The steeper the lapse rate, the stronger the greenhouse effect. If the atmosphere were incompressible like water, and convection maintained a uniform temperature with
temperature temperature temperature
Fig. 5.11 A demonstration of the effect of the lapse rate on the strength of the greenhouse effect. If we increase the greenhouse gas concentration of the atmosphere, the IR radiation to space will originate from a higher altitude (labeled skin altitude), but the skin temperature at the skin altitude will be the same as before. The increase in skin altitude increases the ground temperature. If the temperature of the atmosphere were the same at all altitudes, then raising the skin altitude would have no impact on ground temperature.
temperature temperature temperature
Fig. 5.11 A demonstration of the effect of the lapse rate on the strength of the greenhouse effect. If we increase the greenhouse gas concentration of the atmosphere, the IR radiation to space will originate from a higher altitude (labeled skin altitude), but the skin temperature at the skin altitude will be the same as before. The increase in skin altitude increases the ground temperature. If the temperature of the atmosphere were the same at all altitudes, then raising the skin altitude would have no impact on ground temperature.
altitude, as it did in our pan of water on the stove, there would be no greenhouse effect. To understand why this is so, imagine increasing the concentration of CO2 in an atmosphere. This has the effect of raising the altitude in the atmosphere where light on average escapes to space (Fig. 5.11). You remember from Chapter 4 that reality is a bit complicated in this regard; some IR light goes directly to space from the ground, in the frequency range of the atmospheric window, while at other frequencies, like in the CO2 band, light appears to originate from the coldest part of the atmosphere, at the tropopause. But bunching all of that light together in our minds, we can imagine that, on average, light comes from a higher altitude as the CO2 concentration goes up. Let's call that altitude the skin altitude.
Now think back to Chapter 3 and the layer model. Remember that the outermost part of the atmosphere, the part that radiates directly to space, always had the same temperature in all of those different model configurations. We called that the skin temperature, and it was always 253 K for the albedo and sunlight intensity of the Earth, whatever the model configuration. In solving the layer model for the temperatures of all the atmospheric layers and the ground, it was convenient to start from the outer skin layer and work down toward the ground. Let's take that approach again, assuming that the temperature change with altitude (the lapse rate) has been decided by the physics of convection. If the skin altitude were 5 km high and the lapse rate were 6 K/km, then the temperature of the ground would be
We can visualize this as a line drawn downward from the skin altitude, following the slope of a moist adiabat until it intersects the ground (Fig. 5.11). If we increase the CO2 content of the atmosphere, we raise the skin altitude, and the same moist adiabat slope intersects the ground at a higher temperature. Algebraically, the change in temperature from raising the skin altitude zskin can be calculated as
Here's the point. If the lapse rate were different than 6 K/km, then a change in CO2, driving a change in the skin altitude, would have a different effect on the temperature of the ground. If the atmosphere were incompressible, for example, and convection insisted that the temperature should be uniform with altitude, then it would make no difference how high the skin altitude was:
The temperature of the ground would be the same as the skin temperature no matter what.
The lapse rate in the atmosphere (how quickly temperature decreases with altitude) is determined primarily by convection and the "hidden" heat carried aloft by water vapor. The lapse rate determines the sensitivity of the temperature of the ground to changes in the IR opacity of the atmosphere. If we want to forecast the effect of the rising atmospheric CO2 concentration on the temperature of the ground, we will have to get the lapse rate right, and any changes in the lapse rate that result from future climate change.
Was this article helpful?
This is a set of 3 guides all about surviving disasters. Within this set you will learn the following subjects: Earthquakes, Evacuations, Survivor Family and Tsunami.