[MUSIC] So we now have the building blocks that determine how Earth's climate is controlled. But there's one more piece to this that you need to understand as we think about Earth's climate. And that is the idea of feedbacks. So a feedback is essentially a loop of cause and effect. And there's two basic types of feedbacks. There are positive and negative feedbacks. Positive feedbacks are amplifiers and negative feedbacks are stabilizers. The way to understand this is to imagine bowling in different kinds of bowling alleys. So if you went to a fiendish horrible, wicked bowling alley that had sort of a mound in the middle of the lane, so you're standing here and you roll the ball down the center here. If you're off by a little bit say, in a positive direction here, it's gonna tend to pull the ball even farther in a positive direction. This is why it's called a positive feedback because the response of the feedback is the same as the direction that you're off. So if you have a little bit of a perturbation it gets amplified by this positive feedback. A negative feedback would be like a nice sort of kiddy bowling lane where you've sort of a gentle trough in the lane and you've got the gutters here. And if you roll it and you're off a little bit here say in a positive direction the feedback is gonna go in the opposite direction. It's gonna tend to push it back in the negative direction. That's why this is called a negative feedback and this tends to stabilize the bowling ball in the center of the lane here. So makes it easier to roll. So, it's important to realize that the positive feedback if you roll the ball on the negative side of the lane, it will tend to go also more negative, so positive feedback doesn't always make the Earth warmer because warmer is a positive number a positive feedback could make the Earth get even colder if you did something that made the Earth cold. A positive feedback in general is an amplifier. We've seen a couple of feedbacks already in this class. The kitchen sink analogy is a classic example of a negative feedback. So the analogy again is that you have water flowing into the sink from a faucet and it sort of builds up in the sink until it gets high enough that is can push water down the drain as quickly as it's coming in from the faucet. And this works out to be that there is a negative feedback on the water level in the sink. Because if the water level gets too high, the drain rate goes up and it tends to pull the water level back down again. Where if the water level was to low like when you first start out and turn on the faucet, the drain rate will be too low and you'll tend to build up the water level in the sink. So, if you were to start from a sink that had no water in it and turn on the faucet, the water level in the sink would relax to the equilibrium value. Or if you were to walk up to it and dump a bucket of water in. So you had too much to start with, it would relax back down to that equilibrium value. It's being stabilized by this negative feedback. [SOUND] There's also a negative feedback that we talked about controlling the temperature of the Earth. And that is due to the fact that infrared light leaving the Earth depends on the temperature of the Earth. The warmer the Earth is the faster you're shedding energy out to space according to epsilon sigma T to the fourth. And so the same way if you were to start out with an Earth at absolute zero and turn on the Sun, the Earth would warm up until it reached an equilibrium temperature. Or if you were to crash a big meteorite into it and make it too hot. It would cool down and relax toward that equilibrium value. So those are both examples of negative feedbacks. [MUSIC]
