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Glossary:GWP
A factor which estimates the contribution to global warming of a given mass of a greenhouse gas species, relative to the same mass of CO2 over a particular time frame.
(Note - The time period for any quoted GWP is important as there are significant differences between 1-year, 20-year and 100-year GWPs; for example, the GWP for methane is understood to range from 25 to 84 depending on the timeline adopted. The time period usually used for GHG inventory reporting is 100 years. The IPCC Assessment Reports provide relevant GWPs for methane and other GHGs.)[1]
GWP was developed to allow comparisons of the global warming impacts of different gases. Specifically, it is a measure of how much energy the emissions of one ton of a gas will absorb over a given period, relative to the emissions of one ton of carbon dioxide (CO2). The larger the GWP, the more that a given gas warms the Earth compared to CO2 over that time period. The time period typically used for GWPs is 100 years. GWPs provide a common unit of measure, which allows analysts to add up emissions
estimates of different gases (e.g., to compile a national GHG inventory), and allows policymakers to compare emissions reduction opportunities across sectors and gases.
CO2, by definition, has a GWP of 1 regardless of the time period used, because it is the gas being used as the reference. CO2 remains in the climate system for a very long time: CO2 emissions cause increases in atmospheric concentrations of CO2 that may last thousands of years.
Methane (CH4) is estimated to have a GWP of 28–36 over 100 years. CH4 emitted today lasts about a decade on average, which is much less time than CO2. But CH4 also absorbs much more energy than CO2. The net effect of the shorter lifetime and higher energy absorption is reflected in the GWP. The CH4 GWP also accounts for some indirect effects, such as the fact that CH4 is a precursor to ozone, and ozone is itself a GHG.
Nitrous Oxide (N2O) has a GWP 265–298 times that of CO2 for a 100-year timescale. Chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6) are sometimes called high-GWP gases because, for a given amount of mass, they trap substantially more heat than CO2. The GWPs for these gases can be in the thousands or tens of thousands.
GWP is a major analytical and policy issue impacting how methane emissions are reported concerns how to translate methane emissions into carbon dioxide equivalent. Methane is a much more potent greenhouse gas than carbon dioxide, although it has a much shorter atmospheric life. Emissions are very often reported in terms of carbon dioxide equivalent (CO2e), which requires an assessment of the global warming potential (GWP) of methane. The most-common metrics are that the radiative forcing impact of methane is 28–36 times that of CO2 measured over a 100–year time horizon, and 84–87 times over a 20-year horizon.
The measurement, reporting and verification of methane emissions using a transparent and globally accepted methodology has become a crucial issue. Given the level of public scrutiny and policy focus on this issue, it has become absolutely vital that the gas industry takes proactive steps to create and implement a global plan both to reduce, but first to accurately document, methane emissions.[2]