The Earth's Climate

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Solar energy

Solar energy results from nuclear fusion in the Sun's core, where temperatures exceed 10 million degrees Celsius. This energy is radiated first to the Sun's "surface," where temperatures are still as high as 6,000 degrees Celsius, and then into the solar system in the form of electromagnetic radiation. Solar radiation is the main source of energy that heats the surface of the Earth. Every day the Sun sends us a considerable quantity of energy, of which less than 3% is consumed by man over an entire year. In comparison, geothermal energy, generated by the Earth's internal heat, represents less than a thousandth of the solar energy the planet receives.
The other sources of energy—cosmic radiation like the light from stars—amount to a mere one-millionth of solar energy.
On average (day and night, winter and summer, at the Tropics and at the poles), the Sun emits 342 watts per square meter at the levesl of the upper atmosphere. About 30% of this energy is reflected back into space.
Since 1978, instruments on man-made satellites have been taking precise measurements of the variation in solar irradiance. It is very low—only 0.1%—and mostly concerns ultraviolet rays, which are largely absorbed at high altitudes, in the stratosphere.
The alternation between glacial and interglacial periods is apparently determined by variations in the amount of solar energy that reaches the polar zones in the summer, which in turn are due to slight variations in the Earth's rotation axis and orbit in cycles of 19,000 to 400,000 years.