Global Warming Potential

Radiative forcing is an immediate effect of incremental emission of a greenhouse gas, but, to estimate the long-term effects, the residence time of the gas must be considered as an additional factor. In 1990, the global warming potential (GWP) was introduced to describe the long-term contribution of a gas to global warming; this index is weighted according to the atmospheric lifetime of the gas. According to the IPCC, GWP is defined as an index, describing the radiative characteristics of well-mixed greenhouse gases, which represents the combined effect of the differing times these gases remain in the atmosphere and their relative effectiveness in absorbing outgoing IR radiation. This index is a measure of the relative radiative effect of a given substance compared to another (CO2), integrated over a chosen time horizon, approximating the time-integrated warming effect of a unit mass of a given greenhouse gas in today's atmosphere, relative to that of CO2. GWP is used to estimate the potential future impacts of emissions of different gases upon the climate system in a relative sense; hence, it is a convenient measure of the role that a greenhouse gas plays in global warming. Formally, a, (t )L, (t )dt

aCO2 (t)LCO2 (t)dt where H is the time horizon; a(t) and aCo2(t) are the instantaneous radiative forcing due to a unit increase in the concentration of the gas i and CO2; L(t) and LCo2(t) are the respective residence times. In brief, GWP of a gas is given by the ratio of cumulative radiative forcing due to a unit emission impulse of a gas to that due to a unit impulse of CO2 in a lapse of time. By definition, the GWP of CO2 is 1. GWP would mean, for example, that the effects of the emission of one unit of methane over the next 100 years are equivalent to the effect of the emission of 23 units of CO2.

Table 1 Summary of GWPs and lifetimes of some important greenhouse gases according to the IPCC Third Assessment Report

Lifetime (years) GWP20y GWP,00y GWP5(J0y

Table 1 Summary of GWPs and lifetimes of some important greenhouse gases according to the IPCC Third Assessment Report

Lifetime (years) GWP20y GWP,00y GWP5(J0y

Carbon dioxide

CO2

50-200

1

1

1

Methane

CH4

8.4-12

62

23

7

Nitrous oxide

N2O

114-120

275

296

156

HFC-23

CHF3

260

9400

12 000

10 000

HFC-32

CH2F2

5

1800

550

170

Sulfur hexaflouride

SFe

3200

15100

22200

32 400

GWPs are calculated for three time horizons (20, 100, and 500 years).

GWPs are calculated for three time horizons (20, 100, and 500 years).

GWPs are obtained by a simple calculation procedure and have been calculated for several greenhouse gases and for different time horizons, as shown in the IPCC 1996 Second Assessment Report (subsequently revised in the 2001 Third Assessment Report). A brief list of GWPs and lifetimes is reported in Table 1.

The lifetime of a greenhouse gas affects the calculation of GWP. When a gas has a lifetime comparable with that of CO2, its GWP does not depend on the choice of time horizon (see, e.g., the case of N2O). If the lifetime of a gas and that of CO2 differ, the GWP is also affected by the choice of time horizon. In particular, if the lifetime of a gas is shorter than that of CO2, the GWP decreases during the time horizon (CH4); GWPs increase during the time horizon when gas lifetimes are longer than that of CO2 (SF6).

GWP is useful to convert the emissions of different greenhouse gases to a common unit, according to the formula:

Aemission (gasy) ^ Aemission(CO2)

where Aemission(gasy) is the incremental emission of greenhouse gasy, GWPy its GWP and Aemission (CO2) the corresponding value in terms of CO2 equivalent.

The common unit is thus the CO2 equivalent which is used to devise systems for accounting human impact on climate change and to assign responsibility for emissions.

Assessing the relative importance of the emission of different greenhouse gases on a common basis has fundamental implications in the context of global warming mitigation policies. Its introduction, enabling measurement of the weight of reductions in the emission of different greenhouse gases, is the basis of the IPCC National Greenhouse Gas Inventory (see below). Furthermore, the 1997 Kyoto Protocol explicitly refers to GWP as a useful standard for estimating the effectiveness of emission abatement and for implementing so-called flexible mechanisms, such as international emission trading.

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