Editing IPCC:AR6/SR15/Chapter-2 (section)

==== 2.3.3.1 Emissions of long-lived climate forcers ====

<div id="section-2-3-3-1-block-1"></div>

Climate effects of long-lived climate forcers (LLCFs) are dominated by CO <sub>2</sub> , with smaller contributions of N <sub>2</sub> O and some fluorinated gases (Myhre et al., 2013; Blanco et al., 2014) <sup>[[#fn:r219|219]]</sup> . Overall net CO <sub>2</sub> emissions in pathways are the result of a combination of various anthropogenic contributions (Figure 2.5) (Clarke et al., 2014) <sup>[[#fn:r220|220]]</sup> : (i) CO <sub>2</sub> produced by fossil-fuel combustion and industrial processes, (ii) CO <sub>2</sub> emissions or removals from the agriculture, forestry and other land use (AFOLU) sector, (iii) CO <sub>2</sub> capture and sequestration (CCS) from fossil fuels or industrial activities before it is released to the atmosphere, (iv) CO <sub>2</sub> removal by technological means, which in current pathways is mainly achieved by BECCS and AFOLU-related CDR, although other options could be conceivable (see Chapter 4, Section 4.3.7). Pathways apply these four contributions in different configurations (Figure 2.5) depending on societal choices and preferences related to the acceptability and availability of certain technologies, the timing and stringency of near-term climate policy, and the ability to limit the demand that drives baseline emissions (Marangoni et al., 2017; Riahi et al., 2017; Grubler et al., 2018; Rogelj et al., 2018; van Vuuren et al., 2018) <sup>[[#fn:r221|221]]</sup> , and come with very different implication for sustainable development (Section 2.5.3).

All 1.5°C pathways see global CO <sub>2</sub> emissions embark on a steady decline to reach (near) net zero levels around 2050, with 1.5°C-low-OS pathways reaching net zero CO <sub>2</sub> emissions around 2045–2055 (Table 2.4; Figure 2.5). Near-term differences between the various pathway classes are apparent, however. For instance, Below-1.5°C and 1.5°C-low-OS pathways show a clear shift towards lower CO <sub>2</sub> emissions in 2030 relative to other 1.5°C and 2°C pathway classes, although in all 1.5°C classes reductions are clear (Figure 2.6). These lower near-term emissions levels are a direct consequence of the former two pathway classes limiting cumulative CO <sub>2</sub> emissions until carbon neutrality in order to aim for a higher probability of limiting peak warming to 1.5°C (Section 2.2.2 and 2.3.2.2). In some cases, 1.5°C-low-OS pathways achieve net zero CO <sub>2</sub> emissions one or two decades later, contingent on 2030 CO <sub>2</sub> emissions in the lower quartile of the literature range, that is, below about 18 GtCO <sub>2</sub> yr <sup>−</sup> <sup>1</sup> . Median year-2030 global CO <sub>2</sub> emissions are of the order of 5–10 GtCO <sub>2</sub> yr <sup>−</sup> <sup>1</sup> lower in Below-1.5°C compared to 1.5°C-low-OS pathways, which are in turn lower than 1.5°C-high-OS pathways (Table 2.4). Below-1.5°C and 1.5°C-low-OS pathways combined show a decline in global net anthropogenic CO <sub>2</sub> emissions of about 45% from 2010 levels by 2030 (40–60% interquartile range). Lower-2°C pathways show CO <sub>2</sub> emissions declining by about 25% by 2030 in most pathways (10–30% interquartile range). The 1.5°C-high-OS pathways show emissions levels that are broadly similar to the 2°C-consistent pathways in 2030.

The development of CO <sub>2</sub> emissions in the second half of the century in 1.5°C pathways is characterized by the need to stay or return within a carbon budget. Figure 2.6 shows net CO <sub>2</sub> and N <sub>2</sub> O emissions from various sources in 2050 and 2100 in 1.5°C pathways in the literature. Virtually all 1.5°C pathways obtain net negative CO <sub>2</sub> emissions at some point during the 21st century, but the extent to which net negative emissions are relied upon varies substantially (Figure 2.6, Table 2.4). This net withdrawal of CO <sub>2</sub> from the atmosphere compensates for residual long-lived non-CO <sub>2</sub> GHG emissions that also accumulate in the atmosphere (like N <sub>2</sub> O) or cancels some of the build-up of CO <sub>2</sub> due to earlier emissions to achieve increasingly higher likelihoods that warming stays or returns below 1.5°C (see Section 2.3.4 for a discussion of various uses of CDR). Even non-overshoot pathways that aim at achieving temperature stabilization would hence deploy a certain amount of net negative CO <sub>2</sub> emissions to offset any accumulating long-lived non-CO <sub>2</sub> GHGs. The 1.5°C overshoot pathways display significantly larger amounts of annual net negative CO <sub>2</sub> emissions in the second half of the century. The larger the overshoot the more net negative CO <sub>2</sub> emissions are required to return temperatures to 1.5°C by the end of the century (Table 2.4, Figure 2.1).

N <sub>2</sub> O emissions decline to a much lesser extent than CO <sub>2</sub> in currently available 1.5°C pathways (Figure 2.6). Current IAMs have limited emissions-reduction potentials (Gernaat et al., 2015) <sup>[[#fn:r222|222]]</sup> (Sections 2.3.1.2, 2.4.4, Supplementary Material  2.SM.1.2), reflecting the difficulty of eliminating N <sub>2</sub> O emission from agriculture (Bodirsky et al., 2014) <sup>[[#fn:r223|223]]</sup> . Moreover, the reliance of some pathways on significant amounts of bioenergy after mid-century (Section 2.4.2) coupled to a substantial use of nitrogen fertilizer (Popp et al., 2017) <sup>[[#fn:r224|224]]</sup> also makes reducing N <sub>2</sub> O emissions harder (for example, see pathway ''S5'' in Figure 2.6). As a result, sizeable residual N <sub>2</sub> O emissions are currently projected to continue throughout the century, and measures to effectively mitigate them will be of continued relevance for 1.5°C societies. Finally, the reduction of nitrogen use and N <sub>2</sub> O emissions from agriculture is already a present-day concern due to unsustainable levels of nitrogen pollution (Bodirsky et al., 2012) <sup>[[#fn:r225|225]]</sup> . Section 2.4.4 provides a further assessment of the agricultural non-CO <sub>2</sub> emissions reduction potential.

<div id="section-2-3-3-1-block-2"></div>

<span id="figure-2.6"></span>
<!-- START IMG -->
<!-- IMG TITLE -->
'''Figure 2.6'''

<span id="section-4"></span>
<!-- IMG CAPTION -->
Annual global emissions characteristics for 2020, 2030, 2050, 2100.
<!-- IMG FILE -->
[[File:9943509a36d49b42308928cfd6843289 Figure-2.6-766x1024.jpg]]

Data are shown for (a) Kyoto-GHG emissions, and (b) global total CO <sub>2</sub> emissions, (c) CO <sub>2</sub> emissions from the agriculture, forestry and other land use (AFOLU) sector, (d) global N2O emissions, and (e) CO <sub>2</sub> emissions from fossil fuel use and industrial processes. The latter is also split into (f) emissions from the energy supply sector (electricity sector and refineries) and (g) direct emissions from fossil-fuel use in energy demand sectors (industry, buildings, transport) (bottom row). Horizontal black lines show the median, boxes show the interquartile range, and whiskers the minimum–maximum range. Icons indicate the four pathway archetypes used in this chapter. In case less than seven data points are available in a class, the minimum–maximum range and single data points are shown. Kyoto-GHG, emissions in the top panel are aggregated with AR4 GWP-100 and contain CO <sub>2</sub> , CH4, N2O, HFCs, PFCs, and SF6. NF3 is typically not reported by IAMs. Scenarios with year-2010 Kyoto-GHG emissions outside the range assessed by IPCC AR5 WGIII assessed are excluded (IPCC, 2014b) <sup>[[#fn:r226|226]]</sup> .

Original Creation for this Report using IAMC 1.5°C Scenario Data hosted by IIASA

<!-- END IMG -->
<div id="section-2-3-3-2"></div>

<span id="emissions-of-short-lived-climate-forcers-and-fluorinated-gases"></span>