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Energy Balance
An initial point of clarification is necessary for those students without
background in the earth or physical sciences. When we use the term "radiation"
in this course we are considering components of solar emissions ("sunshine") and
thermal emissions from the earth. These definitions do not include nuclear or
cosmic radiation. Likewise, the term "radiatively active" refers to
emission or
absorption of visible light or thermal (infrared) energy as from a wood-burning
stove. So, except for the ultra violet component of solar radiation, the
radiant energy we discuss in this course is benign to humans.
The earth receives energy from the sun, mostly in the form of visible radiation, as we showed in a previous unit. At the top of the atmosphere, a plane perpendicular to the line from the sun receives about 1360 W m-2. The daily amounts of energy per unit area at different points on the earth are shown in Figure 1. Units used in this figure are cal cm-2, where 1 cal cm-2 min-1 = 697.58 W m-2. Note that the location receiving the maximum daily radiation is the South Pole in December! From the global perspective, some of this solar energy is reflected back to outer space as visible light and the rest is processed in various ways by the earth/atmosphere/ocean system and re-radiated back to space as infrared (IR) radiation. The detailed pathways for this energy transformation determine the climate conditions of our planet. The composition of the planet's atmosphere and the characteristics of the planet surface control the temperature distribution at the planet surface and hence the motions and processes of its atmosphere.
Once the visible solar energy is absorbed by the earth system - be it in clouds, the free atmosphere, or at the surface - it is transformed from electromagnetic energy to heat energy. This absorbed energy either raises the surface temperature or evaporates surface water to create atmospheric water vapor. If an object has a temperature higher than its surroundings, then it will lose energy to its surroundings by infrared radiation. Infrared radiation is not detectable by the human eye but can be detected by absorption by human skin. A heating element in an electric oven radiates with both infrared and visible radiation if it is glowing. If the current is turned off, it gradually loses its glow (its visible radiation) but continues to emit IR radiation, which can be felt by a hand close to the heating element.
The earth, compared with outer space, is very warm, and so it radiates energy away to outer space at a rate that is proportional to the fourth power of the temperature. Some of this energy radiated from the surface of the earth is directly lost to outer space, while some is absorbed by gases and clouds in the atmosphere. This absorption of infrared radiation by a planet's atmosphere is called the "greenhouse effect". Nitrogen, the dominant gas in the atmosphere, does not absorb infrared radiation, but oxygen, the second most abundant gas, does absorb in certain wavelength regions.
