Greenhouse Effect

Sections: Greenhouse Analogies | A Simple Model | Added Realism

What is the Greenhouse Effect?

At its most basic level, the greenhouse effect is a process through which the climate system maintains a higher level of energy within the system due to the atmosphere being opaque to thermal radiation. Now this does not mean that the radiation never leaves the climate system. Instead, once the climate system is in a given equilibrium, the amount of thermal radiation entering the system will be equal to the amount of thermal radiation leaving the system.

However, since the atmosphere is at least partly opaque to thermal radiation, this results in the radiation taking longer to leave the climate system. But while energy takes longer to leave the system, the same amount of energy enters the system at the same rate as before. As such, the average temperature must be higher than it would be if the atmosphere were transparent to thermal radiation in the same way that it is transparent to sunlight for such an equilibrium to be achieved. A higher temperature raises the rate at which energy leaves the system to the level at which energy enters the system despite the fact that energy takes longer to pass through the system.

If this at first seems unclear as it did to me, or you wish to explain the effect to others, there are a few useful Greenhouse Analogies which should help. Afterwards you might like to look at A Simple Model. In any case, I would view understanding the greenhouse effect and the role that it plays in the climate system not as something which one gets immediately but as a process - where elements are added to the explanation which result in a more realistic and accurate view of the climate system as a whole.

But if there are two take away points at present, they should be that without greenhouse gases, the earth would be cooler than it is at present - so cold that life might might never have gotten a foothold, and when we raise the amount of greenhouse gases in the atmosphere, we increase the opacity of the atmosphere to thermal radiation, raising the temperature required for the rate at which energy leaves the system to equal the rate at which energy enters the system. Both of these points are discussed in greater depth in A Simple Model.

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