Global Warming Potential (GWP) : Water Vapor – Exception (WVE)

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  • Compare emission and reduction of different gases
  • Applied to units of mass & not volume
  • 3 Key Factors determining GWP
  • Gases absorption of infrared radiation
  • Where along the electromagnetic spectrum the gas absorbs radiation
  • Atmospheric lifetime of the gas
  • GWP is how much heat a greenhouse gas traps in the atmosphere up to a specific time horizon, relative to carbon dioxide.
  • It is measured in 20,100 or 500 years.
  • High GWP correlates with a large infrared absorption and a long atmospheric lifetime.
  • Global warming potential (GWP) is defined as the cumulative radiative forcing, both direct and indirect effects, over a specified time horizon resulting from the emission of a unit mass of gas related to some reference gas. Carbon dioxide (CO2) was chosen as the reference gas to be consistent with the guidelines of the Intergovernmental Panel on Climate Change.
  • Greenhouse gas emissions are commonly presented in units of billion tonnes of carbon dioxide equivalent (Gt CO2e) . The de facto way of converting non-CO2 emissions to CO2e is to multiply the gas by its GWP100 (global warming potential over 100 years) . The value of GWP100 for methane (CH4) from the last IPCC assessment report is 28. This means that methane has 28 times as much “global warming potential” as CO2, so 1Gt CH4 equates to 28 GtCO2e.

Global Warming Potential

Global Warming Potential
  • Global warming potential (GWP) values relative to CO2
  • Values based on assessment report AR5


  • Methane < Nitrous Oxide < HFC-134a < CFC-11 < CF4 < PFTBA (perfluorotributylamine) < HFC-23 < SF6
  • PFTBA is used in electrical industry, it has radiative efficiency (relative effectiveness of greenhouse gases to restrict long-wave radiation from escaping back into space) of any molecule detected in the atmosphere to date

Water Vapor – Exception

  • Water vapour is GHG but it GWP is not calculated directly. It has a profound infrared absorption spectrum with more and broader absorption bands than CO2, and also absorbs non-zero amounts of radiation in its low absorbing spectral regions.
  • Its concentration in the atmosphere depends on air temperature and water availability; using a global average temperature of ~16 °C, for example, creates an average humidity of ~18,000ppm at sea level (CO2 is ~400ppm and so concentrations of [H2O] / [CO2] ~45x) .
  • Water vapor does not decay in the environment, so an average over some time horizon or some other measure consistent with “time dependent decay,” must be used in lieu of the time dependent decay of artificial or excess CO2 molecules.
  • Also, Earth՚s temperature distribution, and the differing land masses in the Northern and Southern hemispheres complicate calculations.