Pollution and greenhouse effect

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The band absorption properties of the chief absorbing components of the atmosphere (CO2 and H2O) are clustered in the infrared part of the spectrum. Remember that monatomic (argon) and homopolar diatomic (nitrogen and oxygen) gases do not absorb in the parts of the spectrum important for global energy balances. Both CO2 and H2O have strong absorption bands in the IR, and their ability to absorb depends in part on their concentration. Other polyatomic gases (particularly methane) have similar characteristics.

Greenhouse gases trapping emitted radiation from the Earth's surface
Figure 1: Greenhouse gases trapping emitted radiation from the Earth's surface

The discussion on [[Gray surfaces|shields] shows why the greenhouse gases are important in the discussion of global warming. They act as a shield between the radiation emission from the Earth's surface and the heat sink of outer space, adding additional resistance to heat rejection by the Earth, without significantly affecting the shorter wavelength solar radiation absorbed by the Earth (Fig. 1). Increases in the concentration of either CO2 gas or H2O vapor (or both) tend to cause an increase in the overall equilibrium temperature of the Earth.

The same effect occurs in greenhouses and solar collectors, where the glass cover transmits solar energy in the visible spectrum; however, glass is nearly opaque at IR wavelengths, so emission from the plant beds or collector plate cannot escape, and the greenhouse or solar collector temperature remains well above the outdoor ambient temperature. This is actually the origin of the descriptive term "greenhouse" in "greenhouse gases."

The Earth's equilibrium temperature is affected by many factors. Those we have touched on in radiation are the concentrations of the polyatomic gases in the atmosphere (CO2, water vapor, methane and others) and their effect on trapping solar energy, and the changes in the Earth's average and local solar and IR absorptivity as affected by changing patterns of vegetation and ice fields on the Earth's surface. Other important factors include the coupling of surface temperatures with free convection patterns in the atmosphere and water bodies; the effect of the Earth's rotation; the CO2 concentration in the atmosphere as affected by man's activities, plant cycles, and solution of CO2 in the ocean; the effect of particulates in the atmosphere from dust storms, volcanic activity, and emissions from coal, wood and other fossil-fueled sources. All of these form a coupled system that must be considered in modeling global temperature patterns. The material in this text forms a good basis for understanding the complexity of the interactions.


Faghri, A., Zhang, Y., and Howell, J. R., 2010, Advanced Heat and Mass Transfer, Global Digital Press, Columbia, MO.

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