Earth receives energy from the Sun in the form of solar radiation. It then radiates the energy it has received back into space in the form of terrestrial radiation (see "Radiation from the Sun and from the Earth" on pages 128-134). The two are in balance—the amount of outgoing energy from the Earth is equal to the amount of incoming energy from the Sun. If this were not so, the Earth would grow steadily hotter or colder. The global climate does change over time, of course, and at present many people are worried that it may be growing warmer, but these are small changes that cancel out over long periods. In general, the energy budget of the Earth is in balance. The surface absorbs energy and the atmosphere loses energy.
It is a complicated balance, however. Although it is true to say that the Earth absorbs and radiates equal amounts of energy, this is a huge oversimplification of what actually happens.
Neither solar energy nor terrestrial radiation is distributed evenly. Equatorial regions enjoy a much warmer climate than do the polar regions, because they receive more solar energy. In latitudes lower than 40°, however, the surface absorbs more energy in the course of the year than the atmosphere loses. In latitudes higher than 40°, the opposite is the case. There the atmosphere loses more energy than the surface absorbs. Nevertheless, the equatorial regions are not becoming hotter and the Arctic and Antarctic are not growing colder. This is because heat is transferred from the warmer areas to the cooler areas, of course, and this transfer has to be factored into the global energy budget. If it is omitted, the budget will balance only if we assume that the equator is 25°F (14°C) warmer than in fact it is and that the North and South Poles are 45°F (25°C) colder.
Heat is transferred by the movement of air (see "General circulation of the atmosphere" on pages 10-17) and by ocean currents (see "Transport of heat by the oceans" on pages 18-27). Horizontal heat transfer is called advection, and it must also balance. The amount of warmth that moves into high latitudes must be equal to the amount the land and ocean lose by radiation in the same latitudes. If this were not so, some parts of the Earth would grow steadily warmer or cooler. Moving toward the equator from the North or South Pole, the amount of heat arriving by advection increases steadily until it reaches the latitude, about 40°, where the amount of radiant heat being absorbed is large enough to inhibit advection from lower latitudes. The amount of heat moving by advection reaches zero at the equator. This is shown in the diagram on page 136, where the amount of energy is given in megawatts (millions of watts) per square meter (MW m-2). The diagram also illustrates the fact that the Northern and Southern Hemispheres are independent of each other and that energy does not cross from one to the other.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.