The Ocean

The medium that all life on the planet came from, and still depends on.

We named the planet wrong. Seen from outside, it is not Earth but Ocean: more than two-thirds of its surface is water, most of the living space on it is sea, and the dry land we treat as the whole world is the unusual part, a few peaks of rock breaking the surface of a water world. The ocean is where life began, where most of it still lives, and the great slow regulator that keeps the whole biosphere habitable. It is also, in a quite literal sense, what you are mostly made of.

I. The Sea We Came From

Life Origins placed the likely origin of life in water, very possibly at the mineral chimneys of deep-sea hydrothermal vents, where the chemistry and the steady energy gradient were right. Whatever the precise cradle, life began in the sea and stayed there for most of its history. For roughly the first three billion years, everything alive was aquatic. The move onto land, traced in The Road to Sapiens, came late and was difficult, precisely because life was so thoroughly built for water that leaving it meant rebuilding nearly every system.

The mark of that origin is still all over you. Your cells are bathed in salty fluid; your blood plasma carries a balance of sodium, potassium, calcium, and chloride that is, in broad strokes, reminiscent of seawater, because the chemistry of life was set in the sea, and we have carried that internal ocean with us ever onto the land. When a doctor drips saline into a vein, it works because the inside of your body is, at the level of basic salt chemistry, a small enclosed parcel of ancient sea. This is sometimes overblown (your plasma is not simply diluted modern seawater, and the resemblance is partial and shaped by hundreds of millions of years of regulation rather than a frozen relic), but the underlying point holds: terrestrial animals did not escape the ocean so much as internalise it, sealing a portion of salt water inside a skin and carrying it ashore. You are, in part, a way for seawater to walk around on land.

II. The Great Reservoir

The single most important thing the ocean does for the habitability of the planet is hold heat. Water has an enormous capacity to absorb and store thermal energy, far more than air or rock, and the ocean is a vast tank of it, kilometres deep across most of the globe. This makes it the planet’s great thermal flywheel, the thing that steadies the whole system.

This is why coastal climates are milder than inland ones, why the sea smooths out the swing between day and night and between summer and winter, and why the ocean, not the atmosphere, is where most of the heat in the climate system actually resides. The air, from The Air, is the fast, churning, visible part of the solar heat engine; the ocean is the slow, deep, massive part, and the two are coupled. The Sun heats the tropics more than the poles, and the ocean, like the atmosphere, responds by moving that heat around. It does so through great currents, driven by wind at the surface and, in the deep, by differences in temperature and saltiness that change the density of water and set it sinking or rising. This deep circulation, sometimes called the global conveyor, carries warmth from the tropics toward the poles over timescales of centuries, and it is one of the master regulators of regional climate. The warmth that keeps north-western Europe far milder than its latitude would suggest, for instance, is carried there by this oceanic heat transport. That such currents could shift or weaken as the climate changes is one of the genuine concerns of climate science, precisely because so much regional habitability rests on their continuing to run as they have.

The ocean is the slow flywheel of the planetary dissipative system: it takes in concentrated solar heat, stores it, moves it across the world, and releases it gradually, smoothing the dispersal of energy that the whole biosphere rides on. A water world is a thermally stable world, and thermal stability is part of what gave life the long, uninterrupted runway it needed.

III. The Water Cycle Runs Through You

The ocean is also the reservoir of the planet’s water, and that water is in constant motion through a cycle that passes, repeatedly, through living bodies.

The Sun evaporates water from the ocean surface; it rises as vapour, cools, condenses into cloud, and falls as rain and snow over land and sea. What falls on land flows through soil and rock and river, is taken up by plants and animals, and eventually returns to the sea to begin again. This is the water cycle, and it is the supply line for all terrestrial life, the means by which the ocean’s water reaches the land creatures that cannot live in the sea but cannot live without its water either. Every freshwater spring and every drink you take is a way station in water on its way back to the ocean.

You are largely made of that water in transit. Roughly 60 percent of your body is water, the medium in which all your chemistry happens, the solvent that carries nutrients in and waste out, that regulates your temperature through sweat, that fills and shapes your cells. You are not a solid object that contains some water; you are mostly water, organised for a while into a particular shape, with the rest of your substance dissolved or suspended in it. The water moving through you now was, not long ago, in the ocean or a cloud or a river, and before long it will be again. This is the open-system idea from the section overview made vivid: there is no line where the planet’s water ends and your water begins, only a temporary borrowing. 

IV. The Largest Habitat on Earth

Life fills the sea from the sunlit surface to the crushing dark of the deepest trenches. In the lit upper layer, vast quantities of phytoplankton, microscopic photosynthesising drifters, turn sunlight and carbon dioxide into food and, in the process, produce a large share of the oxygen in the air, comparable to all the world’s forests combined. The breath you take is owed as much to the sea as to the land. These drifting organisms are the base of an oceanic food web that runs up through zooplankton, fish, and on to the largest animals ever to exist. The ocean also runs ecosystems with no sunlight at all: around the deep hydrothermal vents, entire communities live on energy drawn from the chemistry of the Earth itself, the same kind of chemistry that may have birthed life in the first place, a reminder that the Sun is the usual engine of life but not the only conceivable one.

This vast living system is tightly coupled to the rest of the biosphere. The ocean absorbs a large fraction of the carbon dioxide added to the atmosphere, which buffers climate change but also changes the chemistry of the sea itself, making it more acidic, with consequences for shell-building and reef-forming life that are an active and serious area of study. The ocean is absorbing carbon and acidifying as a result. The ocean’s health and the atmosphere’s are not two problems but one system, which is the recurring lesson of this entire section: the parts are not really separable.

V. The Body and the Sea

Pull the threads together, and the ocean stops being a place you might visit and becomes something closer to kin. You began, as a species and as a lineage, in it. You carry a regulated parcel of its salt chemistry inside every cell. You are mostly water borrowed from its cycle. You breathe oxygen, which it helped make, and your climate, your rainfall, and your food supply are steadied by its slow movement of heat. The pull many people feel toward the sea, the calming effect of being near it, is hard to study and easy to romanticise, and the careful thing to say is that while there is some evidence that natural environments, including coastlines, support wellbeing, the strong and specific claims sometimes made for the ocean’s healing properties outrun what has been shown. 

We think of ourselves as land animals on a solid world, with the ocean as one feature among others. The bigger picture is that we are a temporarily terrestrial, water-based chemistry that arose in the sea, climbed out carrying the sea inside us, and remains tied to it by every breath, every drink, and every season. The ocean is not the medium for all life on the planet because it is where the fish are. It is the medium because life is, at bottom, something water does, and the ocean is where the water is deepest.

VI. Cross-Links

• The Biosphere for the section overview
• The Air for the atmospheric half of the heat engine
• The Ground for the rock and soil the water cycle runs through
• The Sun for the engine that drives evaporation and currents
• The Earth for how the cycles tie together
• Resources for the reading list