What are the properties of a greenhouse gas?
ACS Climate Science Toolkit | Greenhouse Gases
Temperature is a measure of the average energy of molecular motion in a sample of matter: to and fro translation, intramolecular vibration (and lattice vibration in solids), and rotation (both entire molecules and intramolecular portions). The sum of these motions’ energies can be described as the “thermal energy” of the sample. Thermal energy and, hence, temperature can change as various forms of energy, including electromagnetic (light/photons), interact with the sample and change the average energy of motion.
The electromagnetic spectrum covers a wavelength range of about 24 orders of magnitude, from the shortest, most energetic, high frequency gamma rays to the longest, very low energy, low frequency radio waves). The range of interest for climate science and atmospheric warming is approximately in the middle of this spectrum from about 0.1 to 100 μm (10–1 to 102 μm). This includes the visible region of the spectrum (0.4 to 0.7 μm) and the adjacent higher energy (shorter wavelength) near ultraviolet (UV) and lower energy (longer wavelength) infrared (IR), including much of the “thermal infrared”. Incoming solar radiation is in the shorter wavelength (higher energy) part of this range from UV through near IR, between about 0.1 to 4 μm. Radiation from the warmed Earth is mainly in the thermal IR region between 4 and 30 μm.
Molecular vibrations and some energetic rotations have energy level spacings that correspond to energies in the IR region of the electromagnetic spectrum (most rotations are in the microwave range which runs between thermal IR and radio wavelengths). Thus IR radiation absorbed by molecules causes increased vibration. Collisions between these energized molecules and others in the sample transfer energy among all the molecules, which increases the average thermal energy and, hence, raises the temperature. Conversely, molecules that emit IR radiation lose their vibrational energy and their collisions with other molecules decrease the average thermal energy and lower the temperature.
The wavelength unit used here and in most discussions of greenhouse gases is the micrometer, μm, which is usually called a “micron”. Frequencies of radiation in the IR are often given in units of reciprocal centimeters, cm–1, called “wavenumbers” (number of waves per centimeter). To convert microns to wavenumbers, divide the numerical value in microns into 10,000 μm·cm–1.
In order for molecular vibrations to absorb IR energy, the vibrational motions must change the dipole moment of the molecule. All molecules with three or more atoms meet this criterion and are IR absorbers. While the Earth’s (dry) atmosphere is predominantly composed of non-IR absorbers, N2 (78%), O2 (21%), and Ar (~0.9%), the 0.1% of remaining trace gases contains many species that absorb IR. The absorptions by CO2, CH4, N2O, and O3 are shown in the schematic diagram in the sidebar below.
The atmosphere is, of course, actually “wet” and may contain several percent water vapor, as well as liquid and solid water in clouds, from the natural water cycle (evaporation-condensation-precipitation). Water vapor also has strong absorptions in the IR, as shown in the schematic diagram. The majority of atmospheric warming is due to these absorptions by water vapor that occur at both ends of the thermal IR region.
Note that there is a “window” in the water vapor spectrum from about 8 to 15 μm where there is little IR absorption and hence little contribution to atmospheric warming. The strong absorption by CO2 at the long wavelength end of this region narrows this window a bit and adds to the warming effect. Radiation from the Earth that is in this window region passes through the atmosphere with little absorption and contributes little to atmospheric warming. Other gases that absorb in this window region or in other narrower window regions of the thermal IR (where water vapor and CO2 do not absorb appreciably) can make significant relative contributions to atmospheric warming by absorbing energy that would otherwise be lost to space.
Global Warming Potential
In the context of contributions of different gases to atmospheric warming the concept of global warming potential (GWP) can be useful. GWP is a measure of how much energy a greenhouse gas would add to atmospheric warming in a given time compared to CO2. A molecule’s GWP depends on three factors:
- the wavelengths where the molecule absorbs. (The absorption needs to be in the thermal IR range where the Earth emits and will be more effective if it absorbs where water vapor and CO2 do not.)
- the strength of the relevant absorptions. (The more energy the molecule absorbs, the more effective it will be in warming.)
- the atmospheric lifetime of the molecule. (The longer the gas persists, the more warming it can produce.)
GWP values are calculated as a ratio of the combined effect of these factors if 1 kg of the gas in question is injected into the atmosphere compared to the effect if 1 kg of kilogram of CO2 is injected. CO2 is assigned a value of unity, so the resulting ratio is the GWP. GWPs for a few selected gases are given in the table. To interpret GWPs, consider, for example, the 20 year GWP of 72 for CH4. This means that injecting 1 kg of CH4 into the atmosphere today would have 72 times more atmospheric warming effect over the next 20 years than injecting 1 kg of CO2. However, since the amount of CO2 being injected into the atmosphere is orders of magnitude greater than for these other gases, radiative forcing by CO2 still exceeds their combined effect on atmospheric warming.
| GWP time horizon | ||||
| Gas | Lifetime, yr | 20 yr | 100 yr | 500 yr |
| Carbon Dioxide, CO2 | see text | 1 | 1 | 1 |
| Methane, CH4 | 12 | 72 | 25 | 7.6 |
| Nitrous Oxide, N2O | 114 | 289 | 298 | 153 |
| CFC-12, CCl2F2 | 100 | 11,000 | 10,900 | 5,200 |
| HFC-23, CHF3 | 270 | 12,000 | 14,800 | 12,200 |
| HFC-134a, CH2FCF3 | 14 | 3,830 | 1,430 | 435 |
| Sulfur Hexafluoride, SF6 | 3,200 | 16,300 | 22,800 | 32,600 |
Note that no lifetime is given for CO2 in the atmosphere. The sources and sinks for CO2 involve the complex interplay of CO2 among the hydrosphere (temperature dependent dissolution and release), the biosphere (respiration and photosynthesis), and the lithosphere (weathering and deposition), all of which complicate its rate of disappearance. About half of a CO2 sample emitted today will be gone in a century, but a portion of the rest will persist for 1000s of years.
The table shows that a gas with a lifetime of about a century has about the same 20- and 100-year GWPs, which suggests that the concentrations of this gas and of CO2 disappear roughly in parallel during this period. For the short-lived gases, the GWPs decline with time as they disappear faster than the CO2 standard. Conversely, the GWPs for very long-lived gases, like SF6, increase with time as they remain in the atmosphere longer than CO2.
Although the production and use of CFCs has been phased out, they have substantial lifetimes in the atmosphere and will persist through the 21st century. Similarly, the HFCs, whose phase-out is ongoing, will continue to build up during the century. These halogenated gases have very high GWPs, largely because they have multiple intense absorptions in the thermal IR region. However, the actual contribution of a greenhouse gas to atmospheric warming depends not only on its GWP, but also on its concentration. At present, the concentrations of halogenated gases in the atmosphere are low and their combined radiative forcing is only about one-fifth that of CO2. Because of overlapping absorptions, total radiative forcings and GWPs are not simple sums of values for individual gases, but require calculating the effects for individual gases over narrow wavelength ranges and summing these over the whole thermal IR spectral region.
GWPs for water vapor and tropospheric O3 are not calculated because their atmospheric lifetimes are only days long and their concentrations highly variable. More to the point for water vapor is that human activities have almost no direct influence on its tropospheric concentration, which is controlled by the temperature of the atmosphere and the liquid water from which it evaporates (or ice from which it sublimes). Rising planetary temperature increases the amount of water vapor in the atmosphere, which increases its warming effect. This is a feedback mechanism that adds substantially to the radiative forcings of the other non-condensable greenhouse gases.
All molecules have positive (nuclei) and negative (electron clouds) regions, A molecule is dipolar and has a permanent dipole moment, if the averaged centers of its positively and negatively charged regions do not coincide. If a vibrational motion of the molecule disturbs these averages, its dipole moment can change and an appropriate energy of IR radiation can be absorbed to cause this molecular vibration. As an example, consider the CO2 molecule. The more electronegative oxygen atoms attract electron density that makes the ends of the molecule slightly negative. The central carbon atom is therefore slightly positive, as represented in the diagram. Since the molecule is linear with equal bond lengths, the center of negative charge and the center of positive charge coincide at the central point, the carbon atom, and the molecule has no permanent dipole moment. The symmetrical stretching vibration, top representation, does not change this symmetry, does not change the dipole moment, and does not lead to IR absorption. The molecular bending vibrations, middle two representations, displace the negative charges away from the line of centers of the molecule and create a structure with a dipole moment. Thus, the dipole moment changes (from zero to some value) and these motions can be initiated by the absorption of IR radiation. This absorption gives rise to the prominent absorption band centered at about 15 μm. Likewise, for the asymmetric stretching vibration, bottom representation, the average bond lengths become unequal, which moves the positive and negative centers apart, creates a dipole moment, and leads to the IR absorptions at about 4 μm.
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