Global warming potentials (GWP's) are factors which describe the relative warming effect of greenhouse gases compared with the benchmark warming effect of CO2 (i.e. CO2 has, by definition, a GWP of 1). These factors enable emissions from one or several distinct greenhouse gases to be expressed in terms of single, standardised unit of warming, i.e. the quantity of CO2 which would produce the same effect (CO2e):
mass of gas * GWP = mass of CO2e
For example, methane (CH4) is considered to have a GWP of 21, which is to say that it has an influence on global warming which is 21 times that of CO2. An annual CH4 emission of, say, 2000 kg would be equivalent to 42000 kg CO2 (i.e. 2000 x 21 = 42000), and could be expressed as 42000 kg CO2e. These types of calculations are used frequently within Discover in order to express the emissions of one or several greenhouse gases as a single quantity.
GWP's in Discover
The category global warming potentials provides reference data for over 80 greenhouse gases.
As well as providing a lookup reference for users, this category is used during calculations in many other categories (i.e. all those which include non-CO2 emissions calculations) for converting absolute greenhouse gas emissions into CO2-equivalent emissions.
As can be seen from the data table below, there are several choices of GWP for each gas. This reflects the fact that different greenhouse gases remain in the atmosphere for different periods of time, meaning that the warming effect of each gas - relative to CO2 - changes depending on the timescale under consideration. The IPCC therefore publishes three versions of GWP for each gas, representing their relative warming effect over 20-, 100- and 500-year time horizons. These data, from the 2007 Fourth Assessment Report (4AR), are represented by the gwp_4AR_20, gwp_4AR_100 and gwp_4AR_500 data item values in this category.
Discover also provides GWP's based upon a 100-year time horizon published in the IPCC Second Assessment Report (SAR)(1995), represented by the gwp_SAR_100 data item value. The use of these values for calculating CO2e emissions for many gases has become accepted practice following protocol developed by the United Nations Framework Convention on Climate Change.
When using GWP's for emissions calculations, Discover uses a default choice for each specific gas, represented in this category by the GWP data item value. The default choices follow accepted practice and are consistent with the guidelines published by the UK government agency DEFRA and the The World Resources Institute Greenhouse Gas Protocol. Typically, GWP's defined in the SAR represent the default choice, with those gases only defined subsequently being represented by default values published in the 4AR.