Greenhouse gases

Carbon dioxide, water vapor, methane, nitrous oxides, chlorofluorocarbons, vapor oxides and the other gases that trap heat within the Earth’s atmosphere. The major natural greenhouse gases are water vapor, which causes about 36-70% of the greenhouse effect on Earth (not including clouds); carbon dioxide, which causes between 9-26%; and ozone, which causes between 3-7% (note that it is not really possible to assert that such-and-such a gas causes a certain percentage of the greenhouse effect, because the influences of the various gases are not additive. Other greenhouse gases include, but are not limited to: methane, nitrous oxide, sulfur hexafluoride, and chlorofluorocarbons. Human activity raises levels of greenhouse gases primarily by releasing carbon dioxide, but other gases, e.g. methane, are not negligible. The concentrations of several greenhouse gases have increased over time due to human activities, such as: 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 as is the use of CFCs in refrigeration systems. According to the global warming hypothesis, greenhouse gases from industry and agriculture have played a major role in the recently observed global warming. Carbon dioxide, methane, nitrous oxide and three groups of fluorinated gases are the subject of the Kyoto Protocol, which entered into force in 2005. Methane, nitrous oxide and ozone-depleting gases are also taken into account in the international agreements, but not ozone. The greenhouse gases, once in the atmosphere, do not remain there forever. They can be removed from the atmosphere: as a consequence of a physical change (condensation and precipitation remove water vapor from the atmosphere) as a consequence of chemical reactions within the atmosphere. This is the case for methane. It is oxidized by reaction with the naturally occurring hydroxyl radical, OH· and degraded to CO and water vapor at the end of a chain of reactions (the contribution of the CO from the oxidation of methane is not included in the methane GWP). This also includes solution and solid phase chemistry occurring in atmospheric aerosols, as a consequence of a physical interchange at the interface between the atmosphere and the other compartments of the planet. 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 within other reservoirs such as soil, the oceans, and biological systems, but the mean lifetime refers to the decaying away of the excess. The other 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 CO and evaluated for a specific timescale. Carbon monoxide has an indirect radiative forcing effect by elevating concentrations of methane and tropospheric ozone through chemical reactions with other atmospheric constituents (e.g., the hydroxyl radical, OH) that would otherwise destroy them. Carbon monoxide is created when carbon-containing fuels are incompletely burned. Through natural processes in the atmosphere, it is eventually oxidized to carbon dioxide. Carbon monoxide concentrations are both short-lived in the atmosphere and spatially variable. 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. One of the related effects of global warming is that as the level of carbon dioxide in the atmosphere increases, so does the acidity of the oceans. One of the more alarming potential correlations with Greenhouse Gasses and Global Warming is the notion of Global Dimming which seems to have masked the effect of Global Warming due to the Earth getting cooler through Global Dimming.