Global warming potential
Global warming potential (GWP) is a measure of how much heat a greenhouse gas traps in the atmosphere over a specific time period, relative to carbon dioxide (CO2).
As methane has a much shorter atmospheric lifetime than carbon dioxide, its GWP is much less over longer time periods, with a GWP-100 of 27.9 and a GWP-500 of 7.95.
"[1]: 2232 In turn, radiative forcing is a scientific concept used to quantify and compare the external drivers of change to Earth's energy balance.
[4] As governments develop policies to combat emissions from high-GWP sources, policymakers have chosen to use the 100-year GWP scale as the standard in international agreements.
The Kigali Amendment to the Montreal Protocol sets the global phase-down of hydrofluorocarbons (HFCs), a group of high-GWP compounds.
It requires countries to use a set of GWP100 values equal to those published in the IPCC's Fourth Assessment Report (AR4).
[5] The global warming potential (GWP) depends on both the efficiency of the molecule as a greenhouse gas and its atmospheric lifetime.
Conversely, if a molecule has a longer atmospheric lifetime than CO2 its GWP will increase when the timescale is considered.
[9]: Table 7.15 The decrease in GWP at longer times is because methane decomposes to water and CO2 through chemical reactions in the atmosphere.
Similarly the third most important GHG, nitrous oxide (N2O), is a common gas emitted through the denitrification part of the nitrogen cycle.
[9]: Table 7.15 [11] Estimates of GWP values over 20, 100 and 500 years are periodically compiled and revised in reports from the Intergovernmental Panel on Climate Change.
The values given in the table assume the same mass of compound is analyzed; different ratios will result from the conversion of one substance to another.
This is a net reduction of 22.26 tonnes of GWP, reducing the global warming effect by a ratio of 25:2.74 (approximately 9 times).
[23] A substance's GWP depends on the number of years (denoted by a subscript) over which the potential is calculated.
Its concentration in the atmosphere is limited by air temperature, so that radiative forcing by water vapour increases with global warming (positive feedback).
[31] Because the GWP of a greenhouse gas depends directly on its infrared spectrum, the use of infrared spectroscopy to study greenhouse gases is centrally important in the effort to understand the impact of human activities on global climate change.
Just as radiative forcing provides a simplified means of comparing the various factors that are believed to influence the climate system to one another, global warming potentials (GWPs) are one type of simplified index based upon radiative properties that can be used to estimate the potential future impacts of emissions of different gases upon the climate system in a relative sense.
GWP is based on a number of factors, including the radiative efficiency (infrared-absorbing ability) of each gas relative to that of carbon dioxide, as well as the decay rate of each gas (the amount removed from the atmosphere over a given number of years) relative to that of carbon dioxide.
For those gases, the relative radiative forcing will depend upon abundance and hence upon the future scenario adopted.
Clarifying this, while increasing CO2 has less and less effect on radiative absorption as ppm concentrations rise, more powerful greenhouse gases like methane and nitrous oxide have different thermal absorption frequencies to CO2 that are not filled up (saturated) as much as CO2, so rising ppms of these gases are far more significant.
The following units are commonly used: For example, the table above shows GWP for methane over 20 years at 86 and nitrous oxide at 289, so emissions of 1 million tonnes of methane or nitrous oxide are equivalent to emissions of 86 or 289 million tonnes of carbon dioxide, respectively.
[22] Those 2007 estimates are still used for international comparisons through 2020,[23] although the latest research on warming effects has found other values, as shown in the tables above.
The IPCC Fifth Assessment Report has skipped the 500-year values but introduced GWP estimations including the climate-carbon feedback (f) with a large amount of uncertainty.
While GWP estimates infrared thermal radiation absorbed, GTP estimates the resulting rise in average surface temperature of the world, over the next 20, 50 or 100 years, caused by a greenhouse gas, relative to the temperature rise which the same mass of CO2 would cause.
GWP* therefore assigns an increase in emission rate of an SLCP a supposedly equivalent amount (tonnes) of CO2.
[46] However GWP* has been criticised both for its suitability as a metric and for inherent design features which can perpetuate injustices and inequity.
