Storm in a Teacup: The Physics of Everyday Life(20)

Convection currents like this help move energy around and share it out wherever a fluid is heated from below. They are why fish-tank heaters, underfloor heating, and saucepans on the stove are all so effective—none of those things would work nearly as well without gravity. When we say “heat rises,” that’s not quite true. It’s more that “cooler fluid sinks as it wins the gravitational battle.” But no one thanks you for pointing that out.

Buoyancy doesn’t just matter for hot air balloons, snails, and romantic candlelit dinners. The oceans, the vast engines of our planet, take their marching orders from gravity just like everything else. The depths are not still. Water that hasn’t seen sunlight for centuries is flowing across and around the planet, on a long, slow journey back to the daylight. But before looking down into the depths, look up. Next time you see a tiny moving glint high in the sky on a clear day, a passenger jet at cruising altitude, allow yourself to appreciate just how high up it is: about 6 miles. Then imagine yourself standing at the deepest part of the ocean floor, the bottom of the Marianas Trench. The ocean surface would be the same distance above you as that plane is.?? Even the average depth of the oceans is 2.5 miles, a little under half the distance to that plane. The ocean covers 70 per cent of the surface of the Earth. There is a lot of water out there.

And hidden in those dark depths is a familiar pattern. The same mechanism causing raisins to dance in lemonade is also driving the vast oceans of the Earth on their slow journey around the planet. The scale is different and the consequences are more important, but the principle is exactly the same. The blue of our blue planet is in motion.

But why would it move? The oceans have had millions of years to adjust to their situation. Surely they’d have reached wherever they’re going by now? Two things keep stirring the pot: heat and salinity. They matter because they affect density, and a fluid with areas of different density will flow to adjust as the battle of gravity plays out. We all know that the ocean is salty, but I am still staggered every time I really think about just how much salt is out there. To make a standard full household bath as salty as the ocean, you need to add about 22 pounds of salt, a large bucketful. A whole bucket, just for one bath! It’s not the same everywhere in the ocean—the salinity ranges from about 3.1 percent to about 3.8 percent, and although that difference sounds tiny, it matters. Just as putting sugar in a fizzy drink makes it more dense, the huge amount of salt makes sea water more dense than fresh water. Colder water is more dense than warm water, and the oceans range from about 32°F close to the poles to 86°F close to the equator. So cold, salty water will sink and warmer, fresher water will rise. And that simple principle takes sea water on a continual journey around the planet. It may be thousands of years before one bit of water returns to the same part of the ocean again.

In the North Atlantic,§§ water is cooling as the wind steals heat away. Where the sea surface freezes to form sea ice, the new ice is mostly just water; the salt gets left behind. Together, those processes make the sea water colder, saltier, and denser, and so it starts to sink, pushing the less dense water out of its way as it answers the call of gravity and finds its way to the bottom of the sea. As it slithers slowly along the sea floor, it’s channeled by valleys and blocked by ridges, just like a river. From the North Atlantic, it flows southward along the bottom of the ocean at an inch per second, and after a thousand years it reaches its first obstacle: Antarctica. Unable to creep farther south, it turns to the east as it meets the Southern Ocean. This ocean, the great watery roundabout at the bottom of the planet, links all the sea water on the planet because on its way around the white continent, it merges with the lower edge of the Atlantic, the Indian, and the Pacific oceans. The vast, slow flow of water from the North Atlantic creeps around Antarctica until it turns northward again, journeying far into either the Indian or the Pacific Ocean. Gradual mixing with the water around it reduces its density, and it eventually finds its way back to the surface, after perhaps 1,600 years without a single sunbeam passing through it. There rain, runoff from rivers, and ice-melt dilute the salt again, while wind-driven currents push it along on the rest of its journey until it finds its way back into the North Atlantic, repeating/resuming the cycle. It’s called the thermohaline circulation: “thermo” for heat, and “haline” because of the salt. This ocean overturning is also sometimes referred to as the Ocean Conveyer Belt, and although this conveys a slightly simplistic picture, these flows do girdle the planet and they are driven by gravity. The wind-driven surface currents have carried explorers and traders for centuries. But the ocean conveyer system as a whole carries a cargo of at least equal importance to our civilization: heat.

More heat from the Sun is absorbed at the equator than in any other area on the planet, both because the Sun is higher in the sky there, and because the planet is widest there and so there’s a large area for absorption. Heating up water even a tiny bit takes a lot of energy, so warm oceans are like a giant battery for solar energy. The shifting ocean is redistributing that energy around the planet, and the thermohaline circulation is the hidden mechanism behind our weather patterns. Much of our thin, fickle atmosphere whooshes about on top of a steady heat reservoir that constantly provides energy and moderates extremes.

The atmosphere gets all the glory, but the oceans are the power behind the throne. Next time you look at a globe, or a satellite picture of Earth, don’t think of the oceans as the empty blue bits between all the interesting continents. Imagine the tug of gravity on those giant, slow currents, and see the blue bits for what they are: the biggest engine on the planet.