|
Trapping implies that something is taken out of the system. For example, if you trap a wolf, that wolf no longer interacts with other parts of the system. If you later release the wolf back into the environment then the trap has acted like a system buffer. If the wolf remains trapped, then it is lost to the system. Systems function via exchanges and exchange rates - "trapping heat" implies that heat has been lost to the system. This is incorrect and is a physically imprecise way to describe the overall process. In addition, this gives the lay person a false impression that the Earth's atmosphere can be considered like a "blanket". The more physically description is that the Earths atmosphere is a selective absorber of OLR, heats up to some equilibrium temperature, and then re-emits radiation back to the surface of the Earth, thus warming the surface. The atmosphere thus serves as a systems buffer, not a blanket
The primary GHG in the Earth's atmosphere is water vapor; CO2 is a secondary GHG and there are 4-6 tertiary GHGS. The reason that water vapor is primary can be seen by examining the percentage of the total emitted IR radiation from the Earth as compared to the other gases. This is shown in Figure 1. The spectrum of emission from the Earth is that of a blackbody radiator at a temperature of 255K. At wavelengths > 17 microns, water vapor absorbs all of that emitted energy which comprises about 50% of the total energy emitted by the Earth. But this absorption is all by water vapor in its gaseous form. An additional 25% comes from water vapor in its liquid form -e.g. a cloud. A cloud is composed of very tiny water droplets (radius < 0.1 microns). As such the gravitational force on them is minimal and atmospheric clouds literally float in the air, supported by buoyancy until they are either a) evaporated by the sun or b) form water droplets big enough to fall out as precipitation.
In contrast, CO2 has a much narrower range of IR absorption but its principle absorption feature has the same wavelength as the peak wavelength of the Earth's emission. As a consequence, CO2 can still absorb ~20% of the total energy emitted by the Earth. The bulk of the IR radiation emitted by the Earth that can escape directly to space occurs between 8 and 12 microns. All radiation emitted at wavelengths less than 8 microns is again absorbed by water vapor, but this is a very small percentage of the total emitted radiation. This physical situation means that any increases in water vapor or CO2 will cause the atmosphere to further warm up which in turn will lead to increased surface warming.
