| Revision as of 17:45, 7 August 2006 editVsmith (talk | contribs)Administrators272,635 editsm Reverted edits by 24.166.142.27 (talk) to last version by William M. Connolley← Previous edit | Revision as of 18:30, 7 August 2006 edit undo24.166.142.27 (talk)No edit summaryNext edit → | ||
| Line 3: | Line 3: | ||
| '''Greenhouse gases''' (GHGs) are ]eous components of the ] that contribute to the ]. Like the glass of a ], greenhouse gases are transparent only to some wavelengths of light. When sunlight hits the Earth, some is absorbed and re-emitted at longer wavelengths for which the greenhouse gas is opaque, hindering emission back out into space. This warms the Earth; although it is not the primary ]. | '''Greenhouse gases''' (GHGs) are ]eous components of the ] that contribute to the ]. Like the glass of a ], greenhouse gases are transparent only to some wavelengths of light. When sunlight hits the Earth, some is absorbed and re-emitted at longer wavelengths for which the greenhouse gas is opaque, hindering emission back out into space. This warms the Earth; although it is not the primary ]. | ||
| The major natural greenhouse gases are ], which causes about |
The major natural greenhouse gases are ], which causes about 95% of the greenhouse effect on Earth (]), but that can be converted to ] by nature; ], which causes 3%; ], which causes less than 2%, and ], which causes less than 1%. (Note that it is not really possible to assert that such-and-such a gas causes a certain percentage of the ], 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).<ref>{{cite journal| url=http://www.atmo.arizona.edu/students/courselinks/spring04/atmo451b/pdf/RadiationBudget.pdf| title=Earth’s Annual Global Mean Energy Budget| first=J. T.| last=Kiehl| coauthors= Kevin E. Trenberth| format=PDF| journal=Bulletin of the American Meteorological Society| pages=197-208| volume=78| issue=2| month=February| year=1997| accessdate=2006-05-01}}</ref> <ref>{{cite web| url=http://www.realclimate.org/index.php?p=142| year=6 Apr 2005| title=Water vapour: feedback or forcing?| publisher=RealClimate| accessdate=2006-05-01}}</ref> | ||
| Other greenhouse gases include, but are not limited to: ], ], ]s, ]s and ] - see ]. | Other greenhouse gases include, but are not limited to: ], ], ]s, ]s and ] - see ]. | ||
| The major atmospheric constituents (] and ]) are not greenhouse gases, because homonuclear diatomic ]s (e.g. N<sub>2</sub>, O<sub>2</sub>, H<sub>2</sub> ...) do not absorb in the ] as there is no net change in the ] of these molecules. | The major atmospheric constituents (] and ]) are not greenhouse gases, because homonuclear diatomic ]s (e.g. N<sub>2</sub>, O<sub>2</sub>, H<sub>2</sub> ...) do not absorb in the ] as there is no net change in the ] of these molecules. | ||
| Recent studies cast doubt as to the role CO2 plays in producing the Greenhouse effect. "Recent publications in "Science" and "Paleoceanography" show that CO2 levels were higher at the end of the last ice age than during the much warmer Eocene period, 43 million years earlier. These studies also found that CO2 levels are far higher today than they were during the relatively hot Miocene period, 17 million years ago." | |||
| ==Anthropogenic greenhouse gases== | ==Anthropogenic greenhouse gases== | ||
| There is considerable scientific debate as to how much human activity contributes to Greenhouse gas emissions and therefore the Greenhouse effect, since at most, mankind contributes only 3% of the total increase in atmospheric CO2. This is important to policy makers, because if global warming is attributed overwhelmingly to non-anthropogenic causes, then people and governments cannot impact global climate through the economically costly restriction of carbon and other gas emissions. Yet, even if all people on earth were eliminated, the total reduction of carbon dioxide, itself a minor Greenhouse gas as compared to natural water vapor, would be at most, 3%. | |||
| ] | ] | ||
Revision as of 18:30, 7 August 2006
Greenhouse gases (GHGs) are gaseous components of the atmosphere that contribute to the greenhouse effect. Like the glass of a greenhouse, greenhouse gases are transparent only to some wavelengths of light. When sunlight hits the Earth, some is absorbed and re-emitted at longer wavelengths for which the greenhouse gas is opaque, hindering emission back out into space. This warms the Earth; although it is not the primary mechanism that warms greenhouses.
The major natural greenhouse gases are water vapor, which causes about 95% of the greenhouse effect on Earth (including clouds), but that can be converted to rain by nature; carbon dioxide, which causes 3%; methane, which causes less than 2%, and ozone, which causes less than 1%. (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.) 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 ...) do not absorb in the infrared as there is no net change in the dipole moment of these molecules.
Recent studies cast doubt as to the role CO2 plays in producing the Greenhouse effect. "Recent publications in "Science" and "Paleoceanography" show that CO2 levels were higher at the end of the last ice age than during the much warmer Eocene period, 43 million years earlier. These studies also found that CO2 levels are far higher today than they were during the relatively hot Miocene period, 17 million years ago."
Anthropogenic greenhouse gases
There is considerable scientific debate as to how much human activity contributes to Greenhouse gas emissions and therefore the Greenhouse effect, since at most, mankind contributes only 3% of the total increase in atmospheric CO2. This is important to policy makers, because if global warming is attributed overwhelmingly to non-anthropogenic causes, then people and governments cannot impact global climate through the economically costly restriction of carbon and other gas emissions. Yet, even if all people on earth were eliminated, the total reduction of carbon dioxide, itself a minor Greenhouse gas as compared to natural water vapor, would be at most, 3%.
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.
- the use of CFCs in refrigeration systems. The use of CFCs and halons in fire suppression systems and various manufacturing processes.
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 role of water vapor
Water vapor is a natural greenhouse gas which, of all greenhouse gases, accounts for the largest percentage of the greenhouse effect. Water vapor levels fluctuate regionally, but in general humans do not produce a direct forcing of water vapor levels. 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. This in turn leads to an increase in the greenhouse effect and thus a further increase in temperature, and thus an increase in water vapor, 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.
- 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.(, see B7).
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. From the same ice cores it appears that CO2 concentrations have stayed between 260 and 280 µL/L during the entire preceding 10,000 years. Some studies, using evidence from stomata of fossilized leaves, have found greater variability and CO2 levels above 300 µL/L during the period 7-10 kyr ago. In response, others have argued that these findings are more likely to 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:
| Gas | Current (1998) Amount by volume | Increase over pre-industrial (1750) | Percentage increase | Radiative forcing (W/m) |
|---|---|---|---|---|
| Carbon dioxide | ||||
| Methane | ||||
| Nitrous oxide |
| Gas | Current (1998) Amount by volume |
Radiative forcing (W/m) |
|---|---|---|
| CFC-11 | ||
| CFC-12 | ||
| CFC-113 | ||
| Carbon tetrachloride | ||
| HCFC-22 |
(Source: IPCC radiative forcing report 1994 updated (to 1998) by IPCC TAR table 6.1 ).
Removal from the atmosphere and global warming potential
The greenhouse gases, once in the atmosphere, do not remain there eternally. 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 naturally occurring hydroxyl radical, OH· and degraded to CO2 and water vapor at the end of a chain of reactions (the contribution of the CO2 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. 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 (see ocean acidification, ) (CO2 is chemically stable in the atmosphere).
- as a consequence of a photochemical change driven by sun light. Halocarbons are dissociated by UV light releasing Cl· and F· as free radicals in the stratosphere with harmful effects on ozone (halocarbons are generally too stable to disappear by chemical reaction in the atmosphere).
- 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.
The lifetime of an individual molecule of gas in the atmosphere is frequently much shorter than the lifetime of a concentration anomaly of that gas. Thus, because of large (balanced) natural fluxes to and from the biosphere and ocean surface layer, an individual CO2 molecule may last only a few years in the air, on average; however, the calculated lifetime of an increase in atmospheric CO2 level is hundreds of years.
Aside from water vapor near the surface, which has a residence time of few 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.
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 will often hear 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 that is characterized by atmospheric lifetime.
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 CO2 and evaluated for a specific timescale. Thus, a molecule with a high GWP on a short time scale (say 20 years) but a short lifetime, will have a high 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:
- CO2 has a variable atmospheric lifetime (approximately 200-450 years for small perturbations). 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(100) of 10600.
- HCFC-22 has an atmospheric lifetime of 12.1 years and a GWP(100) of 1700.
- Tetrafluoromethane has an atmospheric lifetime of 50,000 years and a GWP(100) of 5700.
- Sulfur hexafluoride has an atmospheric lifetime of 3,200 years and a GWP(100) of 22000.
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 (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.
See also
- Biofuel
- Carbon sink
- Carbon neutral
- Clean Air Act
- Corporate Average Fuel Economy (CAFE)
- Effects of global warming
- Environmental agreements
- European Climate Change Programme
- Global Atmosphere Watch
- Global warming
- Hydrogen Economy
- United Nations Intergovernmental Panel of Climate Change
- Life cycle cost analysis
- Massachusetts v. Environmental Protection Agency
- North American Carbon Program
- Ocean acidification
- Radiative forcing
- Regional Greenhouse Gas Initiative
- Renewable energy
References
- Kiehl, J. T. (1997). "Earth's Annual Global Mean Energy Budget" (PDF). Bulletin of the American Meteorological Society. 78 (2): 197–208. Retrieved 2006-05-01.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help); Unknown parameter|month=ignored (help) - "Water vapour: feedback or forcing?". RealClimate. 6 Apr 2005. Retrieved 2006-05-01.
{{cite web}}: CS1 maint: year (link) - "Climate Change 2001: Working Group I: The Scientific Basis: figure 6-6". Retrieved 2006-05-01.
- "Climate Change 2001: Working Group I: The Scientific Basis: C.1 Observed Changes in Globally Well-Mixed Greenhouse Gas Concentrations and Radiative Forcing". Retrieved 2006-05-01.
- Friederike Wagner, Bent Aaby and Henk Visscher (2002). "Rapid atmospheric CO2 changes associated with the 8,200-years-B.P. cooling event". PNAS. 99 (19): 12011–12014. doi:10.1073/pnas.182420699.
- Andreas Indermühle, Bernhard Stauffer, Thomas F. Stocker (1999). "Early Holocene Atmospheric CO2 Concentrations". Science. 286 (5446): 1815. doi:10.1126/science.286.5446.1815a.
{{cite journal}}: CS1 maint: multiple names: authors list (link) "Early Holocene Atmospheric CO2 Concentrations". Science. Retrieved May 26.{{cite web}}: Check date values in:|accessdate=(help); Unknown parameter|accessyear=ignored (|access-date=suggested) (help) - H.J. Smith, M Wahlen and D. Mastroianni (1997). "The CO2 concentration of air trapped in GISP2 ice from the Last Glacial Maximum-Holocene transition". Geophysical Research Letters. 24 (1): 1–4.
- Archer, David (2005). "Fate of fossil fuel CO2 in geologic time". Journal of Geophysical Research. 110, C09S05. doi:10.1029/2004JC002625.
- Caldeira, Ken and Wickett, Michael E. (2005). "Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and ocean". Journal of Geophysical Research. 110, C09S04. doi:10.1029/2004JC002671.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - Shindell, Drew T.; Faluvegi, Greg; Bell, Nadine; Schmidt, Gavin A. "An emissions-based view of climate forcing by methane and tropospheric ozone", Geophysical Research Letters, Vol. 32, No. 4
External links
- Greenhouse gas calculator.
- Greenhouse gas reduction technology.
- The NOAA Annual Greenhouse Gas Index (AGGI).
- Well to Wheel analysis.
Carbon dioxide emissions
- EU page about reducing CO2 emissions from 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.
- International Energy Annual: Reserves
- IEA Publications Bookshop - Key World Energy Statistics
- International Energy Annual 2003: Carbon Dioxide Emissions
- International Energy Annual 2003: Notes and Sources for Table H.1co2 (Metric tons of carbon dioxide can be converted to metric tons of carbon equivalent by multiplying by 12/44)
- DOE - EIA - Alternatives to Traditional Transportation Fuels 1994 - Volume 2, Greenhouse Gas Emissions (includes "Greenhouse Gas Spectral Overlaps and Their Significance")
- NOAA Paleoclimatology Program - Vostok Ice Core
- NOAA CMDL CCGG - Interactive Atmospheric Data Visualization NOAA CO2 data
- Carbon Dioxide Information Analysis Centre FAQ Includes links to lots of useful Carbon Dioxide statistics