What Do We Know?
As you move up through the atmosphere, the temperature profile of the atmosphere reverses periodically. As altitude increases, the temperature of the atmosphere will switch from increasing to decreasing temperature, or vice versa.
Data from weather balloons and satellites indicates that reversals in the temperature profile of the atmosphere occur at specific altitudes.
The reversals in temperature profile occur because the chemical composition of the atmosphere as well as the nature of the electromagnetic radiation in that region changes. However, in this lesson, we will only discuss the reversal in the temperature profile between the troposphere and stratosphere.
In the troposphere, many greenhouse gases are present. These gases trap infrared radiation (IR), which possesses radiant energy, within this layer (how they trap IR radiation will be discussed later in the lesson). Close to the Earth's surface (low in the troposhpere) an infrared photon of light initially radiated outward will be absorbed and re-emitted many times. This re-emission can occur in any direction and very few IR photons will escape directly into space. As you move higher in the troposphere, IR photons scattered upward will begin to escape into space. Since the atmosphere is cooler at this position this also defines the temperature at which the atmosphere radiates energy back into space. If you further increase the greenhouse gas concentration the height at which emission back into space takes place increases. Now the IR photons appear to come from an even cooler point in the troposphere and this means that less energy is being emitted back into space. Since less energy is being radiated into space this means that the atmosphere has retained more energy. This is implies that the atmosphere has been warmed.
A stable climate depends on there being an energy balance between incoming solar radiation absorbed by the Earth and outgoing infrared radiation escaping into space. Without greenhouse gases the energy balance would make the Earth's climate too cold to support life as we know it. The net effect of absorption of IR radiation by greenhouse gases is to upset such an energy balance by reducing the amount of energy escaping into space. The imbalance causes the Earth to warm, which increases its emission of IR radiation, until the energy escaping to space is brought back in balance with a climate in which life thrives.