Encyclopedia
Greenhouse gases are gaseous components of the
atmosphere that contribute to the "
greenhouse effect". Although uncertainty exists about exactly how earth's climate responds to these gases, global temperatures are rising. Some greenhouse gases occur naturally in the atmosphere, while others result from human activities. Naturally occuring greenhouse gases include water vapour, carbon dioxide, methane, nitrous oxide, and ozone. Certain human activities, however, add to the levels of most of these naturally occurring gases.
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 ; 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 . 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 ;
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.
Other greenhouse gases include, but are not limited to,
nitrous oxide,
sulfur hexafluoride,
hydrofluorocarbons, perfluorocarbons and
chlorofluorocarbons .
The major atmospheric constituents are not greenhouse gases, because homonuclear diatomic
molecules 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:
- 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;
- use of CFCs in refrigeration systems, and use of CFCs and halons in fire suppression systems and manufacturing processes.
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.
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 CO
2 . Changes in the water vapor may also have indirect effects via cloud formation.
- Most scientists agree that the overall effect of the direct and indirect feedbacks caused by increased water vapor content of the atmosphere significantly enhances the initial warming that caused the increase - that is, it is a strong positive feedback..
Water vapor is a definite part of the
greenhouse gas equation even though not under direct human control:
Intergovernmental Panel on Climate Change 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 CO
2 levels were about 280 µL/L . From the same ice cores it appears that CO
2 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 CO
2 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 CO
2 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:
.
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:
- as a consequence of a physical change .
- as a consequence of chemical reactions within the atmosphere. This is the case for methane. It is oxidized by reaction with naturally occurring hydroxyl radical, OH· and degraded to CO2 and water vapor at the end of a chain of reactions . 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. An example is the mixing of atmospheric gases into the oceans at the boundary layer.
- as a consequence of a chemical change at the interface between the atmosphere and the other compartments of the planet. This is the case for CO2, which is reduced by photosynthesis of plants, and which, after dissolving in the oceans, reacts to form carbonic acid and bicarbonate and carbonate ions .
- as a consequence of a photochemical change
[i]
...
. Halocarbons are dissociated by
UV light releasing Cl
· and F
· as
free radicals in the
stratosphere with harmful effects on
ozone .
- as a consequence of dissociative ionization caused by high energy cosmic rays or lightning discharges, which break molecular bonds. For example, lightning forms N atoms from N2 which then react with O2 to form NO2.
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 CO
2 is only a few years because that is the average time for any CO
2 molecule to stay in the atmosphere before mixing into the ocean, being transformed to oxygen by photosynthesis, etc. This ignores the balancing fluxes of CO
2 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 . 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
2 and evaluated for a specific timescale. Thus, if a molecule has a high GWP on a short time scale 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 CO
2 its GWP will increase with time.
Examples of the atmospheric lifetime and GWP for several greenhouse gases include:
- CO2 has a variable atmospheric lifetime . Recent work indicates that recovery from a large input of atmospheric CO2 from burning fossil fuels will result in an effective lifetime of tens of thousands of years. 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 62 over 20 years, 23 over 100 years and 7 over 500 years. The decrease in GWP associated with longer times is associated with the fact that the methane is degraded to water and CO2 by chemical reactions in the atmosphere.
- Nitrous oxide has an atmospheric lifetime of 120 years and a GWP of 296 over 100 years.
- CFC-12 has an atmospheric lifetime of 100 years and a GWP of 10600.
- HCFC-22 has an atmospheric lifetime of 12.1 years and a GWP of 1700.
- Tetrafluoromethane has an atmospheric lifetime of 50,000 years and a GWP of 5700.
- Sulfur hexafluoride has an atmospheric lifetime of 3,200 years and a GWP of 22000.
Source :
IPCC, .
Related effects
Carbon monoxide has an indirect radiative forcing effect by elevating concentrations of
methane and
tropospheric ozone through chemical reactions with other atmospheric constituents 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 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.
See also
Nations Unies...
Intergovernmental Panel of Climate ChangeReferences
External links
Carbon dioxide emissions
- light-duty vehicles : the EU's aim is to reach - by 2010 at the latest -an average CO2 emission figure of 120 g/km for all new passenger cars marketed in the Union.
- NOAA CO2 data
- Includes links to lots of useful Carbon Dioxide statistics
Methane emissions
