engine, compares it to a fine cocktail, and explains what the next great ocean frontier is.
Alexander McNamara: In your book you describe the ocean as an engine, but what do you actually mean by that?
Helen Chersky: The definition of an engine is something that converts any thermal energy into motion, which is what the ocean does.
There is a warm layer at the top, which can reach a height of 100 metres [330 feet] It’s thick, there’s water at the bottom and it’s much colder, and it has layers in it.
But even though the top and bottom of the ocean are somewhat separate, there are plug holes near the poles where they connect, and this connection drives circulation as the water goes down, slides along the ocean floor for a few hundred years, and then returns somewhere else.
This is what the engine does on a larger scale, transferring heat from the equator to the poles. It is this physical drive, with all these features and all this anatomy, that makes things happen, and then animals and humans are affected by the results. The whole shape of Earth’s civilizations is sort of shaped by what the ocean drive does.
AM: For most people, casual observers will look at the ocean and see that it’s quite flat, maybe if it’s a windy day it’ll be a bit choppy, but from the sounds of it, there’s a lot going on underneath.
High Commissioner: So there are two things that set the engine in motion – the fact of the Earth’s rotation and the density of water.
We tend to think that because we can stir it with a spoon, it is very easy to mix the water, but generally that is not the case, you have to put energy into it and the spoon is just a vehicle to transfer it. The ocean is the same.
Imagine a layered cocktail that you can have if you’re careful [liquid] To sit on another. This only works if you put them in the right order, because the top is less dense than the one below it, and it is less dense than the one below it.
[In the ocean]If you have warmer, less dense water, it will sit at the top. Then if there is cold water underneath it will stay at the bottom, and they won’t mix. It’s just like the layers in a cocktail – there’s no energy to mix it so it stays there.
So, there’s this warm blanket over the ocean – it’s called the mixed layer because the water kind of mixes in there – but it doesn’t mix with what’s underneath, and the important reason is that things fall out of the mixed layer. These are often parts of life, parts of life that carry nutrients. These are like the atoms you need to create life, right? You need some nitrate, you need some phosphorus, you need some iron – you need these things, and they tend to fall out of the mixed layer.
The problem is that if they fall, they can’t get back up. So, in theory, there shouldn’t be any life on Earth because the sunlight is up there and you have this warm blanket from which nutrients fall. Then after a while it runs out, you have a separate ocean with nutrients at the bottom, where there’s no sunlight, and the top, where there aren’t any nutrients and everything is stagnant.
That’s why there’s not a lot of life in the middle of large ocean basins, in the middle of the Pacific Ocean, for example, because that separation is so strong. Nothing can live.
The only reason there is life in the ocean is so you can break this paradox. This happens near the edges, where you get periods of fluctuations, and near the poles, where the top layer and the bottom layer can connect. This is why layers are important, and density is what determines layers.
Then of course the ocean moves much more sideways than up and down, generally because of wind pressure at the surface and because we’re on a rotating planet. Then you enter the world of eddies and circles, where you can create currents of interesting shapes, restricted of course by continents and gaps between continents.
When it comes to heat, obviously the sun is overhead at the equator, so there is a very direct input of a lot of energy, and these circulating currents pull the warm water towards the poles, pushing the cold water deeper. You have a net transfer of heat to the poles, and that’s how the heat is distributed around.
Truly the ocean is the Earth’s battery. This is where the sun’s energy is stored and then used to direct the weather. It can fuel the weather by heating it, which is what fuels hurricanes, and it can affect where ice is located. So, heat is actually a store of energy, and the surroundings determine where that heat is and where it is transferred.
Amy: You say it takes hundreds of years for these currents to move slowly, but how? Because I thought that if you pooled water together it would flow and mix depending on how fast you moved it – does the ocean work in a different, much slower way?
The only reason there is life in the ocean is so you can break this paradox
High Commissioner: Well, you see it. If you have a bathroom where you run hot water, for example, and then decide it’s too hot, you should put some cold water at one end. If you do this when the water is completely still and you stir it, it will be a long time before the two sides start mixing – and that’s just the size of the bathtub.
The ocean is driven primarily by eddies, and the formation of a vortex requires energy. If you don’t have any eddies, there’s no reason for the water masses to mix, so they don’t mix. But the thing about the ocean is that there’s not enough energy in the system to mix the whole thing up. If there’s no panning at all, you’ll have perfect layers and nothing will move and nothing will happen, but if there’s lots and lots of panning, everything will be the same. There’s this kind of moderate piece in the middle where there’s just enough action to make it interesting, but not so much action that it becomes boring again.
AM: Was it always like this?
High Commissioner: No, actually it’s a lot different. And obviously this is the kind of thing where you can get into a very clever dating technique that looks at sediments on the bottom of the oceans and ice samples and things like that.
In almost all of the world’s oceans, not the Arctic but almost everywhere else, the warm water at the top and what’s below are much cooler. For example, at the bottom of the North Atlantic Ocean, the temperature is likely to be 4 or 5 degrees Celsius [39 to 41 degrees Fahrenheit]even when it reaches 30 degrees [C, or 86 F] On the surface, so it’s much cooler. But there were times in Earth’s past when heat mixed below more easily and the depths of the ocean were as hot as 15 degrees Celsius [59 F].
But the place where this rule is violated is the Arctic Ocean because it’s cold on the surface — cold enough to freeze, all right — but there’s a layer underneath that’s much warmer, and it contains enough warmth to melt all the ice. Ice today. This warmth is trapped at depth, and the reason it’s at the bottom is because it’s very salty, making it denser than the fresh water at the top. Even in today’s oceans, it’s not just temperature that causes layers to form, salt also has an effect.
AM: I was surprised by the fact that there is this huge blob of salty water under the North Pole. Is this a problem and how did you get there?
High Commissioner: So it becomes salty due to ice formation. There are two types, land ice and sea ice. Land ice forms when water evaporates from the ocean, is carried aside, falls as rain or snow, and freezes. But sea ice occurs when the surface of the ocean itself freezes.
And the thing about this process is that because the water molecules trap themselves in this solid structure that is ice, and because the water molecules are so foreign, there’s no room in that structure for salt or sodium or chloride, which are the two components, and magnesium and all the other salts, you get in the ocean. What happens is that the water forms a little crystal and all the molecules lock into place and the salt is squeezed out.
Therefore, under the formation of ice, salt water is generated directly beneath it, and it descends. So the thing is, if you stop producing ice, you’re going to generate less salt, and you’re probably going to change that system. It’s really interesting, these different configurations that the ocean can have.
Of course, the whole thing is constantly moving. If you look at the globe at the top of the North Pole, there are these two very narrow entrances, it’s kind of restricted, there’s only a few ways in and out. Whatever was there must have passed through those narrow gaps, and so what we’re starting to see is the influence of the Pacific Ocean creeping into the Arctic Ocean. In the past, the Atlantic and Pacific Oceans were kind of outward, and now they’re starting to squeeze through those narrow gaps, changing the structure of the Arctic.
AM: Thinking about the future of the ocean, where, in general, is the next great frontier for us?
High Commissioner: Obviously the bigger question is how it will change under climate change. There are big questions about how things that the ocean carries, like oxygen, for example, change the shape of the ocean’s engine because it has extra energy, it changes what it does. So, if you slow down that circulation, you change the amount of oxygen, and in the deep ocean that’s going to be important for anything that’s trying to breathe, for example.
So, yes, I think there are still big dramatic questions, but we have to be realistic. We have to understand how the entire Earth machine works, so that we can work with it and not against it, which is what we have failed to do so far.
Editor’s Note: This interview has been edited and condensed for clarity.
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