2 00:00:09,087 --> 00:00:11,231 We now have the building blocks that 3 00:00:11,231 --> 00:00:14,916 determine how Earth's climate is controlled, but there's one 4 00:00:14,916 --> 00:00:16,792 more piece to this that you need to 5 00:00:16,792 --> 00:00:20,421 understand as we think about Earth's climate. 6 00:00:20,421 --> 00:00:23,710 And that is the idea of feedbacks. 7 00:00:25,230 --> 00:00:29,750 A feedback is essentially a loop of cause and effect. 8 00:00:29,750 --> 00:00:34,960 And there's two basic types of feedbacks. There are positive and negative feedbacks. 9 00:00:36,210 --> 00:00:40,570 Positive feedbacks are amplifiers, and negative feed-backs are stabilizers. 10 00:00:40,570 --> 00:00:43,280 The way to understand this is to 11 00:00:43,280 --> 00:00:49,220 imagine bowling in different kinds of bowling alleys. 12 00:00:49,220 --> 00:00:53,530 If you went to a fiendish, horrible, wicked bowling alley 13 00:00:53,530 --> 00:00:58,500 that had a mound in the middle of the lane, 14 00:00:58,500 --> 00:01:01,220 you're standing here, and you're trying to roll the ball down the 15 00:01:01,220 --> 00:01:02,280 center here. 16 00:01:02,280 --> 00:01:06,990 If you're off a by a little bit, say, in a positive direction here. 17 00:01:06,990 --> 00:01:11,160 it's going to tend to pull the ball even farther in the positive direction. 18 00:01:11,160 --> 00:01:14,810 This is why it's called a positive feedback, because the response of 19 00:01:14,810 --> 00:01:19,770 the feedback is the same as the direction of that you're off. 20 00:01:19,770 --> 00:01:22,760 If you have a little bit of 21 00:01:22,760 --> 00:01:27,100 a perturbation, it gets amplified by this positive feedback. 22 00:01:27,100 --> 00:01:29,010 A negative feedback would be like a 23 00:01:29,010 --> 00:01:31,780 nice kiddy bowling lane where you've 24 00:01:31,780 --> 00:01:37,260 got a gentle trough in the in the lane, and you've got the gutters here. 25 00:01:37,260 --> 00:01:40,600 And if you roll it and you're off a little bit here, say 26 00:01:40,600 --> 00:01:46,140 in a positive direction, the the feedback is going to go in the opposite direction. 27 00:01:46,140 --> 00:01:49,290 It's going to tend to push it back in the negative direction. 28 00:01:49,290 --> 00:01:51,900 That's why this is called a negative feedback. 29 00:01:51,900 --> 00:01:54,840 And this tends to stabilize the bowling ball in the center 30 00:01:54,840 --> 00:01:58,350 of the lane here, so it makes it easier to roll. 31 00:01:58,350 --> 00:02:06,000 It's important to realize that if you the positive feedback if you 32 00:02:06,000 --> 00:02:11,850 roll the ball on the negative sign of the lane it'll tend to go also more negative. 33 00:02:11,850 --> 00:02:17,300 Positive feedback doesn't always make the earth 34 00:02:17,300 --> 00:02:19,580 warmer because warmer is a positive number. 35 00:02:19,580 --> 00:02:22,760 A positive feedback could make the earth get even 36 00:02:22,760 --> 00:02:25,580 colder if you did something that made the earth cold. 37 00:02:25,580 --> 00:02:27,810 A positive feedback in general is an amplifier. 38 00:02:29,795 --> 00:02:31,010 [NOISE] 39 00:02:31,010 --> 00:02:34,280 We've seen a couple of feedbacks already in this class. 40 00:02:34,280 --> 00:02:39,390 The kitchen sink analogy is a classic example of a negative feedback. 41 00:02:39,390 --> 00:02:45,180 The analogy again is that you have water flowing into the sink from a faucet. 42 00:02:45,180 --> 00:02:49,110 And it builds up in the sink until it gets high enough that 43 00:02:49,110 --> 00:02:53,860 it can push water down the drain as quickly as it's coming in from the faucet. 44 00:02:53,860 --> 00:02:56,050 And this 45 00:02:56,050 --> 00:03:02,130 works out to be that there is a negative feedback on the water level in the sink. 46 00:03:02,130 --> 00:03:05,250 Because if the water level gets too high, the drain rate goes 47 00:03:05,250 --> 00:03:08,950 up, and it tends to pull the water level back down again. 48 00:03:08,950 --> 00:03:10,820 Where if the water level is too low, like when you 49 00:03:10,820 --> 00:03:14,940 first start out and turn on the faucet the drain rate 50 00:03:14,940 --> 00:03:17,690 would be too low, and you'll tend to build up the 51 00:03:17,690 --> 00:03:21,005 water vapor concentration or, the water level in the sink. 52 00:03:21,005 --> 00:03:21,620 [SOUND] 53 00:03:21,620 --> 00:03:28,480 If you were to start from a sink that had no water in it, and 54 00:03:28,480 --> 00:03:33,260 turn on the faucet, the water level in the sink would relax to the equilibrium value. 55 00:03:33,260 --> 00:03:36,170 Or if you were to walk up to it and dump a bucket of water in. 56 00:03:37,290 --> 00:03:39,510 You have too much to start with it 57 00:03:39,510 --> 00:03:42,450 would have relaxed back down to that equilibrium value. 58 00:03:42,450 --> 00:03:46,810 It's being stabilized by this by this negative feedback. 59 00:03:48,810 --> 00:03:51,090 There's also a negative feedback that we've talked 60 00:03:51,090 --> 00:03:54,340 about controlling the temperature of the Earth and that 61 00:03:54,340 --> 00:03:58,060 is due to the fact that infrared light leaving 62 00:03:58,060 --> 00:04:01,660 the Earth depends on the temperature of the Earth. 63 00:04:01,660 --> 00:04:05,170 The warmer the Earth is, the faster you're shedding energy 64 00:04:05,170 --> 00:04:08,230 out to space, according to epsilon sigma T to the fourth. 65 00:04:09,350 --> 00:04:14,350 And so in the same way, if you were to start out with an Earth 66 00:04:14,350 --> 00:04:17,300 at absolute zero and turn on the sun, the 67 00:04:17,300 --> 00:04:21,220 Earth would warm up until it reached an equilibrium temperature. 68 00:04:21,220 --> 00:04:24,660 Or if you were to crash a big meteorite into it and make 69 00:04:24,660 --> 00:04:28,640 it too hot, it would cool down and, and relax toward that equilibrium value. 70 00:04:28,640 --> 00:04:30,593 Those are both examples of negative feedbacks. 71 00:04:30,593 --> 00:04:36,450 [MUSIC]