The Earth reflects about 30 percent of the incoming solar radiation. The other 70 percent is absorbed and warms the atmosphere, the land, and the oceans. In order to maintain an energy balance, so the Earth does not get too hot or cold, the amount of incoming energy must roughly equal the amount of outgoing energy. The insolation occurs in the visible portion of the spectrum. The radiation that is emitted from objects that have been heated on Earth, however, peaks in the infrared portion of the spectrum in the form of heat. The Earth's energy balance is maintained when the incoming solar radiation is roughly balanced by the outgoing energy being radiated by the Earth back out into space as infrared radiation.
The point of heating and radiation varies between the insolation and the outgoing radiation. The visible solar radiation heats the Earth's surface, not the atmosphere. The infrared radiation the Earth gives off in response, however, is emitted to space from the Earth's upper atmosphere, not the Earth's surface. The greenhouse gases in the atmosphere absorb any infrared radiation that does originate from the Earth's surface, where it then stays.
There are many paths that the energy from the Sun to the Earth's atmosphere and surface can take. Energy interacts with the Earth's surface, clouds, and atmospheric gases as it undergoes various routes: It can be directly reflected off the Earth's surface, absorbed and then rera-diated, and then reabsorbed by the Earth's atmosphere. The illustration on page 8 shows the unique way the Earth's atmosphere is able to capture and redirect the energy.
The atmosphere allows roughly 70 percent of the Sun's radiation to reach the Earth, reflecting the remainder back to outer space. As depicted by the illustration, the majority of insolation is in the short and medium wavelength region of the spectrum. The highest energy, short wavelengths, such as the gamma rays, X-rays, and ultraviolet (UV) light, is absorbed by the mid to high levels of the atmosphere. This is desirable because if these wavelengths traveled to the surface they could cause harm to life on Earth. For example, exposure to UV light can lead to cancer. The medium wavelengths—referred to as visible light—travel to the Earth's surface. These waves can be absorbed or reflected at the Earth's surface, as well as by the CO2 and water vapor in the atmosphere.
When wavelengths from the electromagnetic spectrum reach the Earth, several things can happen. They can be reflected; the Earth's
The Sun's ultraviolet and light radiation pass through the clear atmosphen
Some ultraviolet and light radiation is reflected by the Earth and the atmosphere
Some of the reflected infrared radiation is trapped in the atmosphere by greenhouse gas molecules. The effect of this is to warm the Earth's surface and the lower atmosphere
Source: U.S. Department of State
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Some infrared radiation Earth remains inBhe ground, and some is emitted from the
The greenhouse effect refers to the interaction between the Earth's atmosphere and surface to absorb, transfer, and emit energy as heat, cycling it through the atmosphere and back to the surface. The natural greenhouse effect is necessary for life as it exists on Earth today.
surface, the ocean, the clouds, or the atmosphere can absorb them; and they can be scattered. Roughly one-third of the incoming visible light is directly reflected back into space without ever reaching the Earth's surface. Of that which does enter the Earth's atmosphere, nearly one-quarter of that is reflected by clouds and particulates (tiny suspended particles) in the atmosphere. Of the remaining visible light that does reach the Earth's surface, approximately one-tenth is reflected upward by snow and ice because these surfaces have an extremely high albedo (high reflectivity).
Of the two-thirds of the visible light that is transmitted successfully through the Earth's atmosphere, half is absorbed by the land and water on the Earth's surface, and one-fifth is absorbed by the clouds and atmosphere. When electromagnetic energy is absorbed at the Earth's surface, it heats it up and then reradiates it back as longwave infrared (IR) radiation. The temperature is also much cooler: 210-310 K. Infrared radiation is what keeps the air warm above the Earth's surface.
As the infrared radiation is emitted upward, it does not just escape to space. Water vapor and other gases in the atmosphere absorb it and
The Earth's natural greenhouse effect is often compared to a nursery greenhouse because of its ability to alter the local environment, keeping heat confined in a specific environment. (Nature's Images)
trap it in the atmosphere. This trapped heat then radiates in all directions: upward, down toward the Earth's surface again, and sideward. This trapped heat energy is what constitutes the greenhouse effect. Because the heat is being "held in" by the atmosphere, it functions in a similar manner to the concept of a nursery greenhouse, warming the Earth's environment.
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