Editing IPCC:AR6/SR15/Chapter-1 (section)

====== Lead Authors ======

* Piers Forster (United Kingdom)
* Elmar Kriegler (Germany)
* Joeri Rogelj (Austria, Belgium)
* Seth Schultz (United States)
* Drew Shindell (United States)
* Kirsten Zickfeld (Canada, Germany)

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Emissions of many different climate forcers will affect the rate and magnitude of climate change over the next few decades (Myhre et al., 2013) <sup>[[#fn:r180|180]]</sup> . Since these decades will determine when 1.5°C is reached or whether a warming greater than 1.5°C is avoided, understanding the aggregate impact of different forcing agents is particularly important in the context of 1.5°C pathways. Paragraph 17 of Decision 1 of the 21st Conference of the Parties on the adoption of the Paris Agreement specifically states that this report is to identify aggregate greenhouse gas emission levels compatible with holding the increase in global average temperatures to 1.5°C above pre-industrial levels (see Chapter 2). This request highlights the need to consider the implications of different methods of aggregating emissions of different gases, both for future temperatures and for other aspects of the climate system (Levasseur et al., 2016; Ocko et al., 2017) <sup>[[#fn:r181|181]]</sup> .

To date, reporting of GHG emissions under the UNFCCC has used Global Warming Potentials (GWPs) evaluated over a 100-year time horizon (GWP <sub>100</sub> ) to combine multiple climate forcers. IPCC Working Group 3 reports have also used GWP <sub>100</sub> to represent multi-gas pathways (Clarke et al., 2014) <sup>[[#fn:r182|182]]</sup> . For reasons of comparability and consistency with current practice, Chapter 2 in this Special Report continues to use this aggregation method. Numerous other methods of combining different climate forcers have been proposed, such as the Global Temperature-change Potential (GTP; Shine et al., 2005) <sup>[[#fn:r183|183]]</sup> and the Global Damage Potential (Tol et al., 2012; Deuber et al., 2013) <sup>[[#fn:r184|184]]</sup> .

Climate forcers fall into two broad categories in terms of their impact on global temperature (Smith et al., 2012) <sup>[[#fn:r185|185]]</sup> : long-lived GHGs, such as CO <sub>2</sub> and nitrous oxide (N <sub>2</sub> O), whose warming impact depends primarily on the total cumulative amount emitted over the past century or the entire industrial epoch; and short-lived climate forcers (SLCFs), such as methane and black carbon, whose warming impact depends primarily on current and recent annual emission rates (Reisinger et al., 2012; Myhre et al., 2013; Smith et al., 2013; Strefler et al., 2014) <sup>[[#fn:r186|186]]</sup> . These different dependencies affect the emissions reductions required of individual forcers to limit warming to 1.5°C or any other level.

Natural processes that remove CO <sub>2</sub> permanently from the climate system are so slow that reducing the rate of CO <sub>2</sub> -induced warming to zero requires net zero global anthropogenic CO <sub>2</sub> emissions (Archer and Brovkin, 2008; Matthews and Caldeira, 2008; Solomon et al., 2009) <sup>[[#fn:r187|187]]</sup> , meaning almost all remaining anthropogenic CO <sub>2</sub> emissions must be compensated for by an equal rate of anthropogenic carbon dioxide removal (CDR). Cumulative CO <sub>2</sub> emissions are therefore an accurate indicator of CO <sub>2</sub> -induced warming, except in periods of high negative CO <sub>2</sub> emissions (Zickfeld et al., 2016) <sup>[[#fn:r188|188]]</sup> , and potentially in century-long periods of near-stable temperatures (Bowerman et al., 2011; Wigley, 2018) <sup>[[#fn:r189|189]]</sup> . In contrast, sustained constant emissions of a SLCF such as methane, would (after a few decades) be consistent with constant methane concentrations and hence very little additional methane-induced warming (Allen et al., 2018; Fuglestvedt et al., 2018) <sup>[[#fn:r190|190]]</sup> . Both GWP and GTP would equate sustained SLCF emissions with sustained constant CO <sub>2</sub> emissions, which would continue to accumulate in the climate system, warming global temperatures indefinitely. Hence nominally ‘equivalent’ emissions of CO <sub>2</sub> and SLCFs, if equated conventionally using GWP or GTP, have very different temperature impacts, and these differences are particularly evident under ambitious mitigation characterizing 1.5°C pathways.

Since the AR5, a revised usage of GWP has been proposed (Lauder et al., 2013; Allen et al., 2016) <sup>[[#fn:r191|191]]</sup> , denoted GWP* (Allen et al., 2018) <sup>[[#fn:r192|192]]</sup> , that addresses this issue by equating a permanently sustained change in the emission ''rate'' of an SLCF or SLCF-precursor (in tonnes-per-year), or other non-CO <sub>2</sub> forcing (in watts per square metre), with a one-off ''pulse'' emission (in tonnes) of a fixed amount of CO <sub>2</sub> . Specifically, GWP* equates a 1 tonne-per-year increase in emission rate of an SLCF with a pulse emission of GWP ''<sub>H</sub>'' x ''H'' tonnes of CO <sub>2</sub> , where  is the conventional GWP <sub>''H''</sub> of that SLCF evaluated over time GWP ''<sub>H</sub>'' for SLCFs decreases with increasing time H, GWP <sub>''H''</sub> x ''H'' for SLCFs is less dependent on the choice of time horizon. Similarly, a permanent 1 W m <sup>−2</sup> increase in radiative forcing has a similar temperature impact as the cumulative emission of ''H'' /AGWP <sub>''H''</sub> tonnes of CO <sub>2</sub> , where AGWP ''<sub>H</sub>'' is the Absolute Global Warming Potential of CO <sub>2</sub> (Shine et al., 2005; Myhre et al., 2013; Allen et al., 2018) <sup>[[#fn:r193|193]]</sup> . This indicates approximately how future changes in non-CO <sub>2</sub> radiative forcing affect cumulative CO <sub>2</sub> emissions consistent with any given level of peak warming.

When combined using GWP*, cumulative aggregate GHG emissions are closely proportional to total GHG-induced warming, while the annual rate of GHG-induced warming is proportional to the annual rate of aggregate GHG emissions (see Cross-Chapter Box 2, Figure 1). This is not the case when emissions are aggregated using GWP or GTP, with discrepancies particularly pronounced when SLCF emissions are falling. Persistent net zero CO <sub>2</sub> -equivalent emissions containing a residual positive forcing contribution from SLCFs and aggregated using GWP <sub>100</sub> or GTP would result in a steady decline of GMST. Net zero global emissions aggregated using GWP* (which corresponds to zero net emissions of CO <sub>2</sub> and other long-lived GHGs like nitrous oxide, combined with near-constant SLCF forcing – see Figure 1.5) results in approximately stable GMST (Allen et al., 2018; Fuglestvedt et al., 2018 <sup>[[#fn:r194|194]]</sup> and Cross-Chapter Box 2, Figure 1, below).

Whatever method is used to relate emissions of different greenhouse gases, scenarios achieving stable GMST well below 2°C require both near-zero net emissions of long-lived greenhouse gases and deep reductions in warming SLCFs (Chapter 2), in part to compensate for the reductions in cooling SLCFs that are expected to accompany reductions in CO <sub>2</sub> emissions (Rogelj et al., 2016b; Hienola et al., 2018) <sup>[[#fn:r195|195]]</sup> . Understanding the implications of different methods of combining emissions of different climate forcers is, however, helpful in tracking progress towards temperature stabilisation and ‘balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases’ as stated in Article 4 of the Paris Agreement. Fuglestvedt et al. (2018) <sup>[[#fn:r196|196]]</sup> and Tanaka and O’Neill (2018) <sup>[[#fn:r197|197]]</sup> show that when, and even whether, aggregate GHG emissions need to reach net zero before 2100 to limit warming to 1.5°C depends on the scenario, aggregation method and mix of long-lived and short-lived climate forcers.

The comparison of the impacts of different climate forcers can also consider more than their effects on GMST (Johansson, 2012; Tol et al., 2012; Deuber et al., 2013; Myhre et al., 2013; Cherubini and Tanaka, 2016) <sup>[[#fn:r198|198]]</sup> . Climate impacts arise from both magnitude and rate of climate change, and from other variables such as precipitation (Shine et al., 2015) <sup>[[#fn:r199|199]]</sup> . Even if GMST is stabilised, sea level rise and associated impacts will continue to increase (Sterner et al., 2014) <sup>[[#fn:r200|200]]</sup> , while impacts that depend on CO <sub>2</sub> concentrations such as ocean acidification may begin to reverse. From an economic perspective, comparison of different climate forcers ideally reflects the ratio of marginal economic damages if used to determine the exchange ratio of different GHGs under multi-gas regulation (Tol et al., 2012; Deuber et al., 2013; Kolstad et al., 2014) <sup>[[#fn:r201|201]]</sup> .

Emission reductions can interact with other dimensions of sustainable development (see Chapter 5). In particular, early action on some SLCFs (including actions that may warm the climate, such as reducing sulphur dioxide emissions) may have considerable societal co-benefits, such as reduced air pollution and improved public health with associated economic benefits (OECD, 2016; Shindell et al., 2016) <sup>[[#fn:r202|202]]</sup> . Valuation of broadly defined social costs attempts to account for many of these additional non-climate factors along with climate-related impacts (Shindell, 2015; Sarofim et al., 2017; Shindell et al., 2017) <sup>[[#fn:r203|203]]</sup> . See Chapter 4, Section 4.3.6, for a discussions of mitigation options, noting that mitigation priorities for different climate forcers depend on multiple economic and social criteria that vary between sectors, regions and countries.

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'''Cross Chapter Box 2: Figure 1'''

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'''Implications of different approaches to calculating aggregate greenhouse gas emissions on a pathway to net zero.'''
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[[File:c6d3d62f1a62e7739246a448c8117ec2 box-2-figure-1-1024x461.jpg]]

(a) Aggregate emissions of well-mixed greenhouse gases (WMGHGs) under the RCP2.6 mitigation scenario expressed as CO2-equivalent using GWP100 (blue); GTP100 (green) and GWP* (yellow). Aggregate WMGHG emissions appear to fall more rapidly if calculated using GWP* than using either GWP or GTP, primarily because GWP* equates a falling methane emission rate with negative CO <sub>2</sub> emissions, as only active CO <sub>2</sub> removal would have the same impact on radiative forcing and GMST as a reduction in methane emission rate. (b) Cumulative emissions of WMGHGs combined as in panel (a) (blue, green and yellow lines &amp; left hand axis) and warming response to combined emissions (black dotted line and right hand axis, Millar et al. (2017a) <sup>[[#fn:r204|204]]</sup> . The temperature response under ambitious mitigation is closely correlated with cumulative WMGHG emissions aggregated using GWP*, but with neither emission rate nor cumulative emissions if aggregated using GWP or GTP.

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