Global Warming Potential
Global Warming Potential (GWP) is based on the integrated RF of a pulse emission. It is used as a tool to compare the potency of different greenhouse gases with that of CO2. GWP calculates the integrated RF and lifetime of each gas relative to that of carbon dioxide . Carbon dioxide has an assigned GWP of 1 and is used as the baseline unit (i.e. reference gas) to which all other greenhouse gases are compared. Thus GWP is unit-less. GWP values can be used to convert various greenhouse gas emissions into comparable CO2 equivalents when computing overall sources and sinks; greenhouse gases can thus be expressed in terms of Carbon Dioxide Equivalent (CO2e).
Because they are based on integrated RFs, GWP values depend on the time span over which the potential is calculated. Short-lived GHGs initially have large effects that become less significant over time relative to CO2, since the integrated RF of CO2 increases over time. Methane, for example, has a GWP of approximately 25 over 100 years but 62 over 20 years. The Kyoto Protocol uses the GWP time frame of 100 years. If a climate policy is enacted to limit long-term temperature increase, effects of short-lived emissions may be overestimated if the time horizon chosen is too short. On the other hand, a time horizon of 100 years versus one of 20 years might underestimate the importance of short-lived emissions.
Both integrated RF and GWP do not indicate when the temperature changes occur given the varying residence times and warming capacities of different GHGs. GWPs also fail to account for thermal inertia of the climate system. GWP furthermore assumes that the warming response due to the various GHGs’ forcings are all the same. Yet, as explained earlier, climate efficacy of different GHGs can vary considerably.
While GWP is widely accepted as a reliable proxy for the warming impacts of long-lived, well-dispersed gases such as CO2, GWP with a 100-year time frame, as used in the Kyoto Protocol, is not suitable for measuring the kind of short-lived emissions associated with aviation. Or, as the IPCC states:
To assess the possible climate impacts of short-lived species and compare those with the impacts of the LLGHGs [long-lived greenhouse gases], a metric is needed. However, there are serious limitations to the use of global mean GWPs for this purpose. While the GWPs of the LLGHGs do not depend on location and time of emissions, the GWPs for short-lived species will be regionally and temporally dependent.