Lapse Rate and Effective Radiation Altitude

To maintain its energy balance, Earth must emit some of the energy it receives from the sun. The Earth primarily emits energy in the Infrared (IR) region of the electromagnetic spectrum. As you learned previously, photons of IR energy excite greenhouse gas molecules, which then transfer their energy to surrounding nitrogen or oxygen molecules. Therefore, even though photons of IR get emitted from Earth, many of them get absorbed by greenhouse gas molecules in the troposphere and their energy ends up being absorbed rather than leaving Earth’s atmosphere. The probability is very low that a photon close to Earth’s surface will escape directly into space. However, as we move higher in the troposphere, there are fewer molecules to absorb the IR photons, and the probability that the photons will escape increases.

The effective radiation altitude is the altitude at which IR radiation has a higher probability of escaping upwards into space. Emission that takes place higher up in the troposphere does so at a lower temperature and therefore less energy is leaving the lower atmosphere. Thus as the effective radiation altitude increases, the troposphere retains more energy, causing tropospheric warming.

Greenhouse Gases Raise Effective Radiation Level

To get a better understanding of the lapse rate and effective radiation altitude, open the Lapse Rate learning tool. Take a moment to explore the applet by changing the concentration of CO₂ and observe what happens, and then answer the following questions.

Question: What does the arrow represent, and why does it change colour with changing CO₂ concentrations?

Answer: The arrow represents the relative amount of energy leaving the surface of the Earth. The arrow becomes more transparent as CO₂ concentrations increase, visualizing that less energy is leaving the troposphere.

Question: What effect does increasing the concentration of CO₂ have on the effective radiation altitude?

Answer: As the CO₂ concentration goes up, the effective radiation altitude increases.

Question: In your own words, explain how a change in the effective radiation altitude affects tropospheric warming.

Sample Answer: As the effective radiation altitude increases, the temperature at which emissions occur is lower, and so more energy is trapped in the lower troposphere.