Infrared Absorption by the Earth

Geographical and Seasonal Variation in Solar Radiation Infrared Absorption by the Earth s Atmosphere Atmospheric Concentration of Carbon Dioxide, 1958-2000 Mean Temperatures in the United States, 1900-1992 Global Temperature Trend, 1856-2000 Atmospheric Electricity Speed of Sound in Various Media... 

Infrared Absorption by the Earth s Atmosphere, 14-28 Infrared and Far-Infrared Absorption... 

Greenhouse effect A warming effect exerted by the earth s atmosphere (particularly CO2 and H2O) due to the thermal energy retained by the absorption of infrared radiation. 

The Earth s atmosphere is composed primarily of non-polar molecules like N2 and O2, especially at greater altitudes where the H2O concentrations are small. One would therefore expect collision-induced contributions to the absorption of the Earth s atmosphere from N2-N2, N2-O2 and O2-O2 pairs. The induced rototranslational absorption of nitrogen has not been detected in the Earth s atmosphere, presumably because of strong interference by water absorption bands, but absorption in the various induced vibrational bands is well established (Tipping 1985). Titan (the large moon of Saturn) has a nitrogen atmosphere, somewhat like the Earth methane is also present. Collision-induced absorption by N2-N2 and N2-CH4 is important in the far infrared. 

Greenhouse effect The absorption of reflected infrared radiation from the Earth by gases in the atmosphere such as carbon dioxide (a greenhouse gas) leading to atmospheric or global warming. 

Since the atmosphere shields us from most deep ultraviolet radiation and from infrared radiation, the bulk of visible light (the solar spectrum) ranges from 350 to 750 nm. The 25,000 Frauenhofer15 "dark" lines are interruptions (in the range 295 to 1000 nm) in the continuous solar emission spectrum, due to absorption by the chemical elements present in the sun s atmosphere. Ultraviolet radiation was discovered by Ritter16 in 1801. Some radio waves do penetrate the earth s atmosphere, and they are most intense during solar storms. Infrared radiation also penetrates to some extent. 

As such, it is absolutely clear that the chemical composition of the atmosphere as well as the physical characteristics of condensed phase trace species are of leading importance as determinants of climate. A well-known example is the increase in the temperature of the Earth s surface due to the absorption of infrared radiation from the Earth s surface by CO2 in the air (see box). Without CO2, the Earth s surface would be several degrees cooler than at present, depending on cloud cover, water vapor, and other controlling factors. Of course, there is substantial concern over the secular increase of CO2, which will double from its pre-industrial level by the early to mid-twenty-first century. 

The aim of this section is to discuss the modifications of the atmospheric composition which can be related to the variations of the radiation balance and temperature observed in this century. We shall first deal with the relation between short-range modifications of atmospheric (stratospheric and tropospheric) composition and the transfer of incoming radiation. After this discussion, atmospheric factors influencing the absorption of infrared radiation emitted by the Earth s surface will be presented briefly. 

According to Twomey (1977) an increase of cloud droplet concentration involves an increase of the albedo if other factors (liquid water content, thickness of cloud) remain constant. Thus, for a cloud thickness of 0.1 km (liquid water content 0.3 g m"3) the albedo is raised from 0.25 to 0.43 when the drop concentration is increased by a factor of eight. For a cloud with a thickness of 1 km the corresponding albedo increase is from 0.82 to 0.90. However, an increase in the optical thickness of a cloud also leads to a higher absorption of the infrared radiation emitted by the Earth s surface. 

Ultimately, studies from ground-based observatories are limited by absorption in the Earth s atmosphere. For example, no studies above THz are possible even from mountain-top observatories. Two major instruments, SOFIA and FIRST, are poised to change this situation dramatically. SOFIA (for Stratospheric Observatory For Infrared Astronomy [26]) will carry a 2.7 m telescope in a 747SP aircraft to altitudes of 41... 

Greenhouse effect Absorption of solar radiation by the earth, its conversion and reemission in the infrared, and the absorption of this radiation by the atmosphere resulting in a gradual rise of atmospheric temperature. 

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