When Does a Greenhouse Run Away?
Over at Lumoville , an alleged professor of atmospheric science is claiming that a runaway greenhouse is impossible on Earth, so I thought I might try to discuss what the necessary and sufficient conditions for one are - Cliff Notes version. Opacity The atmosphere, or rather some of the gases in the atmosphere, are fairly transparent to incoming visible radiation but opague to outgoing thermal radiation. This one way transport warms the Earth a good deal beyond what its temperature would be in their absence. The most important such gas is water vapor. Water vapor is the key, but not the only, player in runaway warming. A tale of two feedbacks. The blanket of water vapor around the Earth warms it. Suppose we warm the surface. That increases the amount of water vapor entering the atmosphere and consequently tends to warm it. That's a positive feedback, people, and positive feedbacks are unstable - a little bit of warming produces more warming, which in turn produces still m...