Greenhouse+gas


 * Greenhouse gases (GHG)** are gases in the atmosphere that absorb and give off radiation in a process know as the greenhouse effect, because the gases create conditions similar to a glass green house where light and heat can come in but can not leave. If there were no greenhouse gases present the Earth's temperature drop significatly to about 33 °C (59 °F).The primary greenhouse gases in the Earth's atmosphere are water vapor, c arbon dioxide, methane , nitrous oxide, and ozone . In the universe the atmospheres of Venus, Mars and Titan, contain greenhouse gases. Since the beginning of the Industrial Revolution there has been a significant increase in the burning of fossil fuels, which has contributed to the increase in carbon dioxide in the atmosphere from 280 ppm (particles per million) to 390 ppm, despite the uptake of a large portion of the emissions through various natural "sinks" involved in the carbon cycle. Since the rise of the carbon in the atmosphere many scientists have begun campaigns to spread awareness of Global Warming and reduce carbon emissions. Anthropogenic carbon dioxide (CO 2 ) emissions (i.e., emissions produced by human activities) come from combustion of carbonaceous fuels, principally wood, coal, oil and natural gas.



(%) ||
 * ~ Gas ||~ Formula ||~ Contribution
 * Water vapor || H 2 O || 36 – 72 % ||
 * Carbon dioxide || CO 2 || 9 – 26 % ||
 * Methane || CH 4 || 4 – 9 % ||
 * Ozone || O 3 || 3 – 7 % ||

Atmospheric lifetime
Aside from water vapor, which has a residence time of about nine days, major greenhouse gases are well-mixed, and take many years to leave the atmosphere. Although it is not easy to know with precision how long it takes greenhouse gases to leave the atmosphere, there are estimates for the principal greenhouse gases. Jacob (1999) defines the lifetime of an atmospheric species X in a one-box model as the average time that a molecule of X remains in the box. Mathematically can be defined as the ratio of the mass  (in kg) of X in the box to its removal rate, which is the sum of the flow of X out of the box, chemical loss of X , and deposition of X  (all in kg/sec): .If one stopped pouring any of this gas into the box, then after a time , its concentration would be about halved. The atmospheric lifetime of a species therefore measures the time required to restore equilibrium following a sudden increase or decrease in its concentration in the atmosphere. Individual atoms or molecules may be lost or deposited to sinks such as the soil, the oceans and other waters, or vegetation and other biological systems, reducing the excess to background concentrations. The average time taken to achieve this is the mean lifetime. Carbon dioxide has a variable atmospheric lifetime, and cannot be specified precisely. The atmospheric lifetime of CO 2 is estimated of the order of 30–95 years. This figure accounts for CO 2 molecules being removed from the atmosphere by mixing into the ocean, photosynthesis, and a few other processes. However, this excludes the balancing fluxes of CO 2 into the atmosphere from the geological reservoirs, which have slower characteristic rates. While more than half of the CO 2 emitted is currently removed from the atmosphere within a century, some fraction (about 20%) of emitted CO  2 remains in the atmosphere for many thousands of years.

Global warming potential
The global warming potential (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 CO 2 and evaluated for a specific timescale. Thus, if a gas has a high radiative forcing but also 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 CO 2 its GWP will increase with the timescale considered. Carbon dioxide is defined to have a GWP of 1 over all time periods. Methane has an atmospheric lifetime of 12 ± 3 years and a GWP of 72 over 20 years, 25 over 100 years and 7.6 over 500 years. The decrease in GWP at longer times is because methane is degraded to water and CO 2 through chemical reactions in the atmosphere.

Greenhouse gases
Atmospheric absorption and scattering at different [|electromagnetic wavelengths]. The largest absorption band of carbon dioxide is in the infrared. Greenhouse gases are those that can absorb and emit infrared radiation. In order, the most abundant greenhouse gases in Earth's atmosphere are: Atmospheric concentrations of greenhouse gases are determined by the balance between sources (emissions of the gas from human activities and natural systems) and sinks (the removal of the gas from the atmosphere by conversion to a different chemical compound). The proportion of an emission remaining in the atmosphere after a specified time is the " Airborne fraction " (AF). More precisely, the annual AF is the ratio of the atmospheric increase in a given year to that year’s total emissions. For CO 2   the AF over the last 50 years (1956–2006) has been increasing at 0.25 ± 0.21%/year.
 * water vapor (H 2 O)
 * carbon dioxide (CO 2 )
 * methane (CH 4 )
 * nitrous oxide (N 2 O)
 * ozone (O 3 )

Non-greenhouse gases
Although contributing to many other physical and chemical reactions, the major atmospheric constituents, nitrogen (N 2 ), oxygen (O 2 ), and argon (Ar), are not greenhouse gases. This is because molecules containing two atoms of the same element such as N 2 and O 2 and monatomic molecules such as Argon (Ar) have no net change in their dipole moment when they vibrate and hence are almost totally unaffected by infrared light. Although molecules containing two atoms of different elements such as carbon monoxide (CO) or hydrogen chloride (HCl) absorb IR, these molecules are short-lived in the atmosphere owing to their reactivity and solubility. Because they do not contribute significantly to the greenhouse effect, they are usually omitted when discussing greenhouse gases.




 * Methane comes from landfills, coal mines, oil and gas operations, agriculture (9% total emission)




 * Carbon Dioxide Emissions by region.
 * The US produces about 25% of Carbon Dioxide emissions from burning fossil fuels