Stefanboltzmann

Radiation emission = constant #2 X (Absolute temperature)4

where:

absolute temperature (°K, degrees Kelvin) = °C (degrees celsius) + 273.15;

diation also is absorbed—19% by the atmosphere and 51% by Earth's surfaces (largely in the oceans). This solar energy budget warms the Earth system to around -18°C.

Although much colder than the Sun, the Earth also emits radiation (Box 8.1), but at a much lower wattage and with a maximum wavelength in the infrared region around 10 microns (Fig. 8.8a). This outgoing radiation, which has longer wavelengths than red light (Fig. 8.7), acts much like an infrared lamp that warms food in a restaurant. Rather than escaping into space, however, more than 70% of this infrared radiation is absorbed in the troposphere by ''greenhouse'' gases (Fig. 1.1b, Table 8.1). In the Earth system, it is estimated that the surface temperature is warmed by more than 30°C because of this atmospheric trapping of outgoing radiation—without which the oceans would freeze and life as we know it would not exist.

The most important greenhouse gas—in terms of both total volume and capacity to absorb infrared radiation—is water vapor (Table 8.1). The breadth of infrared wavelengths absorbed by water vapor exceeds all of the other greenhouse gases (Fig. 8.8b). Water vapor can even close the ''atmospheric window'' (infrared wavelengths between 8 and 12 microns that are least absorbed in the atmosphere), which is why it is warmer during calm winter nights with clouds rather than with stars above. Because liquid water evaporates and precipitates regionally (Figs. 8.4 and 8.5), over hours to weeks, it is difficult to estimate the geographic distribution and ''greenhouse effect'' of water vapor on a global scale. This regional effect is compounded by the asymmetric areas of the ocean north and south of the equator (Fig. 1.4)—potentially magnifying climate warming when insolation is greater in the Southern Hemisphere (Fig. 7.3) because of increased water-vapor production.

In contrast, greenhouse gases with longer atmospheric lifetimes become well mixed and uniformly distributed on a global scale. Next to water vapor, carbon

Wavelength (meters)

Gamma Rays

X rays

Infrared lighTelevision and Radar Radio

Wavelength (meters)

Infrared lighTelevision and Radar Radio

X rays

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