Earth's Emission Spectrum and Atmospheric Infrared
Absorption
As the Earth absorbs energy from the Sun, its temperature rises.
However, the temperature of the Earth has remained in a state of
quasi-equilibrium for a very long time. It follows that any energy
absorbed by the Earth is released in some form back to the atmosphere
such that there is no net amount of energy absorbed over time, and
the temperature remains in equilibrium. The Earth absorbs primarily
visible light from the Sun. Plants use visible light in
photosynthesis and leaves appear green because the yellow portion of
the light has been absorbed. The exact amount of light absorbed by
Earth is highly variable in space and time, depending on the cloud,
aerosol, and absorbing constituents (notably ozone) in the
atmosphere. In a cloud-free atmosphere with no aerosols, as much as
50% of the incoming radiation may be absorbed by the surface. The
amount of absorption also depends on the amount of reflection at the
surface. The fraction of light that is reflected at the surface is
known as surface albedo. Snow, ice, and water are more
reflective than land and vegetation so the surface albedo is
generally greater at higher latitudes (near the poles) and lesser at
lower latitudes. The surface albedo contributes to the overall
planetary albedo which is defined as the reflectivity of the
Earth-atmosphere system and also includes the effects of scattering
from molecules, clouds, and aerosols.
The Earth, like all other matter, acts like a blackbody and radiates in proportion to its kelvin temperature. Because the surface of the Earth varies spatially, and there is absorption at the surface, the Earth is not a perfect blackbody. However, the emissions at various wavelengths can be approximated by a blackbody curve. Figure 19 show's the Earth's emissions at the top of the atmosphere measured by the Nimbus-4 satellite in clear sky conditions. This is the amount of longwave (UV-VIS-IR) radiation that reaches space. The figure also shows the blackbody curves for a number of absolute temperatures. The radiance is plotted as function of wavenumber here because it is the convention in infrared radiative transfer studies. Since the wavenumber is proportional to 1/w, increasing wavenumber corresponds to decreasing wavelength. The primary differences between the surface temperature blackbody curve (about 290 K in this figure) and the radiation from the Earth into space are due to absorption of longwave radiation by certain atmospheric constituents. The most important of these absorbers at these wavelengths are carbon dioxide (CO2), water vapor (H2O), and ozone (O3). This figure shows broad areas of absorption due to these molecules but in fact, each band is made up of thousands of specific absorption lines which are very close together. These fine structures are a result of the complexity of the absorbing molecules. Carbon dioxide absorbs in a band centered at 15m, near the peak in the longwave emissions in wavenumber space. There are also some regions of the infrared portion of the spectrum where ozone absorbs. These absorptions are purely vibrational; nothing is happening to the electrons in the ozone molecule when infrared photons are absorbed. Ozone absorbs in the 9.6 m band. Except for this absorption there is a wide wavenumber range where little absorption takes place (800-1200cm-1). This region is known as the atmospheric window because radiation emitted by the Earth at these wavelengths can easily pass through into space. In this wavelength range, the only thing to stop the radiation from escaping back into space is clouds, which reflect a portion back toward the surface. This is why the surface temperature tends to drop significantly on clear winter nights (no clouds with little water vapor to reflect).
Figure 19. Earth Emissions Measured from Nimbus-4
The actual emissions in Figure 19 most closely follow the blackbody curve corresponding to the temperature of the layer that is radiating. For example, the emissions in the atmospheric window are closest to the surface temperature blackbody curve (NEED TEMP HERE) because that radiation originated at the surface. However, the emissions in the active infrared absorption regions follow colder blackbody curves because the effective source of that radiation is at some layer in the atmosphere that is colder than the surface of the Earth. Below a certain layer none of the emitted radiation makes it into space but remains trapped in the atmosphere, and only radiation emitted in the atmosphere above this layer escapes into space.