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Greenhouse gases in the troposphere

What are “greenhouse gases?” The transparent windows of a greenhouse transmit the warming visible rays of the sun, prevent the resulting warm air from leaving, and hence maintain a warmer environment inside than outside the structure. Part of the sunlight is absorbed by Earth and held as thermal energy. This is heat is re-radiated in the form of longer wavelength infrared radiation. While the dominant gases of the atmosphere (nitrogen (N2) and oxygen (O2) compose 98% of ozone) are transparent to the infrared, the greenhouse gasses, water vapor (H2O), CO2, and methane (CH4), absorb some of the infrared radiation. They collect this heat energy and hold it in the atmosphere, delaying its passage back out of the atmosphere. The atmospheric gases and a greenhouse work in quite different ways, but the resulting effect, has been dubbed “greenhouse gas effect”. Molecules are not, however, rigid ball and stick figures as our chemistry class models may lead us to believe. Molecules are in motion; continuously bouncing around and jiggling and vibrating. Consider first a diatomic nitrogen (N2) or oxygen (O2) molecule. A pair of balls attached by a spring is a good model of such a molecule. Pull the balls apart and release them; they alternately move closer together and further apart. This vibrational mode is extremely symmetric, however; the center of mass of the system always remains at the point midway between the two balls/atoms. Electromagnetic "disturbances" (waves) do not tend to interact with, or transfer energy to, such diatomic molecules (such as N2 or O2). Molecules with three or more atoms, however, are a different story. The figure shows three different vibrational modes of carbon dioxide (CO2) molecule. The first mode, (a), is symmetric; it is comparable to the vibrational mode of diatomic molecules. The center of mass, and of charge, of the system is not displaced during vibration. However, such is not the case for the other two modes, (b) and (c). In the latter two cases, the "center of charge" moves as the molecule vibrates, creating a "dipole moment". As explained for the case of water above, electrons are not shared equally between the atoms in the CO2 molecule, so the molecule is not electrically neutral in all places. As the molecule oscillates, the center of charge moves; from side to side in case (b), and up and down in case (c). A passing electromagnetic "disturbance" (wave, or IR photon) can "excite" such a molecule, causing it to vibrate and transferring energy from the photon to the molecule. This is the mechanism by which greenhouse gases absorb energy from infrared photons.

Earth’s atmosphere is 90% opaque to long wave infrared radiation, a very small portion of Earth’s atmospheric gases generate the effect. We can see how powerful the effect is with only 2% of the total atmosphere (98% N2 and O2). Though CO2 has an important role, water vapor is the most dominant greenhouse gas in the atmosphere. Water vapor generates more greenhouse effect than any other single gas. Water, in gaseous (water vapor) and liquid form (droplets in clouds), generates somewhere between 66% and 85% of the effect. Water vapor and droplets intercept most of the long wave infrared heat. Evapotranspiration carries heat upward from the Earth’s surface to the atmosphere. When the water vapor cools and condenses into clouds the heat is released into the atmosphere heating the atmosphere further.

The second largest greenhouse gas contributor is carbon dioxide. Natural sources of CO2 include volcanic outgassing, combustion of organic matter, respiration of living aerobic organisms, and fermentation by microbes. About three-fourths of anthropogenic CO2 emissions result from burning fossil fuels for heating, power generation, and transportation. Most of the rest of human CO2 emissions are a result of land use changes, especially deforestation. Trees and other plants absorb CO2 during photosynthesis; the carbon is converted into plant materials and into soil as the plants die and decay or shed leaves.