Carbon dioxide sink

A carbon dioxide (CO2) sink is a carbon reservoir that is increasing in size, and is the opposite of a carbon dioxide "source". The main natural sinks are the oceans and plants and other organisms that use photosynthesis to remove carbon from the atmosphere by incorporating it into biomass and release oxygen into the atmosphere. This concept of CO2 sinks has become more widely known because the Kyoto Protocol allows the use of carbon dioxide sinks as a form of carbon offset.

Carbon sequestration is the term describing processes that remove carbon dioxide from the atmosphere. To help mitigate global warming, a variety of means of artificially capturing and storing carbon (while releasing oxygen) — as well as of enhancing natural sequestration processes — are being explored.

Forests

Carbon dioxide is incorporated into forests and forest soils by trees and other plants. Through photosynthesis, plants absorb carbon dioxide from the atmosphere, store the carbon in sugars, starch and cellulose, and release the oxygen into the atmosphere. A young forest, composed of growing trees, absorbs carbon dioxide and acts as a sink. Mature forests, made up of a mix of various aged trees as well as dead and decaying matter, may be carbon neutral above ground. In the soil, however, the gradual build-up of slowly decaying organic material will continue to accumulate carbon, but at a slower rate than an immature forest. Organic material in the form of humus in the forest floor accumulates in greater quantity in cooler regions such as the boreal and taiga forests. At warmer temperatures humus is oxidized rapidly; this, in addition to high rainfall levels, is the reason why tropical jungles have very thin organic soils. The forest eco-system may eventually become carbon neutral. Forest fires release absorbed carbon back into the atmosphere, as does deforestation due to rapidly increased oxidation of soil organic matter.

The dead trees, plants, and moss in peat bogs undergo slow anaerobic decomposition below the surface of the bog. This process is slow enough that in many cases the bog grows rapidly and fixes more carbon from the atmosphere than is released. Over time, the peat grows deeper. Peat bogs inter approximately one-quarter of the carbon stored in land plants and soils.

Under some conditions, forests and peat bogs may become sources of CO2, such as when a forest is flooded by the construction of a hydroelectric dam. Unless the forests and peat are harvested before flooding, the rotting vegetation is a source of CO2 and methane comparable in magnitude to the amount of carbon released by a fossil-fuel powered plant of equivalent power

Oceans

Oceans are natural CO2 sinks, and represent the largest active carbon sink on Earth. This role as a sink for CO2 is driven by two processes, the solubility pump and the biological pump. The former is primarily a function of differential CO2 solubility in seawater and the thermohaline circulation, while the latter is the sum of a series of biological processes that transport carbon (in organic and inorganic forms) from the surface euphotic zone to the ocean's interior. A small fraction of the organic carbon transported by the biological pump to the seafloor is buried in anoxic conditions under sediments and ultimately forms fossil fuels such as oil and natural gas.

At the present time, approximately one third[4] of anthropogenic emissions are estimated to be entering the ocean. The solubility pump is the primary mechanism driving this, with the biological pump playing a negligible role. This stems from the limitation of the biological pump by ambient light and nutrients required by the phytoplankton that ultimately drive it. Total inorganic carbon is not believed to limit primary production in the oceans, so its increasing availability in the ocean does not directly affect production (the situation on land is different, since enhanced atmospheric levels of CO2 essentially "fertilize" land plant growth). However, ocean acidification by invading anthropogenic CO2 may affect the biological pump by negatively impacting calcifying organisms such as coccolithophores, foraminiferans and pteropods. Climate change may also affect the biological pump in the future by warming and stratifying the surface ocean, thus reducing the supply of limiting nutrients to surface waters. Although the buffering capacity of sea water is keeping the pH nearly constant at present, eventually pH will drop. At this point, the dissruption of life in the sea may turn it into a carbon source rather than a carbon sink. The characteristic of buffered systems is to hold the pH reasonably constant over a large introduction of acid and then drop suddenly with a small additional amount

Soils

Carbon as plant organic matter is sequestered in soils: Soils contain more carbon than is contained in vegetation and the atmosphere combined. Soils' organic carbon (humus) levels in many agricultural areas have been severely depleted. Organic material in the form of humus accumulates below about 25 degrees Celsius. Above this temperature, humus is oxidized much more rapidly. This is part of the reason why tropical soils under jungles are so thin, despite the rapid accumulation of organic material on the jungle floor (the other being extensive rainfall leaching soluble components vital to organic soil structure). Areas where shifting cultivation or "hack-and-slash" agriculture are practised are generally only fertile for 2-3 years before they are abandoned. These tropical jungles are similar to coral reefs in that they are highly efficient at conserving and circulating necessary nutrients, which explains their lushness in a nutrient desert.

Grasslands contribute to soil organic matter, mostly in the form of their extensive fibrous root mats. Much of this organic matter can remain unoxidized for long periods of time, depending on rainfall conditions, the length of the winter season, and the frequency of naturally occurring lightning-induced grass-fires necessary to recycle inorganic compounds from existing plant material. While these fires release carbon dioxide, they improve the quality of the grass-lands overall, in turn increasing the amount of carbon retained in the retained humic material. They also desposit carbon directly to the soil in the form of char that does not significantly degrade back to carbon dioxide

Enhancing natural sequestration

 Future sea level rise

In 2001, the Intergovernmental Panel on Climate Change's Third Assessment Report predicted that by 2100, global warming will lead to a sea level rise of 9 to 88 cm. At that time no significant acceleration in the rate of sea level rise during the 20th century had been detected. Subsequently, Church and White found acceleration of 0.013 ± 0.006 mm/yr².

These sea level rises could lead to difficulties for shore-based communities: for example, many major cities such as London and New Orleans already need storm-surge defenses, and would need more if sea level rose, though they also face issues such as sinking land.

Future sea level rise, like the recent rise, is not expected to be globally uniform (details below). Some regions show a sea level rise substantially more than the global average (in many cases of more than twice the average), and others a sea level fall. However, models disagree as to the likely pattern of sea level change.

Intergovernmental Panel on Climate Change results

The results from the IPCC (TAR) sea level chapter (convening authors John A. Church and Jonathan M. Gregory) are given below.

The sum of these components indicates a rate of eustatic sea level rise (corresponding to a change in ocean volume) from 1910 to 1990 ranging from –0.8 to 2.2 mm/yr, with a central value of 0.7 mm/yr. The upper bound is close to the observational upper bound (2.0 mm/yr), but the central value is less than the observational lower bound (1.0 mm/yr), i.e., the sum of components is biased low compared to the observational estimates. The sum of components indicates an acceleration of only 0.2 (mm/yr)/century, with a range from –1.1 to 0.7 (mm/yr)/century, consistent with observational finding of no acceleration in sea level rise during the 20th century. The estimated rate of sea level rise from anthropogenic climate change from 1910 to 1990 (from modeling studies of thermal expansion, glaciers and ice sheets) ranges from 0.3 to 0.8 mm/yr. It is very likely that 20th century warming has contributed significantly to the observed sea level rise, through thermal expansion of sea water and widespread loss of land ice.

A common perception is that the rate of sea level rise should have accelerated during the latter half of the 20th century, but tide gauge data for the 20th century show no significant acceleration. We have obtained estimates based on AOGCMs for the terms directly related to anthropogenic climate change in the 20th century, i.e., thermal expansion, ice sheets, glaciers and ice caps... The total computed rise indicates an acceleration of only 0.2 (mm/yr)/century, with a range from -1.1 to 0.7 (mm/yr)/century, consistent with observational finding of no acceleration in sea level rise during the 20th century. The sum of terms not related to recent climate change is -1.1 to 0.9 mm/yr (i.e., excluding thermal expansion, glaciers and ice caps, and changes in the ice sheets due to 20th century climate change). This range is less than the observational lower bound of sea level rise. Hence it is very likely that these terms alone are an insufficient explanation, implying that 20th century climate change has made a contribution to 20th century sea level rise.

Uncertainties and criticisms regarding IPCC results

- Tide records with a rate of 180 mm/century going back to the 19th century show no measurable acceleration throughout the late 19th and first half of the 20th century. The IPCC attributes about 60 mm/century to melting and other eustatic processes, leaving a residual of 120 mm of 20th century rise to be accounted for. Global ocean temperatures by Levitus et al are in accord with coupled ocean/atmosphere modeling of greenhouse warming, with heat-related change of 30 mm. Melting of polar ice sheets at the upper limit of the IPCC estimates could close the gap, but severe limits are imposed by the observed perturbations in Earth rotation. (Munk 2002)

- By the time of the IPCC TAR, attribution of sea level changes had a large unexplained gap between direct and indirect estimates of global sea level rise. Most direct estimates from tide gauges give 1.5–2.0 mm/yr, whereas indirect estimates based on the two processes responsible for global sea level rise, namely mass and volume change, are significantly below this range. Estimates of the volume increase due to ocean warming give a rate of about 0.5 mm/yr and the rate due to mass increase, primarily from the melting of continental ice, is thought to be even smaller. One study confirmed tide gauge data is correct, and concluded there must be a continental source of 1.4 mm/yr of fresh water. (Miller 2004)

- From (Douglas 2002): "In the last dozen years, published values of 20th century GSL rise have ranged from 1.0 to 2.4 mm/yr. In its Third Assessment Report, the IPCC discusses this lack of consensus at length and is careful not to present a best estimate of 20th century GSL rise. By design, the panel presents a snapshot of published analysis over the previous decade or so and interprets the broad range of estimates as reflecting the uncertainty of our knowledge of GSL rise. We disagree with the IPCC interpretation. In our view, values much below 2 mm/yr are inconsistent with regional observations of sea-level rise and with the continuing physical response of Earth to the most recent episode of deglaciation."

- The strong 1997-1998 El Niño caused regional and global sea level variations, including a temporary global increase of perhaps 20 mm. The IPCC TAR's examination of satellite trends says the major 1997/98 El Niño-Southern Oscillation (ENSO) event could bias the above estimates of sea level rise and also indicate the difficulty of separating long-term trends from climatic variability.

Effects of sea level rise

Based on the projected increases stated above, the IPCC TAR WG II report notes that current and future climate change would be expected to have a number of impacts, particularly on coastal systems. Such impacts may include increased coastal erosion, higher storm-surge flooding, inhibition of primary production processes, more extensive coastal inundation, changes in surface water quality and groundwater characteristics, increased loss of property and coastal habitats, increased flood risk and potential loss of life, loss of nonmonetary cultural resources and values, impacts on agriculture and aquaculture through decline in soil and water quality, and loss of tourism, recreation, and transportation functions.

There is an implication that many of these impacts will be detrimental. The report does, however, note that owing to the great diversity of coastal environments; regional and local differences in projected relative sea level and climate changes; and differences in the resilience and adaptive capacity of ecosystems, sectors, and countries, the impacts will be highly variable in time and space and will not necessarily be negative in all situations.

Statistical data on the human impact of sea level rise is scarce. A study in the April, 2007 issue of Environment and Urbanization reports that 634 million people live in coastal areas within 30 feet of sea level. The study also reported that about two thirds of the world's cities with over five million people are located in these low-lying coastal areas.

Are islands "sinking"

IPCC assessments have suggested that deltas and small island states may be particularly vulnerable to sea level rise. Relative sea level rise (mostly caused by subsidence) is causing substantial loss of lands in some deltas. However, sea level changes have not yet been implicated in any substantial environmental, humanitarian, or economic losses to small island states. Previous claims have been made that parts of the island nations of Tuvalu were "sinking" as a result of sea level rise. However, subsequent reviews have suggested that the loss of land area was the result of erosion during and following the actions of 1997 cyclones Gavin, Hina, and Keli. According to climate skeptic Patrick J. Michaels, "In fact, areas...such as [the island of] Tuvalu show substantial declines in sea level over that period."

Reuters has reported other Pacific islands are facing a severe risk including Tegua island in Vanuatu. Claims that Vanuatu data shows no net sea level rise, are not substantiated by tide gauge data. Vanuatu tide gauge data show a net rise of ~50 mm from 1994-2004. Linear regression of this short time series suggests a rate of rise of ~7 mm/y, though there is considerable variability and the exact threat to the islands is difficult to assess using such a short time series.

Numerous options have been proposed that would assist island nations to adapt to rising sea level.

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Climate sensitivity

In Intergovernmental Panel on Climate Change (IPCC) reports, equilibrium climate sensitivity refers to the equilibrium change in global mean surface temperature following a doubling of the atmospheric (equivalent) CO2 concentration. This value is estimated, by the IPCC Fourth Assessment Report as likely to be in the range 2 to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C. Values substantially higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values. This is a slight change from the IPCC Third Assessment Report, which said it was "likely to be in the range of 1.5 to 4.5°C". More generally, equilibrium climate sensitivity refers to the equilibrium change in surface air temperature following a unit change in radiative forcing, expressed in units of °C/(W/m2). In practice, the evaluation of the equilibrium climate sensitivity from models requires very long simulations with coupled global climate models, or it may be deduced from observations.

Gregory et al. (2002) estimate a lower bound of 1.6°C by estimating the change in Earth's radiation budget and comparing it to the global warming observed over the 20th century. Recent work by Annan and Hargreaves combines independent observational and model based estimates to produce a mean of about 3°C, and only a 5% chance of exceeding 4.5°C.

Shaviv (2005) carried out a similar analysis for 6 different time scales, ranging from the 11-yr solar cycle to the climate variations over geological time scales. He found a typical sensitivity of 2.0°C (ranging between 0.9°C and 2.9°C at 99% confidence) if there is no cosmic-ray climate connection, or a typical sensitivity of 1.3°C (between 0.9°C and 2.5°C at 99% confidence), if the cosmic-ray climate link is real.

Andronova and Schlesinger (2001) (using simple climate models) found that it could lie between 1 and 10°C, with a 54 percent likelihood that the climate sensitivity lies outside the IPCC range. The exact range depends on which factors are most important during the instrumental period: "At present, the most likely scenario is one that includes anthropogenic sulfate aerosol forcing but not solar variation. Although the value of the climate sensitivity in that case is most uncertain, there is a 70 percent chance that it exceeds the maximum IPCC value. This is not good news." said Schlesinger.

Forest et al. (2002) using patterns of change and the MIT EMIC estimated a 95% confidence interval of 1.4–7.7°C for the climate sensitivity, and a 30% probability that sensitivity was outside the 1.5 to 4.5°C range.

Frame et al. (2005) and Allen et al. note that the size of the confidence limits are dependent on the nature of the prior assumptions made.

Climate sensitivity is not the same as the expected climate change at, say 2100: the TAR reports this to be an increase of 1.4 to 5.8°C over 1990.

The Transient climate response (TCR) - a term first used in the TAR - is the temperature change at the time of CO2 doubling in a run with CO2 increasing at 1%/year.

The effective climate sensitivity is a related measure that circumvents this requirement. It is evaluated from model output for evolving non-equilibrium conditions. It is a measure of the strengths of the feedbacks at a particular time and may vary with forcing history and climate state.

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Carbon dioxide

Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere as a gas. It is currently at a globally averaged concentration of approximately 385 ppm by volume in the Earth's atmosphere, although this varies both by location and time. Carbon dioxide's chemical formula is CO2.

In general, it is exhaled by animals and utilized by plants during photosynthesis. Additional carbon dioxide is created by the combustion of fossil fuels or vegetable matter, among other chemical processes.
Carbon dioxide is an important greenhouse gas because of its ability to absorb many infrared wavelengths of the Sun's light, and because of the length of time it stays in the Earth's atmosphere. Due to this, and the role it plays in the respiration of plants, it is a major component of the carbon cycle.

In its solid state, carbon dioxide is commonly called dry ice. Carbon dioxide has no liquid state at pressures below 5.1 atm.

In the Earth's atmospher

Atmospheric CO2 concentrations measured at Mauna Loa Observatory.

Carbon dioxide in earth's atmosphere is considered a trace gas, and is measured in parts per million. Current concentration levels average approximately 385 ppm, which represents a total of around 800 gigatons of carbon. Its concentration can vary considerably on a regional basis: in urban areas it is generally higher, and indoors can reach 10 times the atmospheric concentration.

Carbon dioxide is a greenhouse gas; see greenhouse effect for more.

Due to human activities such as the combustion of fossil fuels and deforestation, the concentration of atmospheric carbon dioxide has increased by about 35% since the beginning of the age of industrialization.

Up to 40% of the gas emitted by a volcano during a subaerial volcanic eruption is carbon dioxide. However, human activities currently release more than 130 times the amount of CO2 emitted by volcanoes. According to the best estimates, volcanoes release about 130-230 million tonnes (145-255 million tons) of CO2 into the atmosphere each year. Emissions of CO2 by human activities amount to about 27 billion tonnes per year (30 billion tons).

Biological role

Carbon dioxide is an end product in organisms that obtain energy from breaking down sugars, fats and amino acids with oxygen as part of their metabolism, in a process known as cellular respiration. This includes all plants, animals, many fungi and some bacteria. In higher animals, the carbon dioxide travels in the blood from the body's tissues to the lungs where it is exhaled. In plants using photosynthesis, carbon dioxide is absorbed from the atmosphere.

Role in photosynthesis

Plants remove carbon dioxide from the atmosphere by photosynthesis, also called carbon assimilation, which uses light energy to produce organic plant materials by combining carbon dioxide and water. Free oxygen is released as gas from the decomposition of water molecules, while the hydrogen is split into its protons and electrons and used to generate chemical energy via photophosphorylation. This energy is required for the fixation of carbon dioxide in the Calvin cycle to form sugars. These sugars can then be used for growth within the plant through respiration. Carbon dioxide gas must be introduced into greenhouses to maintain plant growth, as even in vented greenhouses the concentration of carbon dioxide can fall during daylight hours to as low as 200 ppm, at which level photosynthesis is significantly reduced. Venting can help offset the drop in carbon dioxide, but will never raise it back to ambient levels of 340 ppm. Carbon dioxide supplementation is the only known method to overcome this deficiency. Direct introduction of pure carbon dioxide is ideal, but rarely done because of cost constraints. Most greenhouses burn methane or propane to supply the additional CO2, but care must be taken to have a clean burning system as increased levels of nitrogen oxides (NOx) result in reduced plant growth. Sensors for sulfur dioxide (SO2) and NOx are expensive and difficult to maintain; accordingly most systems come with a carbon monoxide (CO) sensor under the assumption that high levels of carbon monoxide mean that significant amounts of NOx are being produced. Plants can potentially grow up to 50 percent faster in concentrations of 1,000 ppm CO2 when compared with ambient conditions.

Plants also emit CO2 during respiration, so it is only during growth stages that plants are net absorbers. For example a growing forest will absorb many tonnes of CO2 each year, however a mature forest will produce as much CO2 from respiration and decomposition of dead specimens (e.g. fallen branches) as used in biosynthesis in growing plants. Regardless of this, mature forests are still valuable carbon sinks, helping maintain balance in the Earth's atmosphere. Additionally, and crucially to life on earth, phytoplankton photosynthesis absorbs dissolved CO2 in the upper ocean and thereby promotes the absorption of CO2 from the atmosphere.

Animal toxicity

Carbon dioxide content in fresh air varies between 0.03% (300 ppm) and 0.06% (600 ppm), depending on the location. A person's exhaled breath is approximately 4.5% carbon dioxide. It is dangerous when inhaled in high concentrations (greater than 5% by volume, or 50,000 ppm). The current threshold limit value (TLV) or maximum level that is considered safe for healthy adults for an eight-hour work day is 0.5% (5,000 ppm). The maximum safe level for infants, children, the elderly and individuals with cardio-pulmonary health issues is significantly less.

These figures are valid for pure carbon dioxide. In indoor spaces occupied by people the carbon dioxide concentration will reach higher levels than in pure outdoor air. Concentrations higher than 1,000 ppm will cause discomfort in more than 20% of occupants, and the discomfort will increase with increasing CO2 concentration. The discomfort will be caused by various gases coming from human respiration and perspiration, and not by CO2 itself. At 2,000 ppm the majority of occupants will feel a significant degree of discomfort, and many will develop nausea and headaches. The CO2 concentration between 300 and 2,500 ppm is used as an indicator of indoor air quality.

Acute carbon dioxide toxicity is sometimes known as by the names given to it by miners: black damp, choke damp, or stythe. Miners would try to alert themselves to dangerous levels of carbon dioxide in a mine shaft by bringing a caged canary with them as they worked. The canary would inevitably die before CO2 reached levels toxic to people. Choke damp caused a great loss of life at Lake Nyos in Cameroon in 1986, when an upwelling of CO2-laden lake water quickly blanketed a large surrounding populated area. The heavier carbon dioxide forced out the life-sustaining oxygen near the surface, killing nearly two thousand people.

Carbon dioxide ppm levels (CDPL) are a surrogate for measuring indoor pollutants that may cause occupants to grow drowsy, get headaches, or function at lower activity levels. To eliminate most Indoor Air Quality complaints, total indoor CDPL must be reduced to below 600. NIOSH considers that indoor air concentrations that exceed 1,000 are a marker suggesting inadequate ventilation. ASHRAE recommends they not exceed 1,000 inside a space. OSHA limitsconcentrations in the workplace to 5,000 for prolonged periods. The U.S. National Institute for Occupational Safety and Health limits brief exposures (up to ten minutes) to 30,000 and considers CDPL exceeding 40,000 as "immediately dangerous to life and health." People who breathe 50,000 for more than half an hour show signs of acute hypercapnia, w hile breathing 70,000 – 100,000 can produce unconsciousness in only a few minutes. Accordingly, carbon dioxide, either as a gas or as dry ice, should be handled only in well-ventilated areas.

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Greenhouse effect

The greenhouse effect, discovered by Joseph Fourier in 1829 and first investigated quantitatively by Svante Arrhenius in 1896, is the process in which the emission of infrared radiation by the atmosphere warms a planet's surface. The name comes from an analogy with the warming of air inside a greenhouse compared to the air outside the greenhouse. The Earth's average surface temperature is about 20-30°C warmer than it would be without the greenhouse effect. In addition to the Earth, Mars and especially Venus have greenhouse effects

A schematic representation of the exchanges of energy between outer space, the Earth's atmosphere, and the Earth surface. The ability of the atmosphere to capture and recycle energy emitted by the Earth surface is the defining characteristic of the greenhouse effect.

Anthropogenic greenhouse effect

CO2 production from increased industrial activity (fossil fuel burning) and other human activities such as cement production and tropical deforestation has increased the CO2 concentrations in the atmosphere. Measurements of carbon dioxide amounts from Mauna Loa observatory show that CO2 has increased from about 313 ppm (parts per million) in 1960 to about 375 ppm in 2005. The current observed amount of CO2 exceeds the geological record of CO2 maxima (~300 ppm) from ice core data (Hansen, J., Climatic Change, 68, 269, 2005 ISSN 0165-0009).
Because it is a greenhouse gas, elevated CO2 levels will increase global mean temperature; based on an extensive review of the scientific literature, the Intergovernmental Panel on Climate Change concludes that "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations".

Over the past 800,000 years, ice core data shows unambiguously that carbon dixoide has varied from values as low as 180 parts per million (ppm) to the pre-industrial level of 270ppm. Certain paleoclimatologists consider variations in carbon dioxide to be a fundamental factor in controlling climate variations over this time scale.

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Greenhouse gases

Greenhouse gases (GHG) are components of the atmosphere that contribute to the greenhouse effect. Some greenhouse gases occur naturally in the atmosphere, while others result from human activities such as burning of fossil fuels such as coal. Greenhouse gases include water vapor, carbon dioxide, methane, nitrous oxide, and ozone.

The "Greenhouse effect"

When sunlight reaches the surface of the Earth, some of it is absorbed and warms the Earth. Because the Earth's surface is much cooler than the sun, it radiates energy at much longer wavelengths than does the sun. The atmosphere absorbs these longer wavelengths more effectively than it does the shorter wavelengths from the sun. 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 a misnomer, as this process is not the 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%. It is not possible to state 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.

The major atmospheric constituents (nitrogen, N2 and oxygen, O2) are not greenhouse gases. This is because homonuclear diatomic molecules such as N2 and O2 neither absorb nor emit infrared radiation, as there is no net change in the dipole moment of these molecules when they vibrate. Molecular vibrations occur at energies that are of the same magnitude as the energy of the photons on infrared light. Heteronuclear diatomics such as CO or HCl absorb IR; however, these molecules are short-lived in the atmosphere owing to their reactivity and solubility. As a consequence they do not contribute significantly to the greenhouse effect.

Late 19th century scientists experimentally discovered that N2 and O2 did not absorb infrared radiation (called, at that time, "dark radiation") and that CO2 and many other gases did absorb such radiation. It was recognized in the early 20th century that the known major greenhouse gases in the atmosphere caused the earth's temperature to be higher than it would have been without the greenhouse gases.
 

Anthropogenic greenhouse gases

The projected temperature increase for a range of greenhouse gas stabilization scenarios (the coloured bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4 2007.


 

The concentrations of several greenhouse gases have increased over time. Human activity increases the greenhouse effect primarily through release of carbon dioxide, but human influences on other greenhouse gases can also be important. 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 chlorofluorocarbons (CFCs) in refrigeration systems, and use of CFCs and halons in fire suppression systems and manufacturing processes.

-agricultural activities, including the use of fertilizers, that lead to higher nitrous oxide concentrations.

The seven sources of CO2 from fossil fuel combustion are (with percentage contributions for 2000-2004):

1-Solid fuels (e.g. coal): 35%

2-Liquid fuels (e.g. petrol): 36%

3-Gaseous fuels (e.g. natural gas): 20%

4-Flaring gas industrially and at wells: <1%</p>

5-Cement production: 3%

6-Non-fuel hydrocarbons: <1%</p>

7-The "international bunkers" of shipping and air transport not included in national inventories: 4%

Greenhouse gas emissions from industry, transportation (1/3 of total US global warming pollution) and agriculture are very likely the main cause of recently observed global warming. Major sources of an individual's GHG include home heating and cooling, electricity consumption, and automobiles. Corresponding conservation measures are improving home building insulation, cellular shades, Compact fluorescent lamps, and choosing high miles per gallon vehicles.

Carbon dioxide, methane, nitrous oxide and three groups of fluorinated gases (sulfur hexafluoride, HFCs, and PFCs) are the major greenhouse gases and the subject of the Kyoto Protocol, which entered into force in 2005.

CFCs, although greenhouse gases, are regulated by the Montreal Protocol, which was motivated by CFCs' contribution to ozone depletion rather than by their contribution to global warming. Note that ozone depletion has only a minor role in greenhouse warming though the two processes often are confused in the popular media.

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