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If you've ever floated on an ocean swell,

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you'll know that the sea moves constantly.

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Zoom out, and you'll see the larger picture:

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our Earth, covered by 71 percent water,

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moving in one enormous current around the planet.

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This intimidating global conveyor belt

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has many complicated drivers,

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but behind it all is a simple pump

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that moves water all over the earth.

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The process is called thermohaline circulation,

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and it's driven by a basic concept:

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the concentration gradient.

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Let's leave the ocean for one moment

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and imagine we're in an empty room

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with lots of Roombas sardined together

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in one corner.

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Turn them all on at once

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and the machines glide outwards

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bumping into and away from each other

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until the room is filled with an evenly spaced distribution.

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The machines have moved randomly

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towards equilibrium,

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a place where the concentration of a substance

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is equally spread out.

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That's what happens along a concentration gradient,

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as substances shift passively from a high,

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or squashed, concentration,

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to a lower, more comfortable one.

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How does this relate to ocean currents and thermohaline circulation?

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Thermo means temperature,

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and haline means salt

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because in the real world scenario of the sea,

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temperature and salinity drive the shift

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from high to low concentrations.

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Let's put you back in the ocean

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to see how this works.

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Snap!

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You're transformed into a molecule of surface water,

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off the temperate coast of New York

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surrounded by a zillion rowdy others.

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Here, the sun's rays act as an energizer

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that set you and the other water molecules

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jostling about, bouncing off each other

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like the Roombas did.

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The more you spread out,

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the less concentrated the water molecules

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at the surface become.

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Through this passive motion,

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you move from a high to a lower concentration.

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Let's suspend the laws of physics for a moment,

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and pretend that your molecular self

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can plunge deep down into the water column.

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In these colder depths,

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the comparative lack of solar warmth

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makes water molecules sluggish,

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meaning they can sit quite still at high concentrations.

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No jostling here.

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But seeking relief

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from the cramped conditions they're in,

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they soon start moving upwards

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towards the roomier situation at the surface.

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This is how temperature

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drives a shift of water molecules

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from high to low concentrations,

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towards equilibrium.

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But sea water is made up of more than just H2O.

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There are a great deal of salt ions in it as well.

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And like you, these guys have a similar desire

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for spacious real estate.

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As the sun warms the sea,

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some of your fellow water molecules

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evaporate from the surface,

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increasing the ration of salt to H2O.

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The crowded salt ions left behind

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notice that lower down,

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salt molecules seem to be enjoying more space.

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And so an invasion begins,

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as they too move downwards in the water column.

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In the polar regions,

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we see how this small local process

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effects global movement.

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In the arctic and antarctic,

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where ice slabs decorate the water's surface,

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there's little temperature difference

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between surface and deeper waters.

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It's all pretty cold.

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But salinity differs,

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and in this scenario,

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that's what triggers the action.

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Here, the sun's rays melt surface ice,

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depositing a new load of water molecules

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into the sea.

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That not only increases the proximity

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between you and other water molecules,

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leaving you vying for space again,

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but it also conversely dilutes

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the concentration of salt ions.

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So, down you go,

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riding along the concentration gradient

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towards more comfortable conditions.

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For salt ions, however,

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their lower concentration at the surface,

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acts like an advertisement

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to the clamoring masses of salt molecules below

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who begin their assent.

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In both temperate and polar regions,

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this passive motion along a concentration gradient,

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can get a current going.

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And that is the starting point

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of the global conveyor

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called thermohaline circulation.

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This is how a simple concept

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becomes the mechanism underlying

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one of the largest

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and most important systems on our planet.

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And if you look around,

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you'll see it happening everywhere.

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Turn on a light, and it's there.

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Concentration gradients govern

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the flow of electricity,

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allowing electrons squashed together in one space

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to travel to an area of lower concentration

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when a channel is opened,

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which you do by flipping a switch.

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Right now, in fact, there's some gradient action going on

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inside you as you breath air into your lungs

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letting the concentrated oxygen in that air

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move passively out of your lungs

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and into your blood stream.

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We know that the world is filled

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with complex physical problems,

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but sometimes the first step

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towards understanding them can be simple.

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So when you confront the magnitude

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of the ocean's currents,

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or have to figure out how electricity works,

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remember not to panic.

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Understanding can be as simple as flipping a switch.