J 0ar [Crtdt

where TH is the time horizon, RFi is the global mean RF (radiative forcing) of gas i, ai is the RF per unit mass increase in atmospheric abundance of gas i (radiative efficiency), [C(t)] is the time-dependent abundance of i, and the corresponding quantities for the reference gas (r) in the denominator. The numerator and denominator are called the absolute global warming potential (AGWP) of i and r, respectively (IPCC, 2007).

The use of a pulse emission in the GWP calculation has been the subject of considerable debate (for example O'Neill, 2000), but the concept, even with its limitations, makes possible a useful evaluation of the relative climate change impacts of the long-lived greenhouse gases. Table 4.2 shows the GWPs of CO2, methane and N2O, calculated over three timescales: 20, 100 and 500 years. The 100-year time horizon is the one most commonly used in climate change assessments, and over this period N2O is almost 300 times as potent as CO2 as a global warming agent.

Table 4.2 Global warming potentials of the long-lived greenhouse gases CO2, CH4 and N2O

Greenhouse gas

GWP for each time horizon

20 years

100 years

500 years

CO2

1

1

1

CH4

72

25

7.6

N2O

289

298

153

Source: IPCC (2007)

Source: IPCC (2007)

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