Global-warming potential
The (relative) GWP ( global warming potential well, english Global warming potential, greenhouse warming potential or GWP) or CO2 -equivalent of a chemical compound is a measure of the relative effect of the contribution to the greenhouse effect. So you are on, how much contributes a fixed mass of a greenhouse gas to global warming. Is the comparative value of carbon dioxide; the abbreviation is CO2e ( for equivalent). The value is the average heating effect over a specific period; often 100 years are considered.
For example, the CO2 equivalent of methane at a time horizon of 100 years, 25: This means that one kilogram of methane within the first 100 years after the release 25 times as much to the greenhouse effect contributes like a kg of CO2.
But the global warming potential is not the same as the actual proportion of the global warming since differ strongly with the emission levels of various gases. With this concept, the different contributions of individual greenhouse gases can be compared with known quantities of emissions.
In the first commitment period of the Kyoto Protocol emission levels are measured using the CO2 equivalents of the individual gases and weighted according to their global warming potentials. This means that, for example, a methane emission reduction of one tonne is equivalent to a CO2 reduction of 21 tons, since in both cases emissions incurred in the amount of 21 tons of CO2 equivalent less. Decisive here are the numbers according to the second assessment report of the IPCC in 1995, with a time horizon of 100 years.
However, the IPCC itself admits GWP values for time horizons of 20 years, 100 years and 500 years, and stresses that the choice is determined by political considerations. For example, is a long time horizon to choose if preferred curbing the long-term consequences of global warming will be sought.
Values of greenhouse gas potentials
Factors
The relative global warming potential (GWP ) of a greenhouse gas is determined by various factors. Contrary to occasionally raised objection is in addition to the atmosphere emitted carbon dioxide ( CO2) in a position to contribute to the greenhouse effect, although the existing CO2 already almost completely absorbs the heat radiation within its absorption bands.
The Earth emits on average at a height of 5500 m heat into space from, not at sea level. An increase in greenhouse gas concentrations causes there that the area in which the earth radiates its heat into space, moves upwards. Thus, there is equally strong thermal radiation, also the temperature field has to move upwards. The temperature thus increases according to the temperature gradient.
Influence the absorption behavior
The effect of a greenhouse is based on its ability, the heat radiation reflected from the surface to absorb in the infrared region and so as to heat the atmosphere ( greenhouse effect). Here, since the additional heating effect of the gas is considered, in particular its absorption behavior in those spectral regions of importance in which the naturally occurring greenhouse gases ( mainly water vapor and carbon dioxide ) or not absorb very little. This is particularly the so-called atmospheric window in the region 8-13 microns in wavelength.
Effect of concentration and molecular geometry
The radiative forcing of a greenhouse gas depends nonlinearly on its concentration. This non-linear dependence is approximately a logarithmic function. This means that a change in concentration of for example 2 to 3 ppm of the same effect as a change in concentration of 20 ppm to 30 ppm ( or 200 ppm to 300 ppm, etc.) has. Apart from the greater in comparison with, for example, CO2 number of possible modes of vibration of complex molecules, this is another reason that the change in concentration of an absorbent in the atmospheric window trace gas that natürlichserseits not or only exists in extremely small concentrations, so a strong effect, as indicated in the table.
The absorption behavior of a greenhouse gas, that is, in which the wavelength regions, it may absorb the heat radiation depends on the molecular structure of the respective gas.
Influence of the residence time
Also essential is the mean residence time of the gas in the atmosphere. Here, also, the selected time horizon has an important role. Thus, fluorine-containing greenhouse gases due to their high residence time (eg 3200 years for SF6) in the atmosphere a much higher GWP than greenhouse gases without fluorine atoms in the molecule, even for long time horizons. Methane ( residence time about 12 years) acts on the other hand the short term, its GWP is therefore much greater than for a long time for a short time horizons.