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Greenhouse gases add to global warming.
Greenhouse gases are gaseous components of the atmosphere. Greenhouse gases contribute to the "greenhouse effect". Although uncertainty exists about exactly how earth's climate responds to greenhouse gases, global temperatures are rising. Some greenhouse gases occur naturally in the atmosphere. Other say greenhouse gases result from human activities. Naturally occurring greenhouse gases include water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Certain human activities, however, add to the levels of most of these naturally occurring Greenhouse gases.
Greenhouse gases and the "greenhouse effect".
When sunlight reaches the Earth's surface, some is absorbed and warms the earth. Because the earth is much cooler than the sun, it radiates energy at much longer wavelengths than the sun (see Black body radiation and Wien's displacement law); some of these longer wavelengths are absorbed by greenhouse gases in the atmosphere before they are lost to space. 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 in fact a misnomer, as this process is not the primary mechanism that warms greenhouses.
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 9-26%; methane, which causes 4-9%, and Ozone, which causes 3-7%. Note that it is not really possible to assert 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 (see IPCC list of greenhouse gases).
The major atmospheric constituents (N2 and O2) are not greenhouse gases, because homonuclear diatomic molecules (e.g. N2, O2, H2) neither absorb nor emit Infrared radiation as there is no net change in the dipole moment of these molecules.
Anthropogenic greenhouse gases.
The concentrations of several greenhouse gases have increased over time. human activity raises levels of greenhouse gases primarily by releasing carbon dioxide, but human influences on other gases, e.g., methane, are not negligible. Some of the main sources of greenhouse gases due to human activity include:
According to the global warming trend, 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. Note that ozone depletion has only a minor role in greenhouse warming, though the two processes often are confused in the popular media.
Greenhouse gases and the role of water vapor.
Water vapor is a natural greenhouse gas and accounts for the largest percentage of the greenhouse effect. Water vapor concentrations fluctuate regionally, but human activity does not directly affect water vapor concentrations except at very local scales.
In climate models 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 (but has never, so far, acted on Earth as part of a runaway feedback). Changes in the water vapor may also have indirect effects via cloud formation.
Water vapor is a definite part of the greenhouse gas equation even though not under direct human control: Intergovernmental Panel on Climate Change (IPCC) TAR chapter lead author Michael Mann considers citing "the role of water vapor as a greenhouse gas" to be "extremely misleading" as water vapor can not be controlled by humans; see also and. The IPCC discusses the water vapor feedback in more detail.
Increase of greenhouse gases.
Based on measurements from Antarctic ice cores, it is widely accepted that just before industrial emissions began, atmospheric CO2 levels were about 280 µL/L (note the units µL/L are identical to parts per million by volume). From the same ice cores it appears that CO2 concentrations have stayed between 260 and 280 µL/L during the preceding 10,000 years. Some studies, using evidence from stomata of fossilized leaves, have found greater variability with CO2 levels above 300 µL/L during the period 7-10 kyr ago, though others have argued that these findings more likely reflect calibration/contamination problems rather than actual CO2 variability.
Since the beginning of the Industrial Revolution, the concentrations of many of the greenhouse gases have increased. Most of the increase in carbon dioxide occurred after 1945. Those with the largest radiative forcing are:
(Source: IPCC radiative forcing report 1994 updated (to 1998) by IPCC TAR table 6.1).
Greenhouse gases: 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. It is not easy to know with precision how long, because the atmosphere is a very complex system. However, 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 ones. 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. One may encounter claims 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 mixing into the ocean, being transformed to oxygen by photosynthesis, etc. 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.
Related effects of greenhouse gases.
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 burned incompletely. 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 gases 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.
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