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Greenhouse gases are components of the , Carbon Dioxide , Methane , Nitrous Oxide , and Ozone . THE "GREENHOUSE EFFECT" s created by greenhouse gases in the atmosphere and their effect on both Solar Radiation and upgoing thermal radiation]] See Also: Greenhouse effect 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 does the sun. 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.)12 Other greenhouse gases include, but are not limited to, Nitrous Oxide , Sulfur Hexafluoride , Hydrofluorocarbon s, Perfluorocarbon s and Chlorofluorocarbons (see IPCC List Of Greenhouse Gases ). 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 for a range of greenhouse gas stabilization scenarios (the coloured bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4, 2007 .]] The concentrations of several greenhouse gases have increased over time.3 Human activity increases the greenhouse effect primarily through release of carbon dioxide, but human influences on other greenhouse gases can also be important.4 Some of the main sources of greenhouse gases due to human activity include:
The seven sources of CO2 from fossil fuel combustion are (with percentage contributions for 2000-2004): # Solid fuels (e.g. Coal ): 35% # Liquid fuels (e.g. Gasoline ): 36% # Gaseous fuels (e.g. Natural Gas ): 20% # Flaring gas industrially and at wells: <1% # Cement production: 3% # Non-fuel hydrocarbons: <1% # The "international Bunkers " of shipping and air transport not included in national inventories: 4% Greenhouse gas emissions from industry, , cellular shades, Compact Fluorescent Lamp s and choosing high Miles Per Gallon vehicles. Carbon Dioxide , Methane , Nitrous Oxide and three groups of Fluorinated gases (''' Sulfur Hexafluoride , HFC s, and PFCs ''') are the major greenhouse gases and the subject of the Kyoto Protocol , which entered into force in 2005.6 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. 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% {Link without Title} . 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 . 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. INCREASE OF GREENHOUSE GASES Measurements From Antarctic Ice Cores show that just before industrial emissions started, atmospheric CO2 levels were about 280 parts per million by volume (ppm; the units µL/L are occasionally used and are identical to parts per million by volume). From the same ice cores it appears that CO2 concentrations stayed between 260 and 280 ppm during the preceding 10,000 years. Studies using evidence from stomata of fossilized leaves suggest greater variability, with CO2 levels above 300 ppm during the period 7,000-10,000 years ago,7 }} though others have argued that these findings more likely reflect calibration/contamination problems rather than actual CO2 variability.8 }} 910 Since the beginning of the Industrial Revolution , the concentrations of many of the greenhouse gases have increased. The concentration of CO2 has increased by about 100 ppm (i.e., from 280 ppm to 380 ppm). The first 50 ppm increase took place in about 200 years, from the start of the Industrial Revolution to around 1973; the next 50 ppm increase took place in about 33 years, from 1973 to 2006. . Many observations are available on line in a variety of Atmospheric Chemistry Observational Databases . The greenhouse gases with the largest radiative forcing are: emissions 1751 – 2000 .]] (Source: IPCC radiative forcing report 1994 updated (to 1998) by IPCC TAR table 6.1 {Link without Title} {Link without Title} ). Recent rates of change and emission The sharp acceleration in CO2 emissions since 2000 of >3% y-1 (>2 ppm y-1) from 1.1% y-1 during the 90's is attributable to the lapse of formerly declining trends in '' 104(24): 10288-93.'' In comparison, methane has not increased appreciably, and N2O by 0.25% y-1 {Link without Title} . The United States emitted 16.3% more GHG in 2005 than it did in 1990.Emissions inventory from the EPA, cited in ''Science News'', vol. 171, p. 318 According to a preliminary estimate by the Netherlands Environmental Assessment Agency, the largest national producer of CO2 emissions since 2006 has been China with an estimated annual production of about 6200 megatonnes. It is followed by the United States with about 5,800 megatonnes. Relative to 2005, China's fossil CO2 emissions of China increased in 2006 by 8.7%, while in the USA, comparable CO2 emissions decreased in 2006 by 1.4%. The agency notes that its estimates do not include some CO2 sources of uncertain magnitude11. REMOVAL FROM THE ATMOSPHERE AND GLOBAL WARMING POTENTIAL Aside from water vapor near the surface, which has a residence time of days, most greenhouse gases take a very long time to leave the atmosphere. Although it is not easy to know with precision how long, there are estimates of the duration of stay, i.e., the time which is necessary so that the gas disappears from the atmosphere, for the principal greenhouse gases. For the first five years of this century, 48% of total anthropogenic CO2 emissions remained in the atmosphere, a figure that is increasing and diagnostic of weakening Carbon Sink s. Greenhouse gases can be removed from the atmosphere by various processes:
Two scales can be used to describe the effect of different gases in the atmosphere. The first, the atmospheric lifetime, describes how long it takes to restore the system to equilibrium following a small increase in the concentration of the gas in the atmosphere. Individual molecules may interchange with other reservoirs such as soil, the oceans, and biological systems, but the Mean Lifetime refers to the decaying away of the excess. It is sometimes erroneously claimed that the atmospheric lifetime of CO2 is only a few years because that is the average time for any CO2 molecule to stay in the atmosphere before being removed by mixing into the ocean, uptake by photosynthesis, or other processes. This ignores the balancing fluxes of CO2 into the atmosphere from the other reservoirs. It is the net concentration changes of the various greenhouse gases by ''all sources and sinks'' that determines atmospheric lifetime, not just the removal processes. The second scale is Global Warming Potential (GWP). The GWP depends on both the efficiency of the molecule as a greenhouse gas and its atmospheric lifetime. GWP is measured relative to the same mass of CO2 and evaluated for a specific timescale. Thus, if a molecule has a high GWP on a short time scale (say 20 years) but has only a short lifetime, it will have a large GWP on a 20 year scale but a small one on a 100 year scale. Conversely, if a molecule has a longer atmospheric lifetime than CO2 its GWP will increase with time. Examples of the atmospheric lifetime and GWP for several greenhouse gases include:
Source : IPCC , table 6.7 . RELATED EFFECTS 2000 global Carbon Monoxide ]] 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)Shindell, Drew T.; Faluvegi, Greg; Bell, Nadine; Schmidt, Gavin A. "An emissions-based view of climate forcing by methane and tropospheric ozone", ''Geophysical Research Letters'', Vol. 32, No. 4 argue that the contribution to climate change from methane is at least double previous estimates as a result of this effect.[http://www.nasa.gov/vision/earth/lookingatearth/methane.html Methane's Impacts on Climate Change May Be Twice Previous Estimates SEE ALSO
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