E2 Greenhouse Effect and Atmospheric Opacity

Topic

The actual greenhouse effect is a phenomenon of radiative transfer: greenhouse gases absorb infrared radiation in specific spectral bands and re-emit it in all directions; this absorption and re-emission increases the atmosphere's opacity precisely within those specific bands. The more opaque the atmosphere becomes in a specific spectral band, the less infrared radiation within that band manages to pass directly from the surface to space without being absorbed and re-emitted at least once along the way.

The physics describing this absorption and re-emission of infrared radiation relies on two fundamental relationships: one describing how infrared radiation loses intensity—layer by layer—as it passes through an atmosphere capable of absorbing it; and a set of transfer equations that track, for each atmospheric layer, how much infrared radiation is absorbed, emitted, and simply allowed to pass through.

Among greenhouse gases, the radiative forcing produced by carbon dioxide does not increase in direct proportion to its concentration, but rather logarithmically, because carbon dioxide's primary absorption bands become saturated: once a specific spectral band is already absorbing nearly all the incoming infrared radiation, adding more carbon dioxide to the atmosphere yields little additional absorption in that band, even if the concentration continues to rise. Consequently, each time the carbon dioxide concentration doubles, that doubling produces approximately the same amount of additional radiative forcing—close to 3.7 watts per square meter—regardless of whether the doubling starts from a low concentration or an already high one. As the concentration of greenhouse gases rises, the atmosphere becomes opaque to infrared radiation at higher altitudes than before; consequently, the effective emission altitude—the height from which the atmosphere finally emits infrared radiation into space without it being reabsorbed by an overlying layer—shifts upward. Since atmospheric temperature decreases with altitude, a higher effective emission altitude corresponds to a lower emission temperature; this lower temperature reduces outgoing longwave radiation, as a cooler layer emits less infrared radiation than a warmer one. This reduction in outgoing longwave radiation is precisely what drives surface warming: as long as outgoing longwave radiation remains lower than the solar energy absorbed by Earth, the planet's surface continues to accumulate energy and its temperature continues to rise.

The entire greenhouse effect described—from the absorption and re-emission of infrared radiation by greenhouse gases to the resulting surface warming—can be summarized by this chain of events: increased greenhouse gases raise the effective emission altitude; a higher effective emission altitude implies a lower emission temperature; and a lower emission temperature reduces the outgoing longwave radiation that previously balanced the absorbed solar energy, forcing the Earth's surface to warm until a new equilibrium—at a higher surface temperature—once again balances outgoing longwave radiation with absorbed solar energy.

ID:665