Photographs of Earth taken by astronauts in space more than half a century ago revealed a blue planet dominated by oceans and billowing with clouds. Since then, says British oceanographer and physicist Helen Czerski, scientists have been documenting how global warming is changing the seas in ways that are transforming weather patterns worldwide and, in some cases, imperiling the agricultural systems upon which humanity depends.

In her new book, The Blue Machine: How the Ocean Works, Czerski — a frequent science presenter on the BBC — offers a lyrical primer on the natural forces that power the global ocean and how human activities are putting some of these processes at risk. While reporting on the ocean often focuses on the well-known scourges of plastic, chemical pollution, and overfishing, Czerski examines our impacts on the physics of the ocean system, which she describes as a gigantic and highly complex engine.

In an interview with Yale Environment 360, Czerski warned that critical ocean currents may slow down or change course as surface waters continue to warm. Oxygen levels in the sea have been declining, she said, potentially turning some parts of the sea into biological deserts. However, she remains skeptical of ambitious ocean geoengineering schemes designed to mitigate the effects of climate change, which she said risk further destabilizing a natural system that we imperfectly understand.

“The ocean humbles you all the time,” said Czerski, who is an associate professor at University College London. “There’s this illusion that we are in control of our planet and that we’ve got some handles on the driving levers. But we should be very, very careful before we use them.”

Yale Environment 360: The oceans are vast. But even people who are concerned about the environment rarely focus on them. You’ve gone so far as to say that we just “don’t see the ocean.” What do you mean by that?

Helen Czerski: People talk about fish and whales and pollution. They talk about the things in the ocean. Almost nobody talks about the [physical reality of the] ocean itself. That has started to change in the past year because we’ve been having these marine heat waves. And suddenly people are like, “Oh, the ocean has a temperature, and maybe that temperature matters.” It’s a perspective shift once you start to see the ocean not just as the canvas that these stories are written on, but as the engine that drives the whole thing.

e360: It is sometimes said that we know less about the deep sea than we do about the moon. I understand you don’t appreciate comparing the ocean to the moon. Why not?

Czerski: Yeah, it drives me nuts. Our image of the moon comes from those pictures taken by the astronauts. And what you see is a gray, pockmarked surface that literally has not changed for 2 billion years. Nothing happens there except it gets hit by the occasional asteroid. The fundamental problem with comparing the moon to the deep sea is it assumes that the ocean is a dead place where nothing ever changes.

We forget that the ocean is a three-dimensional place, that it’s full of life doing quite weird things, that it’s changing seasonally and doing lots of things that impact the rest of the planet. And so to compare it with this dead rock that hasn’t changed, it’s basically insulting. It’s also dangerous because it says we can put it in that category of things that are cold and dead and not important. We can’t afford to do that anymore.

e360: Early photographs of the Earth from the moon woke us up to the fact that our planet is mostly water. Why has it taken us so long to appreciate how important the ocean is to the climate system and, indeed, for sustaining life on Earth?

Czerski: Here we are living on our blue planet, and yet we never look at the blue. There’s this phrase in the Merchant Marine, they say people are “sea blind.” And I think that our Western society is sea blind. The great tragedy of the ocean is that light doesn’t travel through it. We’re very visual creatures, and we don’t believe things are there if we can’t see them. So if light only travels a couple of hundred meters into the ocean on a good day, well, the rest of it’s not real. You can’t see it. And so we assume it doesn’t matter.

e360: You characterize the ocean as a massive engine. In what sense is it an engine?

Czerski: A definition of an engine is something that changes heat energy into movement, and that is actually what the ocean is doing. The ocean is an engine because it’s dynamic, and the source of that dynamism is fundamentally heat energy from the sun.

e360: You’ve also called the ocean the beating heart of the planetary life-support system. That’s a beautiful image. What makes the ocean so central in your view?

Czerski: There’s a few things. One of them really is just that water is a liquid. And the thing that makes it useful as a liquid is that it has the opportunity to gain energy and become a gas or lose a bit of energy and become a solid.

But the most important thing for the planet is that both of those processes take a colossal amount of energy. And that keeps the system very stable. The actual liquid doesn’t change very much. And the ocean is a really good reservoir of heat: It’s a heat battery. That’s the fundamental thing that makes Earth habitable. That’s why being a blue planet matters. If it’s cloudy for a few days, the ocean doesn’t freeze over. As Mars turns away from the sun, temperatures drop like a stone. Same thing on the moon. But we’ve got a bank of energy to draw on. So that keeps Earth stable.

e360: That brings up the question of ocean currents, which have a huge impact on our weather.

Czerski: One recent study predicted that what’s called the AMOC [the Atlantic Meridional Overturning Circulation] was going to shut down by 2025 because of instabilities. I think the general consensus is that that was based on an extrapolation of an idea that is quite far-fetched. But really the important bit isn’t that they said it will happen in 2025. Almost everyone agrees [that’s unlikely]. The story is not that it might happen next year, or the year after, or 10 years after. It’s that [the possibility is] even on the table.

e360: If the AMOC circulation further weakens, as it seems to be doing, what impact will that have on the Gulf Stream and on our weather?

Czerski: It would change it quite a lot. It changes the pattern of the engine. It almost doesn’t matter what the change is, because if you look at where our farms are, they depend on the weather being like it is. And if that weather moves 200 miles down the road, it’s no good saying, “Oh, well, it’s only 200 miles.” Then all your farms are in the wrong place, or your migrating animals are in the wrong place. The risk is not so much we’re all going to freeze or we’re all going to burn; it’s that we absolutely depend on the pattern being like it is now.

e360: You’ve spoken about the deoxygenation of parts of the ocean. That could have a big impact on marine life.

Czerski: Globally, about 2 percent of the oxygen content in the ocean has been lost since the 1960s. That’s a big deal. If you live down in the dark where there’s no photosynthesis and you need to breathe oxygen, then you are reliant on oxygen coming from the surface. If that slows or stops, then you’ve got a problem.

e360: Why exactly are oxygen levels falling?

Czerski: We don’t have enough science to really pick apart what’s going on. There are a few things that probably contribute. One is that a warmer ocean will take in less gas from the air. So as you warm the ocean, you make it more likely to give gas back rather than take gas in. It might be because the amount of mixing between the surface and the depths is changing. It could also have something to do with the level of photosynthesis changing at the surface. There isn’t one simple answer that’s emerged.

e360: What is it about climate change that’s making the ocean layers mix less and become more highly stratified?

Czerski: The ocean is heated from above by the sun, and warm water is buoyant. Mixing takes energy. The warmer you make this upper layer, the harder it is for water to get out of that layer.

e360: And when there’s less mixing there’s less life, in general, in the sea. Is that right?

Czerski: Yeah, so there’s large areas of the ocean, generally at the surface, that are considered deserts. And the reason for that is that you tend to get downwelling of water, which keeps nutrients from coming up from underneath. That’s why those areas are big deserts. There is this paradox that there shouldn’t really be life in the ocean in a perfectly layered system because the sunlight is all at the top and the nutrients [including those needed by phytoplankton near the surface] are all down at the bottom.

e360: We’ve talked about how oceans are being impacted by climate change. But they can also help us respond to the climate crisis. We can get renewable energy from wind, waves, currents, and offshore solar farms. Conserved and restored coastal and marine ecosystems can store excess carbon. We can also grow more food in the oceans. Do any of these possibilities give you hope?

Czerski: A lot of these things are very worthwhile. Seagrass beds are great for biodiversity. Healthy kelp beds, natural ones, are great for biodiversity. But the question of whether you are going to start to meddle with the ocean in order to deliberately geoengineer it — there’s no evidence that any of these schemes, like farming seaweed and then deliberately sinking it to sequester carbon, will work. Because the ocean’s really complicated.

Our challenge is to understand how the planet works and then, with humility, to understand how to fit in with it instead of kind of stomping off across the thing, just doing whatever we want. We really need to clean up our act on land before we go wading out into the ocean and mucking about. It’s not that none of it will ever be necessary, but we have to sort ourselves out first.

e360: You’ve done a lot of fieldwork in places like the Arctic and in Hawaii. What has your own personal connection with the ocean taught you?

Czerski: As an ocean scientist, I’ve learned that the ocean humbles you all the time. It does things you didn’t expect. It knocks your gear overboard. You’re stuck in places. It does weird things, and your experiments go wrong. You’re not in control. There’s this illusion that we are in control of our planet, and that we’ve got some handles on the driving levers. But we should be very, very careful before we use them.