Analogy: Think of a partially stretched "toy slinky" - if you bump the slinky, the energy of the bump is absorbed by the vibrations in the slinky. Greenhouse Analogy: Energy from the sun in the form of some ultraviolet and visible light short wavelength passes through the glass of the greenhouse. As the light strikes various surfaces in the greenhouse and they are heated.
These surfaces in turn re-radiate the heat in the form of infrared radiation long wavelength. However, the IR radiation is blocked from escaping by the glass. IR is not able to pass through the glass, hence the greenhouse air heats up fairly dramatically. The greenhouse gases have the same property as the glass towards the IR radiation.
Think of the greenhouse gases acting as an invisible glass shield around the earth. Click for larger image Greenhouse Gas Molecules:. The greenhouse gas molecules are shown in the next series of figures along with the IR spectra and the bending and vibrations caused by absorbing the IR radiation.
The arrows on the molecules indicate the direction of the bends and vibrations of the bonds. The IR spectra indicates the specific energies at certain wavelengths which are absorbed. Concentrations of CFC gases in the atmosphere are the highest of any of the halocarbons, and they can absorb more infrared radiation than any other greenhouse gas.
The impact of 1 molecule of a CFC gas is equivalent to 10, molecules of carbon dioxide. Nitrous oxide N2O , a relatively long-lived gas, has increased in atmospheric concentration due mainly to agriculture.
Bacteria convert a small amount of this nitrate and ammonia into the form of nitrous oxide. Internal combustion engines also produce nitrous oxide. Ozone O3 is also a relatively minor greenhouse gas because it is found in relatively low concentrations in the troposphere the lowest layer of the atmosphere. In the troposphere, it is produced by a combination of pollutants — mostly hydrocarbons and nitrogen oxide compounds. In the s, John Tyndall, an Irish scientist who was fascinated by the growth and formation of glaciers, wanted to test his ideas explaining how Earth maintained a fairly constant temperature.
He began a series of experiments to measure the amount of radiant heat infrared radiation that certain gases could absorb and transmit. Tyndall found that water vapor and carbon dioxide were good absorbers and emitters of infrared radiation. The relative importance of a greenhouse gas depends on its abundance in Earth's atmosphere and how much the gas can absorb specific wavelengths of energy.
An effective absorber of infrared radiation has a broader absorption profile, which means that it can absorb a wider spectrum of wavelengths. The sun's ultraviolet wavelengths are strongly absorbed by ozone in the stratosphere. The sun's visible wavelengths of radiation pass easily through the atmosphere and reach Earth. Some of this energy is emitted back from the Earth's surface in the form of infrared radiation. Water vapor, carbon dioxide, methane, and other trace gases in Earth's atmosphere absorb the longer wavelengths of outgoing infrared radiation from Earth's surface.
These gases then emit the infrared radiation in all directions, both outward toward space and downward toward Earth. This process creates a second source of radiation to warm to surface — visible radiation from the sun and infrared radiation from the atmosphere — which causes Earth to be warmer than it otherwise would be. Some of this energy is emitted from Earth's surface back into space in the form of infrared radiation.
Much of this infrared radiation does not reach space, however, because it is absorbed by greenhouse gases in atmosphere, and is then emitted as infrared radiation back toward the Earth's surface. This process is known as the greenhouse effect.
The upper atmosphere is much less dense and contains much less water vapor than near the ground, which means that adding more carbon dioxide significantly influences how much infrared radiation escapes to space. Have you ever noticed that deserts are often colder at night than forests, even if their average temperatures are the same?
Without much water vapor in the atmosphere over deserts, the radiation they give off escapes readily to space. In more humid regions radiation from the surface is trapped by water vapor in the air. Similarly, cloudy nights tend to be warmer than clear nights because more water vapor is present. The influence of carbon dioxide can be seen in past changes in climate.
Ice cores from over the past million years have shown that carbon dioxide concentrations were high during warm periods — about 0. During ice ages, when the Earth was roughly 7 to 13 F C cooler than in the 20th century, carbon dioxide made up only about 0. Even though water vapor is more important for the natural greenhouse effect, changes in carbon dioxide have driven past temperature changes.
In contrast, water vapor levels in the atmosphere respond to temperature. We take pills that are a tiny fraction of our body mass and expect them to affect us. Today the level of carbon dioxide is higher than at any time in human history. Without action to control emissions, carbon dioxide might reach 0.
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