The temperature of the Earth is determined by an equilibrium, a balance between the energy that is received from the light of the Sun and is absorbed by the Earth, and the energy that is emitted as infrared radiation from the Earth to space. In order to keep the temperature constant, the energy that comes in from the Sun and the energy that goes out in infrared radiation need to balance each other. How does the carbon dioxide (and the other greenhouse gases) alter that balance? Carbon dioxide absorbs infrared light. As its abundance in the atmosphere increases, more of the infrared light emitted from the ground is absorbed by the carbon dioxide in the atmosphere, and sent back to the surface. The atmosphere becomes more opaque to the infrared light, and therefore acts like a blanket covering the Earth, preventing the heat from escaping, and warming up the Earth.
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