Earths Energy Balance
Though sunlight may appear white and nondescript, it consists of electromagnetic waves that have a wide range of wavelengths. One can separate these wavelengths by simply holding up a prism to sunlight, which causes light rays of shorter wavelengths to bend at larger angles. The various purples, blues, greens, yellows, and reds that emerge from the prism represent all the wavelengths of light that are visible to the human eye, which only detects wavelengths between 400 and 700 nanometers (billionths of a meter). The visible spectrum, however, accounts for just under half of the Suns total energy. Much of the Suns energy is made up of ultraviolet (UV) radiation, which has shorter wavelengths (higher energy levels) than visible light and extends off of the purple end of the visible spectrum. An even larger amount of this invisible energy can be found in the longer infrared wavelengths (lower energy levels) of light that extend off the opposite end of the visible spectrum.
Not all of this light is absorbed by the Earth. Roughly 30 percent
of the total solar energy that strikes the Earth is reflected back into
space by clouds, atmospheric aerosols, snow, ice, desert sand, rooftops,
and even ocean surf. The remaining 70 percent of the TSI is absorbed by
the land, ocean, and atmosphere. In addition, different layers of the
Earth and atmosphere tend to absorb different wavelengths of light. Only
one percent of the TSI, mostly in the form of UV radiation, is absorbed
by the upper atmosphere, mainly by stratospheric ozone. Twenty to 24
percent of the TSI and a majority of the near infrared radiation is
absorbed in the lower atmosphere (troposphere), mainly by water vapor,
trace gases, clouds, and darker aerosols. The remaining 46 to 50 percent
of predominately visible light penetrates the atmosphere and is taken in
by the land and the oceans.
The absorption of solar energy heats up our planets surface and atmosphere and makes life on Earth possible. But the energy does not stay bound up in the Earths environment forever. If it did, then the Earth would be as hot as the Sun. Instead, as the rocks, the air, and the sea warm, they emit thermal radiation (heat). This thermal radiation, which is largely in the form of long-wave infrared light, eventually finds its way out into space, leaving the Earth and allowing it to cool. For the Earth to remain at a stable temperature, the amount of longwave radiation streaming from the Earth must be equal to the total amount of absorbed radiation from the Sun.