The sun radiates energy in all directions. Most of it dissipates into space, but the tiny fraction of the sun's energy that reaches Earth is enough to heat the planet and drive the global weather system by warming the atmosphere and oceans. The delicate balance between the amount of heat Earth receives from the sun and the heat that Earth radiates back into space makes it possible for the planet to sustain life.
Solar radiation is created by nuclear fusion reactions in the sun's core, which causes it to emit a large amount of electromagnetic radiation, mostly in the form of visible light. This radiation is the energy that heats the Earth. The sun's surface emits about 63 million watts of energy per square meter. By the time the energy reaches the Earth, after traveling 150 million kilometers, or 93 million miles, it has diminished to 1,370 watts per square meter at the top of the atmosphere directly facing the sun.
Electromagnetic radiation, including visible light, infrared radiation, ultraviolet light and X-rays, can travel through the vacuum of space. Other forms of energy require a physical media to move through. For example, sound energy needs air or another substance to be transmitted, and the wave energy of the oceans needs water. Solar energy, however, can travel from the sun to the Earth without the need for a physical substance to transmit the energy. This feature of electromagnetic energy makes it possible for the Earth to receive solar energy, including heat.
Heating the Earth
Some of the solar energy that arrives at the Earth bounces off the atmosphere and clouds and back into space. The surface of the Earth receives about half of the incoming solar radiation. The solar energy takes the form of heat and visible light as well as ultraviolet rays, the type of energy that causes sunburn. The energy is absorbed by matter, including air, water, rocks, buildings, pavement and living things, and the matter is heated as a result. The Earth does not heat evenly, chiefly because some areas receive more solar radiation than others. The differences in energy drive the winds and ocean currents across the entire planet.
If the Earth constantly received solar energy without any means of losing energy, it would continually grow hotter. The Earth radiates heat back into space, preventing the planet from overheating. The amount of reradiated heat is sensitive to the type of gases in the atmosphere; some gases absorb heat more effectively than others and interfere with reradiation. One of these gases is carbon dioxide. As atmospheric carbon dioxide concentrations increase, the Earth's heat budget is altered, with more energy stored in the atmosphere and less heat radiating back into space, a phenomenon known as the greenhouse effect.
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