By Andy May
We can define the total greenhouse effect (GHE) as the difference between what the surface temperature of the Earth would be without an atmosphere (its blackbody temperature, about 254K) and the actual average surface temperature (roughly 288K). Using this definition, the average total greenhouse effect on the Earth is 34°C. The blackbody temperature is the temperature of the planet without an atmosphere, but with the same Bond albedo (reflectivity). Because the atmosphere is transparent to most solar radiation and the Earth’s surface is opaque, the surface of the Earth absorbs twice as much radiation as the atmosphere. Per the laws of thermodynamics, a planet must emit as much radiation as it receives. The Earth’s temperature controls the type of radiation it emits and it emits in the thermal infrared, which is called outgoing longwave infrared radiation, usually abbreviated as “OLR.” This means the Earth’s average surface temperature is warmer than the atmosphere.
One can see many other definitions of the GHE. Common descriptions include both the effect and a cause. Some describe the GHE as a blanket, keeping some of the radiation emitted by the Earth from reaching outer space warming the Earth. Fourier proposed this idea in 1824. Others, like Arrhenius, propose that infrared-active gases (“greenhouse” gases) absorb radiation emitted by the Earth and re-emit some of it back toward the Earth’s surface. Some modern climate scientists like to define a theoretical “radiative forcing” as the GHE. This is defined in IPCC AR5 as the theoretical (but unmeasured) “change in net irradiance at the tropopause” over some period.
We prefer to use the term to describe the effect and leave the cause for later discussion. First, we need to make some reasonable assumptions about heat transfer in the atmosphere. Ultimately all heat energy emitted by the Earth’s surface must make its way to outer space. This is a reasonable assumption because the Earth’s surface temperature has been reasonably stable over geological time, only varying from about 282K to 302K for the last 500 million years per paleontological data. Our current temperature is about 288K, see Figure 1.
Figure 1 (source: Glen Fergus, click the name or figure for a high-resolution version)