Another Type of Positive Feedback

Suppose the planet got too cold.
- Ice reflects light, and therefore heat. If it gets colder, you get more ice, and more heat is reflected to space.
- Since it’s colder there will be less evaporation.
- Less evaporation means less water vapor in the atmosphere, reducing the greenhouse effect, which makes it colder.
- If it gets cold enough CO2 can freeze out further reducing the greenhouse effect. This would be permanent.
In the distant past (a billion or more years ago) the Earth went through periods where all the oceans froze over (called snowball earth). Fortunately it didn’t get cold enough to freeze CO2. Volcanism eventually released enough CO2 to raise temperatures above freezing.

Suppose the planet got too cold.