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.
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 feed-backs are stabilizers.
The way to understand this is to
imagine bowling in different kinds of bowling alleys.
If you went to a fiendish, horrible, wicked bowling alley
that had a mound in the middle of the lane,
you're standing here, and you're trying to roll the ball down the
center here.
If you're off a by a little bit, say, in a positive direction here.
it's going to tend to pull the ball even farther in the positive direction.
This is why it's called a positive feedback, because the response of
the feedback is the same as the direction of that you're off.
If you have a little bit of
a perturbation, it gets amplified by this positive feedback.
A negative feedback would be like a
nice kiddy bowling lane where you've
got a gentle trough in the 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 the feedback is going to go in the opposite direction.
It's going to 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 it makes it easier to roll.
It's important to realize that if you the positive feedback if you
roll the ball on the negative sign of the lane it'll tend to go also more negative.
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.
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We've seen a couple of feedbacks already in this class.
The kitchen sink analogy is a classic example of a negative feedback.
The analogy again is that you have water flowing into the sink from a faucet.
And it builds up in the sink until it gets high enough that
it 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 is too low, like when you
first start out and turn on the faucet the drain rate
would be too low, and you'll tend to build up the
water vapor concentration or, the water level in the sink.
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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.
You have too much to start with it
would have relaxed back down to that equilibrium value.
It's being stabilized by this by this negative feedback.
There's also a negative feedback that we've 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 in 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, and relax toward that equilibrium value.
Those are both examples of negative feedbacks.
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