Greenhouse gases

1. Greenhouse gases • Greenhouse gases are the gases present in the atmosphere which reduce the loss of heat into space and therefore contribute to global temperatures. • Greenhouse gases are essential to maintaining the temperature of the earth; without them the planet would be so cold as to be uninhabitable. • Likewise, an excess of greenhouse gases can raise the temperature of the planet to unlivable levels. • The term greenhouse gas is applied to, in order of relative abundance: water vapour, carbon dioxide, methane, nitrous oxide, ozone and CFCs. • Greenhouse gases are produced by many natural and industrial processes.

2. Carbon dioxide Carbon dioxide is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state. It is currently at a globally averaged concentration of approximately 383 ppm by volume in the Earth's atmosphere, although this varies both by location and time. Carbon dioxide is an important greenhouse gas because it transmits visible light but absorbs strongly in the infrared. Carbon dioxide is produced by all animals, plants, fungi and microorganisms during respiration and is used by plants during photosynthesis. Carbon dioxide is generated as a byproduct of the combustion of fossil fuels or vegetable matter, among other chemical processes. And also as a byproduct in lime production. Inorganic carbon dioxide is output by volcanoes and other geothermal processes such as hot springs.

3. Methane • Methane is the principal component of natural gas. • Methane is a relatively potent greenhouse gas with a high global warming potential. • Methane in the atmosphere is eventually oxidized, producing carbon dioxide and water. As a result, methane in the atmosphere has a half life of seven years (every seven years, the amount of methane halves). • The abundance of methane in the Earth's atmosphere in 1998 was 1745 parts per billion, up from 700 ppb in 1750. • In addition, there is a large, but unknown, amount of methane in methane clathrates in the ocean floors. Global warming could release this methane, which could cause a further sharp rise in global temperatures. Such releases of methane may have been a major factor in previous major extinction events. • The Earth's crust also contains huge amounts of methane. Large amounts of methane are produced in swamps. Other sources include mud volcanoes which are connected with deep geological faults.

4. Nitrous Oxide • Nitrous oxide (also known as laughing gas) is a chemical compound with the chemical formulaN2O. • It is used in motor racing as an oxidizer to increase the power output of engines. • Despite its relatively small concentration in the atmosphere, nitrous oxide is the fourth largest greenhouse gas contributor to overall global warming, behind carbon dioxide, methane and water vapour. • Nitrous oxide is emitted by bacteria in soils and oceans, and thus has been a part of Earth's atmosphere for aeons. Agriculture is the main source of human-produced nitrous oxide: cultivating soil, the use of nitrogen fertilizers, and animal waste handling can all stimulate naturally occurring bacteria to produce more nitrous oxide. • The livestock sector (primarily cows, chickens, and pigs) produces 65% of human-related nitrous oxide. • Industrial sources make up only about 20% of all anthropogenic sources, and include the production of nylon and nitric acid, and the burning of fossil fuel in internal combustion engines.

5. Ozone • Ozone (O3) is a triatomic molecule, consisting of three oxygenatoms. • Ground-level ozone is an air pollutant with harmful effects on the respiratory systems of animals. Ozone in the upper atmosphere filters potentially damaging ultraviolet light from reaching the Earth's surface. • It is not emitted directly by car engines or by industrial operations. It is formed by the reaction of sunlight on air containing hydrocarbons and nitrogen oxides that react to form ozone directly at the source of the pollution or many kilometers down wind. • Although ozone was present at ground level before the industrial revolution, peak concentrations are now far higher than the pre-industrial levels and even background concentrations well away from sources of pollution are substantially higher. This increase in ozone is of further concern as ozone present in the upper troposphere acts as a greenhouse gas.

6. Chlorofluorocarbons (CFCs) • Chlorofluorocarbons (CFC) are compounds containing chlorine, fluorine and carbon only, that is they contain no hydrogen. • They were formerly used widely in industry, for example as refrigerants, propellants, and cleaning solvents. Their use has been regularly prohibited by the Montreal Protocol, because of effects on the ozone layer. • They are also powerful greenhouse gases. • CFC's have half-lives between 50-100 years, so their presence in the atmosphere and reactivity with ozone is long lived. One CFC molecule typically degrades around 10,000 ozone molecules before its removal, but this number can sometimes be in the millions. • Hydrochlorofluorocarbons (HCFCs) are of a class of haloalkanes where not all hydrogen has been replaced by chlorine or fluorine. They are used primarily as chlorofluorocarbon (CFC) substitutes, as the ozone depleting effects are only about 10% of the CFCs.

7. 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). Water vapor is special in the sense that the amount of water vapor depends directly on the temperature. Vapor pressure is the pressure of a vapor in equilibrium with its liquid. Water has a tendency to evaporate to a gaseous form, and water vapor has a tendency to condense back into liquid form. At any given temperature, for a particular substance, there is a pressure at which the gas of that substance is in dynamic equilibrium with its liquid or solid forms. This is the vapor pressure of that substance at that temperature. The air cannot hold more water vapor than is allowed by the vapor pressure.

8. Clausius-Clapeyron Relation The Clausius-Clapeyron relation gives a relationship between the saturation vapor pressure and the temperature. This equation establishes that warmer air has the potential to hold more water vapor per unit volume. As a simple example, the air at 30 oC can hold about 3.5 times more water vapor than air at 10 oC. Current state-of-the-art climate models predict that increasing water vapor concentrations in warmer air will amplify the greenhouse effect created by anthropogenic greenhouse gases. Thus water vapor acts as a strong positive feedback to the forcing provided by greenhouse gases such as CO2.

9. Radiative Forcing of Greenhouse Gases Most greenhouse gases have both natural and anthropogenic sources. During the pre-industrial holocene, levels of the gases were roughly constant. Since the industrial revolution, levels of some gases have increased due to human actions. Gas 1750 Level Current Level   Increase   Forcing (Wm2) Carbon dioxide 280 ppm 384ppm 104 ppm 1.53 Methane 700 ppb 1,745 ppb 1,045 ppb 0.48 Nitrous oxide 270 ppb 314 ppb 44 ppb 0.15 CFC-12 0 533 ppt 533 ppt 0.17 As you can see, even though the concentration of CFCs is low, its effect is very high, so we need a different measure for the effect of these gases.

10. Global Warming Potential • Global warming potential (GWP) is a measure of how much a given mass of greenhouse gas is estimated to contribute to global warming. It is a relative scale which compares the gas in question to that of the same mass of carbon dioxide. • The GWP depends on the following factors: • the absorption of infrared radiation by a given species • the spectral location of its absorbing wavelengths • the atmospheric lifetime of the species