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Global warming potential

From Wikipedia, Global warming potential

Global warming potential (GWP) is a measure of how much a given mass of greenhouse gas is estimated to contribute to global warming. It is a relative scale which compares the gas in question to that of the same mass of carbon dioxide. A GWP is calculated over a specific time interval.

The GWP depends on the following factors:

  • the absorption of infrared radiation by a given species
  • the spectral location of its absorbing wavelengths
  • the atmospheric lifetime of the species

The GWP is defined via the notion of radiative forcing (RF) (p.133, section 2.2 in below source Forster et al):

The definition of RF from the TAR and earlier IPCC assessment reports is retained. Ramaswamy et al. (2001) define it as “the change in net (down minus up) irradiance (solar plus longwave; in W m^2) at the tropopause after allowing for stratospheric temperatures to readjust to radiative equilibrium, but with surface and tropospheric temperatures and state held fixed at the unperturbed values”.

The GWP is then defined on page 210 section 2.10 as “the time-integrated global mean RF of a pulse emission of 1 kg of some compound (i) relative to that of1 kg of the reference gas CO2”. It was developed by (IPCC, 1990) and adopted for use in the Kyoto Protocol.

A diagram of the socalled “instantanous all-sky” RF can be found in Fig. 2.23 on p. 208. It is however not clear wether these are the values, which entered the formula for the GWP.

On p. 211 it is noted that:

“The adequacy of the GWP concept has been widely debated since its introduction (O’Neill, 2000; Fuglestvedt et al., 2003).”

and

“However, as long as it has not been determined, neither scientifically, economically nor politically, what the proper time horizon for evaluating ‘dangerous anthropogenic interference in the climate system’ should be, the lack of temporal equivalence does not invalidate the GWP concept or provide guidance as to how to replace it. Although it has several known shortcomings, a multi-gas strategy using GWPs is very likely to have advantages over a CO2-only strategy (O’Neill, 2003). Thus, GWPs remain the recommended metric to compare future climate impacts of emissions of long-lived climate gases.”

It is sofar not allowed to cite from the IPCC report Nr. 5. The draft is however available (Okt 2013). On p. 58 in table 8.7 new values of GWPs can be found.

Some data

A list of six greenhouse gases with their 100 yr global warming potential.

CO 2CO_2 1

CH 4CH_4 25

N 2O\mathrm{N}_2\!\mathrm{O} 298

HFCs 124 - 14800

SF 6SF_6 22800

PFCs 7390 - 12200

Source: Forster et al Changes in Atmospheric Constituents and in Radiative Forcing in: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Solomon et al (eds.) CUP 2007. p. 212 table 2.14.

Suggested reading

Note: the table in the article above differs from the IPCC table abovementioned. This is because the IPCC looked at indirect effects as well as the direct effects. E.g. for methane they report:

Four indirect radiative effects of CH4 emissions have been identified (see Prather et al., 2001; Ramaswamy et al., 2001). Methane enhances its own lifetime through changes in the OH concentration: it leads to changes in tropospheric ozone, enhances stratospheric water vapour levels, and produces CO2. The GWP given in Table 2.14 includes the first three of these effects.

Historical reference

S. Arrhenius, On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground Philosophical Magazine and Journal of Science (fifth series) 41 (1896) 237-276.

category: climate