Editing IPCC:AR6/WGI/Chapter-5 (section)

=== 5.2.4 The Relative Importance of CO <sub>2</sub> , CH <sub>4</sub> , and N <sub>2</sub> O ===

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The total influence of anthropogenic greenhouse gases (GHGs) on the Earth’s radiative balance is driven by the combined effect of those gases, and the three most important – carbon dioxide (CO <sub>2</sub> ), methane (CH <sub>4</sub> ), nitrous oxide (N <sub>2</sub> O) – were discussed in the previous sections. This section compares the balance of the sources and sinks of these three gases and their regional net flux contributions to the radiative forcing. CO <sub>2</sub> has multiple residence times in the atmosphere – from one year to many thousands of years (Box 6.1 in [[#Ciais--2013|Ciais et al., 2013]] ) – and N <sub>2</sub> O has a mean lifetime of 116 years. They are both long-lived GHGs, while CH <sub>4</sub> has a lifetime of 9.1 years and is considered a short-lived GHG (see [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] for lifetime of GHGs, [[IPCC:Wg1:Chapter:Chapter-6|Chapter 6]] for CH <sub>4</sub> chemical lifetime, and [[IPCC:Wg1:Chapter:Chapter-7|Chapter 7]] for effective radiative forcing of all GHGs).

Figure 5.18 shows the contribution to radiative forcing of CO <sub>2</sub> , CH <sub>4</sub> , N <sub>2</sub> O, and the halogenated species since the 1900s to more recent decades. For the period 1960–2019, the relative contribution to the total effective radiative forcing (ERF) was 63% for CO <sub>2</sub> , 11% for CH <sub>4</sub> , 6% for N <sub>2</sub> O, and 17% for the halogenated species (Chapter 7; Figure 5.18). The systematic decline in the relative contribution to ERF for CH <sub>4</sub> since 1850 is caused by a slower increase rate of CH <sub>4</sub> in the recent decades, at 6, 10 and 5 ppb yr <sup>–1</sup> during 1850–2019, 1960–2019 and 2000–2019, respectively, in comparison with the increasing rate of CO <sub>2</sub> (at 0.7, 1.6 and 2.2 ppm yr <sup>–1</sup> , respectively) and N <sub>2</sub> O (at 0.4, 0.7 and 0.9 ppb yr <sup>–1</sup> , respectively; Figure 5.4). Owing to the shorter lifetime of CH <sub>4</sub> , the effect of a reduction in the emissions increase rate on the ERF increase is evident at inter-decadal time scales.

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'''Figure 5.18 |''' '''Contributions of carbon dioxide (CO''' <sub>2</sub> '''), methane (CH''' <sub>4</sub> '''), nitrous oxide (N''' <sub>2</sub> '''O) and halogenated species to the total effective radiative forcing (ERF) increases in 2019 since 1850, 1960 and 2000, respectively''' . ERF data are taken from [[IPCC:Wg1:Chapter:Annex-iii|Annex III]] (based on calculations from Chapter 7). Note that the sum of the ERFs exceeds 100% because there are negative ERFs due to aerosols and clouds. Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

Atmospheric abundance of GHGs is proportional to their emissions-loss budgets in the Earth’s environment. There are multiple metrics to evaluate the relative importance of different GHGs for the global atmospheric radiation budget and the socio-economic impacts ( [[IPCC:Wg1:Chapter:Chapter-7#7.6|Section 7.6]] ). Metrics for weighting emissions are further developed in AR6 WGIII. Figure 5.19 shows the regional emissions of the three main GHGs. For North Asia, Europe, Temperate North America and West Asia, the most dominant GHG source is CO <sub>2</sub> ( ''high confidence'' ) (Figure 5.19) while, for East Asia, South Asia, South East Asia, Tropical South America, Temperate North America and Central Africa, the source is CH <sub>4</sub> (Figure 5.19). The N <sub>2</sub> O emissions are dominant in regions with intense use of nitrogen fertilizers in agriculture. Only boreal North America showed net sinks of CO <sub>2</sub> , while close to flux neutrality is observed for North Asia, Southern Africa, and Australasia. Persistent emissions of CO <sub>2</sub> are observed for Tropical and South America, northern Africa, and South East Asia ( ''medium confidence'' ). The ''medium confidence'' arises from large uncertainties in the estimated non-fossil fuel CO <sub>2</sub> fluxes over these regions due to the lack of high-quality atmospheric measurements.

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'''Figure 5.19 |''' '''Regional distributions of net fluxes of carbon dioxide (CO''' <sub>2</sub> '''), methane (CH''' <sub>4</sub> '''), nitrous oxide (N''' <sub>2</sub> '''O) on the Earth’s surface.''' The region divisions, shown as the shaded map, are made based on ecoclimatic characteristics of the land. The fluxes include those from anthropogenic activities and natural causes that result from responses to anthropogenic greenhouse gases and climate change (feedbacks) as in the three budgets shown in Sections 5.2.1.5, 5.2.2.5, and 5.2.3.5. The CH <sub>4</sub> and N <sub>2</sub> O emissions are weighted by arbitrary factors of 50 and 500, respectively, for depiction by common y-axes. Fluxes are shown as the mean of the inverse models as available from Thompson et al. (2019); Friedlingstein et al. (2020); Saunois et al. (2020). Further details on data sources and processing are available in the chapter data Table (Table 5.SM.6).

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