The Global Temperature change Potential (GTP) is another way to compare gases. GWP definition is also based on emissions, and anthropogenic emissions of water vapour ( cooling towers, irrigation) are removed via precipitation within weeks, so its GWP is negligible. But the GWP definition excludes indirect effects. Its concentration in the atmosphere is limited by air temperature, so that radiative forcing by water vapour increases with global warming (positive feedback). H 2O is the strongest greenhouse gas, because it has a profound infrared absorption spectrum with more and broader absorption bands than CO 2. Water vapour does contribute to anthropogenic global warming, but as the GWP is defined, it is negligible for H 2O. Ĭommonly, a time horizon of 100 years is used by regulators. The GWP for a mixture of gases can be obtained from the mass-fraction-weighted average of the GWPs of the individual gases. For this reason when quoting a GWP it is important to give a reference to the calculation. This is often not precisely known and hence the values should not be considered exact. The GWP value depends on how the gas concentration decays over time in the atmosphere. Thus methane has a potential of 25 over 100 years (GWP 100 = 25) but 86 over 20 years (GWP 20 = 86) conversely sulfur hexafluoride has a GWP of 22,800 over 100 years but 16,300 over 20 years (IPCC Third Assessment Report). A gas which is quickly removed from the atmosphere may initially have a large effect, but for longer time periods, as it has been removed, it becomes less important. Trifluoromethyl sulphur pentafluoride (SF 5CF 3)Ī substance's GWP depends on the number of years (denoted by a subscript) over which the potential is calculated. Those 2007 estimates are still used for international comparisons through 2020, although the latest research on warming effects has found other values, as shown in the table above. They published these values in Annex III, and they took them from the 4th Assessment Report of the Intergovernmental Panel on Climate Change, which had been published in 2007. Īfter some intermediate updates, in 2013 this standard was updated by the Warsaw meeting of the UN Framework Convention on Climate Change (UNFCCC, decision 24/CP.19) to require using a new set of 100-year GWP values. Under the Kyoto Protocol, in 1997 the Conference of the Parties standardized international reporting, by deciding (decision 2/CP.3) that the values of GWP calculated for the IPCC Second Assessment Report were to be used for converting the various greenhouse gas emissions into comparable CO 2 equivalents. 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). For a starting amount of 1 tonne of methane, which has a GWP of 25, after combustion there would be 2.74 tonnes of CO 2, each tonne of which has a GWP of 1. For instance, burning methane to carbon dioxide would reduce the global warming impact, but by a smaller factor than 25:1 because the mass of methane burned is less than the mass of carbon dioxide released (ratio 1:2.74). 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.
The total impact of all fluorinated gases is estimated at 3% of all greenhouse gas emissions.
Some have high GWP but only a low concentration in the atmosphere. The IPCC lists many other substances not shown here. AR5 has skipped 500 year values but introduced GWP estimations including the climate-carbon feedback (f) with a large amount of uncertainty.
Though recent reports reflect more scientific accuracy, countries and companies continue to use SAR and AR4 values for reasons of comparison in their emission reports. Įstimates of GWP values over 20, 100 and 500 years are periodically compiled and revised in reports from the Intergovernmental Panel on Climate Change: CO 2 emissions cause increases in atmospheric concentrations of CO 2 that will last thousands of years. It has a GWP of 1 regardless of the time period used.
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