How the Ocean Works by Denny Mark;

Author:Denny, Mark;
Language: eng
Format: epub
Publisher: Princeton University Press

Figure 6.3. The intensity of light (watts per square meter) impinging on a surface depends on the angle between light and surface. Light is most intense when the surface is perpendicular to the direction in which the light approaches (A) and least intense when the surface is parallel to the direction in which light approaches (C).

The same ideas apply to earth's surface. The projected area the earth as a whole exposes to the sun is equal to π times the square of the earth's radius, approximately 1.3 × 1014 square meters. When this area is multiplied by a factor of 1370 watts per square meter we find that the sunny side of the earth intercepts 1.8 × 1017 joules of sunlight every second, the value noted in chapter 4. That's 180 billion megawatts of solar energy. As sunlight strikes the planet, about 40% of it is reflected back into space (by clouds and snow, for instance), but roughly 1.0 × 1017 watts, still a staggeringly large number, is absorbed by the atmosphere, land, and ocean.

Thermal Equilibrium

Because the earth absorbs this amount of heat every second, it is reasonable to think that it might be heating up. If one were to start with a cold earth and instantly “turn on” the sun, this would indeed be true. Such a scenario would in a sense be analogous to turning on an incandescent electric light bulb. The instant you flip the switch, energy is delivered to the bulb (in this case in the form of electrical current rather than light), and the bulb rapidly heats up. But at the same time that it heats up, the bulb radiates energy out. For the bulb, some of this radiated energy is in the form of visible light (after all, it is a light bulb), but a similar process is true of any object that is heated. The hotter the object, the more energy it radiates. (The rate at which energy is radiated is proportional to the fourth power of the object's temperature. Thus, a small increase in temperature results in a large increase in radiated energy.) Soon after the switch is turned on, the bulb reaches an equilibrium in which the rate at which energy radiates out is just equal to the rate at which electrical energy is supplied. Once at equilibrium, the temperature of the bulb stays constant.

The same physics applies to the earth taken as a whole. In the course of its 4.6-billion-year history, earth's temperature has risen until the energy it radiates out into space is just equal to the light energy it absorbs from sunlight. This equilibrium results in an average surface temperature of about 15°C. Whereas the temperature of a light bulb filament is several thousand degrees Celsius, resulting in radiation of visible light, at the relatively low surface temperature of the earth, the ground radiates infrared light.

In summary, the earth has the average temperature it does because it has reached an equilibrium with the energy delivered to it by sunlight.

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