0:00:06.442,0:00:09.527 If you've ever floated on an ocean swell, 0:00:09.527,0:00:12.749 you'll know that the sea moves constantly. 0:00:12.749,0:00:14.962 Zoom out, and you'll see the larger picture: 0:00:14.962,0:00:18.046 our Earth, covered by 71 percent water, 0:00:18.046,0:00:21.608 moving in one enormous current around the planet. 0:00:21.608,0:00:23.761 This intimidating global conveyor belt 0:00:23.761,0:00:25.692 has many complicated drivers, 0:00:25.692,0:00:28.192 but behind it all is a simple pump 0:00:28.192,0:00:30.941 that moves water all over the earth. 0:00:30.941,0:00:34.228 The process is called thermohaline circulation, 0:00:34.228,0:00:36.192 and it's driven by a basic concept: 0:00:36.192,0:00:43.135 the concentration gradient. 0:00:43.150,0:00:44.825 Let's leave the ocean for one moment 0:00:44.825,0:00:46.610 and imagine we're in an empty room 0:00:46.610,0:00:49.156 with lots of Roombas sardined together 0:00:49.156,0:00:50.739 in one corner. 0:00:50.739,0:00:52.243 Turn them all on at once 0:00:52.243,0:00:53.991 and the machines glide outwards 0:00:53.991,0:00:56.204 bumping into and away from each other 0:00:56.204,0:01:00.241 until the room is filled with an evenly spaced distribution. 0:01:00.241,0:01:02.126 The machines have moved randomly 0:01:02.126,0:01:03.742 towards equilibrium, 0:01:03.742,0:01:05.824 a place where the concentration of a substance 0:01:05.824,0:01:07.658 is equally spread out. 0:01:07.658,0:01:10.526 That's what happens along a concentration gradient, 0:01:10.526,0:01:13.054 as substances shift passively from a high, 0:01:13.054,0:01:14.906 or squashed, concentration, 0:01:14.906,0:01:17.601 to a lower, more comfortable one. 0:01:17.601,0:01:21.187 How does this relate to ocean currents and thermohaline circulation? 0:01:21.187,0:01:22.939 Thermo means temperature, 0:01:22.939,0:01:24.524 and haline means salt 0:01:24.524,0:01:26.777 because in the real world scenario of the sea, 0:01:26.777,0:01:29.612 temperature and salinity drive the shift 0:01:29.612,0:01:33.168 from high to low concentrations. 0:01:33.168,0:01:34.792 Let's put you back in the ocean 0:01:34.792,0:01:36.707 to see how this works. 0:01:36.707,0:01:37.704 Snap! 0:01:37.704,0:01:40.660 You're transformed into a molecule of surface water, 0:01:40.660,0:01:42.542 off the temperate coast of New York 0:01:42.542,0:01:44.914 surrounded by a zillion rowdy others. 0:01:44.914,0:01:47.791 Here, the sun's rays act as an energizer 0:01:47.791,0:01:49.876 that set you and the other water molecules 0:01:49.876,0:01:52.126 jostling about, bouncing off each other 0:01:52.126,0:01:53.626 like the Roombas did. 0:01:53.626,0:01:54.721 The more you spread out, 0:01:54.721,0:01:56.852 the less concentrated the water molecules 0:01:56.852,0:01:58.763 at the surface become. 0:01:58.763,0:02:00.281 Through this passive motion, 0:02:00.281,0:02:03.865 you move from a high to a lower concentration. 0:02:03.865,0:02:05.948 Let's suspend the laws of physics for a moment, 0:02:05.948,0:02:07.697 and pretend that your molecular self 0:02:07.697,0:02:10.870 can plunge deep down into the water column. 0:02:10.870,0:02:12.198 In these colder depths, 0:02:12.198,0:02:14.446 the comparative lack of solar warmth 0:02:14.446,0:02:16.489 makes water molecules sluggish, 0:02:16.489,0:02:19.698 meaning they can sit quite still at high concentrations. 0:02:19.698,0:02:21.446 No jostling here. 0:02:21.446,0:02:22.615 But seeking relief 0:02:22.615,0:02:24.249 from the cramped conditions they're in, 0:02:24.249,0:02:26.364 they soon start moving upwards 0:02:26.364,0:02:29.310 towards the roomier situation at the surface. 0:02:29.310,0:02:30.422 This is how temperature 0:02:30.422,0:02:32.341 drives a shift of water molecules 0:02:32.341,0:02:34.447 from high to low concentrations, 0:02:34.447,0:02:36.135 towards equilibrium. 0:02:36.135,0:02:39.198 But sea water is made up of more than just H2O. 0:02:39.198,0:02:42.614 There are a great deal of salt ions in it as well. 0:02:42.614,0:02:44.811 And like you, these guys have a similar desire 0:02:44.811,0:02:46.532 for spacious real estate. 0:02:46.532,0:02:48.365 As the sun warms the sea, 0:02:48.365,0:02:50.198 some of your fellow water molecules 0:02:50.198,0:02:52.032 evaporate from the surface, 0:02:52.032,0:02:55.113 increasing the ration of salt to H2O. 0:02:55.113,0:02:57.030 The crowded salt ions left behind 0:02:57.030,0:02:58.657 notice that lower down, 0:02:58.657,0:03:01.364 salt molecules seem to be enjoying more space. 0:03:01.364,0:03:03.114 And so an invasion begins, 0:03:03.114,0:03:06.310 as they too move downwards in the water column. 0:03:06.310,0:03:07.615 In the polar regions, 0:03:07.615,0:03:09.615 we see how this small local process 0:03:09.615,0:03:11.447 effects global movement. 0:03:11.447,0:03:13.116 In the arctic and antarctic, 0:03:13.116,0:03:15.466 where ice slabs decorate the water's surface, 0:03:15.466,0:03:16.927 there's little temperature difference 0:03:16.927,0:03:19.310 between surface and deeper waters. 0:03:19.310,0:03:20.512 It's all pretty cold. 0:03:20.512,0:03:22.058 But salinity differs, 0:03:22.058,0:03:23.266 and in this scenario, 0:03:23.266,0:03:24.976 that's what triggers the action. 0:03:24.976,0:03:27.431 Here, the sun's rays melt surface ice, 0:03:27.431,0:03:29.696 depositing a new load of water molecules 0:03:29.696,0:03:31.280 into the sea. 0:03:31.280,0:03:33.407 That not only increases the proximity 0:03:33.407,0:03:35.742 between you and other water molecules, 0:03:35.742,0:03:37.886 leaving you vying for space again, 0:03:37.886,0:03:39.885 but it also conversely dilutes 0:03:39.885,0:03:42.166 the concentration of salt ions. 0:03:42.166,0:03:43.416 So, down you go, 0:03:43.416,0:03:45.084 riding along the concentration gradient 0:03:45.084,0:03:47.386 towards more comfortable conditions. 0:03:47.386,0:03:48.968 For salt ions, however, 0:03:48.968,0:03:50.969 their lower concentration at the surface, 0:03:50.969,0:03:52.676 acts like an advertisement 0:03:52.676,0:03:55.470 to the clamoring masses of salt molecules below 0:03:55.470,0:03:57.469 who begin their assent. 0:03:57.469,0:03:59.469 In both temperate and polar regions, 0:03:59.469,0:04:02.385 this passive motion along a concentration gradient, 0:04:02.385,0:04:04.190 can get a current going. 0:04:04.190,0:04:05.855 And that is the starting point 0:04:05.855,0:04:07.136 of the global conveyor 0:04:07.136,0:04:09.385 called thermohaline circulation. 0:04:09.385,0:04:10.803 This is how a simple concept 0:04:10.803,0:04:12.637 becomes the mechanism underlying 0:04:12.637,0:04:13.737 one of the largest 0:04:13.737,0:04:15.906 and most important systems on our planet. 0:04:15.906,0:04:16.992 And if you look around, 0:04:16.992,0:04:18.635 you'll see it happening everywhere. 0:04:18.635,0:04:20.495 Turn on a light, and it's there. 0:04:20.495,0:04:22.165 Concentration gradients govern 0:04:22.165,0:04:23.802 the flow of electricity, 0:04:23.802,0:04:26.219 allowing electrons squashed together in one space 0:04:26.219,0:04:28.837 to travel to an area of lower concentration 0:04:28.837,0:04:30.587 when a channel is opened, 0:04:30.587,0:04:32.508 which you do by flipping a switch. 0:04:32.508,0:04:35.382 Right now, in fact, there's some gradient action going on 0:04:35.382,0:04:39.137 inside you as you breath air into your lungs 0:04:39.137,0:04:41.219 letting the concentrated oxygen in that air 0:04:41.219,0:04:43.266 move passively out of your lungs 0:04:43.266,0:04:45.195 and into your blood stream. 0:04:45.195,0:04:46.969 We know that the world is filled 0:04:46.969,0:04:48.553 with complex physical problems, 0:04:48.553,0:04:50.053 but sometimes the first step 0:04:50.053,0:04:52.719 towards understanding them can be simple. 0:04:52.719,0:04:54.070 So when you confront the magnitude 0:04:54.070,0:04:55.468 of the ocean's currents, 0:04:55.468,0:04:57.534 or have to figure out how electricity works, 0:04:57.534,0:04:59.284 remember not to panic. 0:04:59.284,0:05:02.328 Understanding can be as simple as flipping a switch.