The Earth's Climate

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Solar radiation

The Sun has been in existence for several billion years. The Earth is constantly exposed to its rays, which warm the surface of our planet. Without this energy input, the Earth would be a frozen wasteland. But what exactly is this solar radiation that makes our planet inhabitable?

Solar energy reaches the Earth in the form of electromagnetic radiation, part of which is perceptible as visible light. Electromagnetic radiation can be broken down into millimeter and radio waves, infrared emissions, visible and ultraviolet light and, beyond these frequencies, X-rays and gamma rays. A cold mass like the Earth primarily emits radio, millimeter and infrared radiation, whereas a hot mass like the Sun emits waves covering the entire electromagnetic spectrum. It sends us a steady flow of energy dominated by the visible part of the spectrum. Part of this radiation is absorbed by the atmosphere. Ultraviolet and X-rays are mainly absorbed at altitudes above 100 kilometers, while infrared rays and visible light are partly reflected by the atmosphere and clouds.

The Earth's surface, which with an average temperature of 15 degrees Celsius is not very warm, re-emits part of the sun's energy, which can be absorbed by the atmosphere (greenhouse gases) and clouds. The amount of solar energy that reaches the Earth is not constant over time. The Sun goes through a "solar cycle" of about 11 years, a phenomenon that has been observed by astronomers for centuries through variations in the number of sunspots (of dark patches on the Sun's surface). Still, the energy flow emitted by the Sun varies only by about one one-thousandth over the course of such a cycle.