Greenhouse Gas
Greenhouse gases are components of the atmosphere that contribute to the greenhouse effect. Without the greenhouse effect the Earth would be uninhabitable; in its absence, the mean temperature of the earth would be about −19 °C (−2 °F, 254 K) rather than the present mean temperature of about 15 °C (59 °F, 288 K). Greenhouse gases include in the order of relative abundance water vapour, carbon dioxide, methane, nitrous oxide, and ozone. Greenhouse gases come from natural sources and human activity.
Greenhouse gases naturally blanket the Earth and keep it about 33 degrees Celsius warmer than it would be without these gases in the atmosphere. This is called the Greenhouse Effect. Over the past century, the Earth has increased in temperature by about .5 degrees Celsius and many scientists believe this is because of an increase in concentration of the main greenhouse gases: carbon dioxide, methane, nitrous oxide, and fluorocarbons. People are now calling this climate change over the past century the beginning of Global Warming. Fears are that if people keep producing such gases at increasing rates, the results will be negative in nature, such as more severe floods and droughts, increasing prevalence of insects, sea levels rising, and Earth’s precipitation may be redistributed. These changes to the environment will most likely cause negative effects on society, such as lower health and decreasing economic development. However, some scientists argue that the global warming we are experiencing now is a natural phenomenon, and is part of Earth’s natural cycle. Presently, nobody can prove if either theory is correct, but one thing is certain; the world has been emitting greenhouse gases at extremely high rates and has shown only small signs of reducing emissions until the last few years. After the 1997 Kyoto Protocol, the world has finally taken the first step in reducing emissions.
The Greenhouse Effect:
The “greenhouse effect” is the heating of the Earth due to the presence of greenhouse gases. It is named this way because of a similar effect produced by the glass panes of a greenhouse. Shorter-wavelength solar radiation from the sun passes through Earth’s atmosphere, then is absorbed by the surface of the Earth, causing it to warm. Part of the absorbed energy is then reradiated back to the atmosphere as long wave infared radiation. Little of this long wave radiation escapes back into space; the radiation cannot pass through the greenhouse gases in the atmosphere. The greenhouse gases selectively transmit the infared waves, trapping some and allowing some to pass through into space. The greenhouse gases absorb these waves and reemits the waves downward, causing the lower atmosphere to warm.(www.eb.com:180)
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When sunlight reaches the surface of the Earth, some of it is absorbed and warms the surface. Because the Earth’s surface is much cooler than the sun, it radiates energy at much longer wavelengths than the sun does, peaking in the infrared at about 10µm. The atmosphere absorbs these longer wavelengths more effectively than it does the shorter wavelengths from the sun. The absorption of this longwave radiant energy warms the atmosphere; the atmosphere also is warmed by transfer of sensible and latent heat from the surface. Greenhouse gases also emit longwave radiation both upward to space and downward to the surface. The downward part of this longwave radiation emitted by the atmosphere is the “greenhouse effect.” The term is a misnomer, as this process is not the mechanism that warms greenhouses.
The major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect on Earth (not including clouds); carbon dioxide, which causes 9–26%; methane, which causes 4–9%, and ozone, which causes 3–7%. It is not possible to state that a certain gas causes a certain percentage of the greenhouse effect, because the influences of the various gases are not additive. (The higher ends of the ranges quoted are for the gas alone; the lower ends, for the gas counting overlaps.)Other greenhouse gases include, but are not limited to, nitrous oxide, sulfur hexafluoride, hydrofluorocarbons, perfluorocarbons and chlorofluorocarbons .
The major atmospheric constituents (nitrogen, N2 and oxygen, O2) are not greenhouse gases. This is because homonuclear diatomic molecules such as N2 and O2 neither absorb nor emit infrared radiation, as there is no net change in the dipole moment of these molecules when they vibrate. Molecular vibrations occur at energies that are of the same magnitude as the energy of the photons on infrared light. Heteronuclear diatomics such as CO or HCl absorb IR; however, these molecules are short-lived in the atmosphere owing to their reactivity and solubility. As a consequence they do not contribute significantly to the greenhouse effect.
Late 19th century scientists experimentally discovered that N2 and O2 did not absorb infrared radiation (called, at that time, “dark radiation”) and that CO2 and many other gases did absorb such radiation. It was recognized in the early 20th century that the known major greenhouse gases in the atmosphere caused the earth’s temperature to be higher than it would have been without the greenhouse gases.
Anthropogenic greenhouse gases:
The concentrations of several greenhouse gases have increased over time.Human activity may increase the greenhouse effect through release of carbon dioxide, but human influences on other greenhouse gases can also be important.Some of the main sources of greenhouse gases due to human activity include:
*burning of fossil fuels and deforestation leading to higher carbon dioxide concentrations;
*livestock and paddy rice farming, land use and wetland changes, pipeline losses, and covered vented landfill emissions leading to higher methane atmospheric concentrations. Many of the newer style fully vented septic systems that enhance and target the fermentation process also are major sources of atmospheric methane;
*use of chlorofluorocarbons (CFCs) in refrigeration systems, and use of CFCs and halons in fire suppression systems and manufacturing processes.
*agricultural activities, including the use of fertilizers, that lead to higher nitrous oxide concentrations.
The seven sources of CO2 from fossil fuel combustion are (with percentage contributions for 2000–2004):
1.Solid fuels (e.g. coal): 35%
2.Liquid fuels (e.g. gasoline): 36%
3.Gaseous fuels (e.g. natural gas): 20%
4.Flaring gas industrially and at wells: <1%
5.Cement production: 3%
6.Non-fuel hydrocarbons: <1%
7.The "international bunkers" of shipping and air transport not included in national inventories: 4%
The U.S. EPA ranks the major greenhouse gas contributing end-user sectors in the following order: industrial, transportation, residential, commercial and agricultural. Major sources of an individual's GHG include home heating and cooling, electricity consumption, and transportation. Corresponding conservation measures are improving home building insulation, compact fluorescent lamps and choosing high miles per gallon vehicles.
Carbon dioxide, methane, nitrous oxide and three groups of fluorinated gases (sulfur hexafluoride, HFCs, and PFCs) are the major greenhouse gases and the subject of the Kyoto Protocol, which entered into force in 2005.
CFCs, although greenhouse gases, are regulated by the Montreal Protocol, which was motivated by CFCs' contribution to ozone depletion rather than by their contribution to global warming. Note that ozone depletion has only a minor role in greenhouse warming though the two processes often are confused in the popular media.
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The role of water vapor:
Water vapor is a naturally occurring greenhouse gas and accounts for the largest percentage of the greenhouse effect, between 36% and 66% . Water vapor concentrations fluctuate regionally, but human activity does not directly affect water vapor concentrations except at local scales (for example, near irrigated fields).
Current state-of-the-art climate models include fully interactive clouds. They show that an increase in atmospheric temperature caused by the greenhouse effect due to anthropogenic gases will in turn lead to an increase in the water vapor content of the troposphere, with approximately constant relative humidity. The increased water vapor in turn leads to an increase in the greenhouse effect and thus a further increase in temperature; the increase in temperature leads to still further increase in atmospheric water vapor; and the feedback cycle continues until equilibrium is reached. Thus water vapor acts as a positive feedback to the forcing provided by human-released greenhouse gases such as CO2.
Total Related effects:
Carbon monoxide has an indirect radiative effect by elevating concentrations of methane and tropospheric ozone through scavenging of atmospheric constituents (e.g., the hydroxyl radical, OH) that would otherwise destroy them. Carbon monoxide is created when carbon-containing fuels are burned incompletely. Through natural processes in the atmosphere, it is eventually oxidized to carbon dioxide. Carbon monoxide has an atmospheric lifetime of only a few months and as a consequence is spatially more variable than longer-lived gases.
Another potentially important indirect effect comes from methane, which in addition to its direct radiative impact also contributes to ozone formation. Shindell et al (2005) argue that the contribution to climate change from methane is at least double previous estimates as a result of this effect.
Resources:
http://en.wikipedia.org/wiki/Greenhouse_gases
http://www.umich.edu/~gs265/society/greenhouse.htm