Greenhouse gas

Top: Increasing atmospheric CO₂ levels as measured in the atmosphere and ice cores. Bottom: The amount of net carbon increase in the atmosphere, compared to carbon emissions from burning fossil fuel.

Greenhouse gases (GHG) are gaseous components of the atmosphere that contribute to the greenhouse effect. 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 GHE, because the influences of the various gases are not additive. The higher ends of the ranges quoted are for the gas alone; the lower end, for the gas counting overlaps). [1] [2].

Other greenhouse gases include, but are not limited to: methane, nitrous oxide, sulfur hexafluoride, and chlorofluorocarbons - see IPCC list of greenhouse gases.

The major atmospheric constituents ( N₂ and O₂) are not greenhouse gases, because homonuclear diatomic molecules (eg N₂, O₂, H₂ ...) do not absorb in the infrared as there is no net change in the dipole moment of these molecules.

Anthropogenic greenhouse gases

Human activity raises levels of greenhouse gases primarily by releasing carbon dioxide, but other gases, e.g. methane, are not negligible [3].

The concentrations of several greenhouse gases have increased over time [4] 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

Increasing water vapor at Boulder, Colorado.

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 (but not a runaway feedback) to the forcing provided by human-released greenhouse gases such as CO₂. Changes in the water vapour 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 vapour content of the atmosphere significantly enhances the initial warming that caused the increase - that is, it is a strong positive feedback.( [5], see B7). Water vapor is a definite part of the greenhouse gas equation even though not under direct human control: 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 [6]; see also [7] and [8].

The IPCC discuss the water vapor feedback [9].

It is not really possible to assert that such-and-such a gas causes a certain percentage of the GHE, because the influences of the various gases are not additive. The 1990 IPCC report says "If H₂O were the only GHG present, then the GHE of a clear-sky midlatitude atmosphere... would be about 60-70% of the value with all gases included; by contrast, if CO₂ alone was present, the corresponding value would be about 25%".

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:

Global carbon dioxide emissions 1751– 2000.

(Source: IPCC radiative forcing report 1994 updated (to 1998) by IPCC TAR table 6.1 [13] [14]).

Removal from the atmosphere and global warming potential

Major greenhouse gas trends

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. In what follows below we will quote the GWP on a 100 year time scale.

Some examples of the atmospheric lifetime the different greenhouse gases include:

• CO₂s atmospheric lifetime is variable (approximately 200-450 years). It is defined to have a global warming potential (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 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

Source : IPCC, table 6.7.

Related effects

MOPITT 2000 global carbon monoxide

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 [16].

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.