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The greenhouse effect is the absorption of infrared radiation by certain atmospheric gases, which causes the atmosphere to become warmer. The concept was proposed by Swedish chemist Svante Arrhenius at the end of the 19th century. When sunlight shines upon the Earth, most of the visible light penetrates the atmosphere. Some of the sunlight (albedo) reflects off of light-colored surfaces such as ice and snow, directly back into outer space. Reflected sunlight does not contribute to warming the Earth. The rest of the sunlight is absorbed by things like rocks, oceans, plants, and animals and causes them to become warmer. They conduct heat into the air, causing the air to become warmer. This is the reason that air is warmest near the average surface level of the ground (low altitudes and elevations) rather than at high altitudes or high elevations. Rocks, oceans, plants, and animals also glow with invisible photons known as infrared radiation, at wavelengths just beyond the red end of the visible light spectrum. Most of these infrared photons radiate into outer space. Certain atmospheric gases, such as carbon dioxide, methane, and water, absorb these photons on their way out. This causes the gases to become warmer, and they impart their warmth to the other gas molecules in the atmosphere. In this way they act metaphorically like the glass roof of a greenhouse which holds in the heat of the sunlight. This is how the presence of greenhouse gases causes the atmosphere to become warmer.
The greenhouse effect is essential to the survival of life on this planet. Without the greenhouse effect, the Earth would have an average temperature similar to that of Mars.
Three atmospheric gases are important in the greenhouse effect:
- Water is the most abundant greenhouse gas. Clouds block sunlight but also absorb infrared radiation; the net effect, according to climatological calculations, is that clouds have a slight cooling effect on the Earth.
- The most potent major naturally occurring greenhouse gas is methane. Methane is produced mainly by bacterial fermentation but becomes carbon dioxide when it reacts with oxygen gas. Although methane is continually produced, it does not accumulate in the atmosphere.
- Most atmospheric scientists consider carbon dioxide to be the most important greenhouse gas because it absorbs more infrared radiation than water and is more stable than methane.
The importance of atmospheric carbon dioxide to the greenhouse effect is demonstrated by studies of ice cores. In Greenland and in Antarctica, annual layers of ice have accumulated for over many thousands of years. For example, ice cores from Vostok, Antarctica, contain ice layers from the recent past to over 400,000 years ago. By counting back from the present, the age of each layer of ice can be determined. The oxygen and deuturium isotope ratios of each layer of ice is an estimate of the global average temperature. In addition, each layer of ice contains dissolved atmospheric gases. When these gases are released in a laboratory and analyzed, scientists can directly measure the concentration of methane and carbon dioxide that was in the air at that time. There is a close correlation between atmospheric carbon dioxide and global temperature over the past 400,000 years. Both temperature and carbon dioxide have fluctuated dramatically during that time, but always together, at least in the past 400,000 years. . .
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