The Sun

The earth moves in an elliptical orbit around the sun, being closest to the sun, a distance of 1.47 x 10⁸ kilometers, in December. The distance to the sun is maximum at about 1.52 x 10⁸ kilometers in June. The eccentricity of this elliptical orbit is about 0.016. The time of closest approach is called perihelion (see Figure 1) and occurs when the Northern Hemisphere is having its winter, and we call this time the winter solstice. The earth is at aphelion, the furthest distance from the sun, in July when the Southern Hemisphere is having its winter solstice and the Northern Hemisphere has its summer solstice. The midpoints between solstices are called the vernal equinox (equal length of day and night) in spring and autumnal equinox in autumn. The plane of the earth's equator is tipped at an angle of 23.5° to the plane of its orbit around the sun. The earth wobbles slightly so that the tilt actually changes between about 22 and 25 degrees cyclically with a period of about 41,000 years. These seemingly slight changes lead to the so-called Milankovitch effects on climate.

The earth/atmosphere/ocean system can be considered as a very large thermodynamic engine that takes energy from the sun, converts it to many other forms, and then releases it back to outer space. The intensity of the sun's radiation reaching the "top" of the atmosphere is 1,380 Wm^-2. More power per square meter reaches the earth at low latitudes (closer to the equator) than in the polar latitudes. A simple calculation shows that the power of this engine, shown in Figure 2, is about 1.76 x 10¹¹ megawatts. A large power plant in a major city might produce 100 megawatts, so the sun provides the earth with the equivalent of about 2 billion such power plants. Wallace and Hobbs (1977) have calculated magnitudes of natural and anthropogenic heat sources that might alter this energy balance at the earth's surface.

This energy from the sun is absorbed preferentially in low latitudes in the tropical regions and subtropics (see Figure 3). It is transmitted from the tropics toward the polar regions as thermal energy or as latent heat in the form of water vapor. Eventually, this energy is radiated back to outer space in an amount equal to the input, giving the earth/atmosphere/ocean system as a whole a thermodynamic balance. The global warming we will discuss later in the course is not a matter of the atmosphere gaining more energy than it is losing, but rather a change in the redistribution of energy in the atmosphere. The earth is not observed to be heating up or cooling down rapidly, and even if we changed the composition of the gases in our atmosphere we don't change the fact that the earth loses the same amount of energy it receives from the sun. When we change the gases in the earth's atmosphere, we change the processes of redistribution: more heat is retained in the lower atmosphere.

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