One of the simplest positive feedbacks in the climate system is called the ice
albedo feedback. Albedo is a word that
means the fraction of light that gets reflected back out to space.
When visible light comes in and gets reflected back to space,
it doesn't deposit its energy as heat to the planet.
We saw the albedo in the layer model,
the higher the albedo of the planet the colder the planet is
because it doesn't use that energy, that light
to fuel its own thermal energy, it just loses it off the top.
The feedback with ice is that ice and snow are very reflective.
If you have a layer of snow over ground,
it will cause the visible light to
be reflected back out to space; it'll have a high albedo.
And then, you make it warmer, and the light
will hit the darker ground and be absorbed.
This will have a lower albedo.
If you absorb that light, it makes the Earth warmer.
Going from a cold state to a warm state tends to be
amplified by the melting of the ice. And so in this feedback
diagram, the state variable is the temperature, which gets feed back to.
A perturbation to the temperature that makes it a little bit warmer....
causes ice to melt which makes it warm up even more.
This is an amplifier.
This also works for sea ice because
sea ice is very reflective
and the ocean is some of the lowest albedo material on Earth.
And so if you have sea ice, it's going to reflect the light back to space,
you melt the sea ice, the light will be absorbed in the ocean, it's
suddenly getting added to the Earth's heat budget.