Stuart Staniford
The very basic physics of the greenhouse effect is probably familiar to most people, but let me take a second to review it for the rest of our readers. The sun, being very hot, emits enormous amounts of electromagnetic radiation. The spectrum of that radiation peaks in the range that our eyes can detect as visible radiation. The earth’s atmosphere is mostly transparent to the sun’s radiation, and so most of it reaches the ground and the parts that aren’t reflected back upwards are absorbed and heat the earth. Because the earth is much warmer than space, it also radiates electromagnetic radiation out in all directions. However, since the earth is much less warm than the sun, it emits much less energetic (longer wavelength) infrared radiation. If the earth’s atmosphere were dry and free from greenhouse gases, it would be transparent and the earth would be quite a lot colder (permanently below freezing everywhere). Instead, certain gases absorb some of the infrared radiation, which warms the air more than it otherwise would be, and keeps the planet warmer. … So in general, if there is more CO2, then it is harder for infrared radiation to get out of the atmosphere. That means the earth cannot shed heat as effectively, and thus will warm up. As it warms up, it will release more radiation. Eventually, it will get enough warmer that the outgoing radiation will balance the incoming sunlight (referred to as radiative equilibrium). Normally the earth is in radiative equilibrium to a very good approximation, but that is less true at the moment (as we shall see). So the key question is how much temperature rise do we get from any given increase in CO2.
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