Editing IPCC:AR6/SR15/Chapter-4 (section)

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

* Paolo Bertoldi (Italy)
* James Ford (United Kingdom, Canada)
* Richard Klein (Netherlands, Germany)
* Debora Ley (Guatemala, Mexico)
* Timmons Roberts (United States)
* Joeri Rogelj (Austria, Belgium)

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'''Mitigation'''

''1. Introduction''

There is ''high agreement'' that Nationally Determined Contributions (NDCs) are important for the global response to climate change and represent an innovative bottom-up instrument in climate change governance (Section 4.4.1), with contributions from all signatory countries (den Elzen et al., 2016; Rogelj et al., 2016; Vandyck et al., 2016; Luderer et al., 2018; Vrontisi et al., 2018) <sup>[[#fn:r927|927]]</sup> . The global emission projections resulting from full implementation of the NDCs represent an improvement compared to business as usual (Rogelj et al., 2016) <sup>[[#fn:r928|928]]</sup> and current policies scenarios to 2030 (den Elzen et al., 2016; Vrontisi et al., 2018) <sup>[[#fn:r929|929]]</sup> . Most G20 economies would require new policies and actions to achieve their NDC targets (den Elzen et al., 2016; Vandyck et al., 2016; UNEP, 2017b; Kuramochi et al., 2018) <sup>[[#fn:r930|930]]</sup> .

''2. The effect of NDCs on global greenhouse gas (GHG) emissions''

Several studies estimate global emission levels that would be achieved under the NDCs (e.g., den Elzen et al., 2016; Luderer et al., 2016; Rogelj et al., 2016, 2017; Vandyck et al., 2016; Rose et al., 2017; Vrontisi et al., 2018) <sup>[[#fn:r931|931]]</sup> . Rogelj et al. (2016) <sup>[[#fn:r932|932]]</sup> and UNEP (2017b) <sup>[[#fn:r933|933]]</sup> concluded that the full implementation of the unconditional and conditional NDCs are expected to result in global GHG emissions of about 55 (52–58) and 53 (50–54) GtCO <sub>2</sub> -eq yr <sup>−</sup> <sup>1</sup> , respectively (Cross-Chapter Box 11, Figure 1 below).

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'''Cross-Chapter Box 11. Figure 1'''

<span id="ghg-emissions-are-all-expressed-in-units-of-co-2--equivalence-computed-with-100-year-global-warming-potentials-gwps-reported-in-ipcc-sar-while-the-emissions-for-the-1.5c-and-2c-scenarios-in-table-2.4-are-reported-using-the-100-year-gwps-reported-in-ipcc-ar4-and-are-hence-about-3-higher."></span>
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'''GHG emissions are all expressed in units of CO <sub>2</sub> -equivalence computed with 100-year global warming potentials (GWPs) reported in IPCC SAR, while the emissions for the 1.5°C and 2°C scenarios in Table 2.4 are reported using the 100-year GWPs reported in IPCC AR4, and are hence about 3% higher.'''
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[[File:6e05089191580a32991c50b1a3713d3c Cross-Chapter-Box-11-Figure-1-1024x450.jpg]]

Using IPCC AR4 instead of SAR GWP values is estimated to result in a 2–3% increase in estimated 1.5°C and 2°C emissions levels in 2030. Source: based on Rogelj et al. (2016) <sup>[[#fn:r934|934]]</sup> and UNEP (2017b) <sup>[[#fn:r935|935]]</sup> .

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''3. The effect of NDCs on temperature increase and carbon budget''

Estimates of global average temperature increase are 2.9°C–3.4°C above preindustrial levels with a greater than 66% probability by 2100 (Rogelj et al., 2016; UNEP, 2017b) <sup>[[#fn:r936|936]]</sup> , under a full implementation of unconditional NDCs and a continuation of climate action similar to that of the NDCs. Full implementation of the conditional NDCs would lower the estimates by about 0.2°C by 2100. As an indication of the carbon budget implications of NDC scenarios, Rogelj et al. (2016) <sup>[[#fn:r937|937]]</sup> estimated cumulative emissions in the range of 690 to 850 GtCO <sub>2</sub> for the period 2011–2030 if the NDCs are successfully implemented. The carbon budget for post-2010 till 2100 compatible with staying below 1.5°C with a 50–66% probability was estimated at  550–600 GtCO <sub>2</sub> (Clarke et al., 2014; Rogelj et al., 2016) <sup>[[#fn:r938|938]]</sup> , which will be well exceeded by 2030 at full implementation of the NDCs (Chapter 2, Section 2.2 and Section 2.3.1).

''4.'' ''The 2030 emissions gap with 1.5'' ''ºC and urgency of action''

As the 1.5°C pathways require reaching carbon neutrality by mid-century, the NDCs alone are not sufficient, as they have a time horizon until 2030. Rogelj et al. (2016) <sup>[[#fn:r939|939]]</sup> and Hof et al. (2017) <sup>[[#fn:r940|940]]</sup> have used results or compared NDC pathways with emissions pathways produced by integrated assessment models (IAMs) assessing the contribution of NDCs to achieve the 1.5°C targets. There is ''high agreement'' that current NDC emissions levels are not in line with pathways that limit warming to 1.5°C by the end of the century (Rogelj et al., 2016, 2017; Hof et al., 2017; UNEP, 2017b; Vrontisi et al., 2018) <sup>[[#fn:r941|941]]</sup> . The median 1.5°C emissions gap (&gt;66% chance) for the full implementation of both the conditional and unconditional NDCs for 2030 is 26 (19–29) to 28 (22–33) GtCO <sub>2</sub> -eq (Cross-Chapter Box 11, Figure 1 above).

Studies indicate important trade-offs of delaying global emissions reductions (Chapter 2, Sections 2.3.5 and 2.5.1). AR5 identified flexibility in 2030 emission levels when pursuing a 2°C objective (Clarke et al., 2014) <sup>[[#fn:r942|942]]</sup> indicating that strongest trade-offs for 2°C pathways could be avoided if emissions are limited to below 50 GtCO <sub>2</sub> -eq yr <sup>−1</sup> in 2030 (here computed with the GWP–100 metric of the IPCC SAR). New scenario studies show that full implementation of the NDCs by 2030 would imply the need for deeper and faster emission reductions beyond 2030 in order to meet 2ºC, and also higher costs and efforts of negative emissions (Fujimori et al., 2016; Sanderson et al., 2016; Rose et al., 2017; van Soest et al., 2017; Luderer et al., 2018) <sup>[[#fn:r943|943]]</sup> . However, no flexibility has been found for 1.5°C-consistent pathways (Luderer et al., 2016; Rogelj et al., 2017) <sup>[[#fn:r944|944]]</sup> , indicating that if emissions through 2030 are at NDC levels, the resulting post-2030 reductions required to remain within a 1.5°C-consistent carbon budget during the 21st century (Chapter 2, Section 2.2) are not within the feasible operating space of IAMs. This indicates that the chances of failing to reach a 1.5°C pathway are significantly increased (Riahi et al., 2015) <sup>[[#fn:r945|945]]</sup> , if near-term ambition is not strengthened beyond the level implied by current NDCs.<br />
Accelerated and stronger short-term action and enhanced longer-term national ambition going beyond the NDCs would be needed for 1.5°C-consistent pathways. Implementing deeper emissions reductions than current NDCs would imply action towards levels identified in Chapter 2, Section 2.3.3, either as part of or over-delivering on NDCs.

''5. The impact of uncertainties on NDC emission levels''

The measures proposed in NDCs are not legally binding (Nemet et al., 2017) <sup>[[#fn:r946|946]]</sup> , further impacting estimates of anticipated 2030 emission levels. The aggregation of targets results in high uncertainty (Rogelj et al., 2017) <sup>[[#fn:r947|947]]</sup> , which could be reduced with clearer guidelines for compiling future NDCs focused more on energy accounting (Rogelj et al., 2017) <sup>[[#fn:r948|948]]</sup> and increased transparency and comparability (Pauw et al., 2018) <sup>[[#fn:r949|949]]</sup> .

Many factors would influence NDCs global aggregated effects, including: (1) variations in socio-economic conditions (GDP and population growth), (2) uncertainties in historical emission inventories, (3) conditionality of certain NDCs, (4) definition of NDC targets as ranges instead of single values, (5) the way in which renewable energy targets are expressed, and (6) the way in which traditional biomass use is accounted for. Additionally, there are land-use mitigation uncertainties (Forsell et al., 2016; Grassi et al., 2017) <sup>[[#fn:r950|950]]</sup> . Land-use options play a key role in many country NDCs; however, many analyses on NDCs do not use country estimates on land-use emissions, but use model estimates, mainly because of the large difference in estimating the ‘anthropogenic’ forest sink between countries and models (Grassi et al., 2017) <sup>[[#fn:r951|951]]</sup> .

''6. Comparing countries’ NDC ambition (equity, cost optimal allocation and other indicators)''

Various assessment frameworks have been proposed to analyse, benchmark and compare NDCs, and indicate possible strengthening, based on equity and other indicators (Aldy et al., 2016; den Elzen et al., 2016; Höhne et al., 2017; Jiang et al., 2017; Holz et al., 2018) <sup>[[#fn:r952|952]]</sup> .There is large variation in conformity/fulfilment with equity principles across NDCs and countries. Studies use assessment frameworks based on six effort sharing categories in the AR5 (Clarke et al., 2014) <sup>[[#fn:r953|953]]</sup> with the principles of ‘responsibility’, ‘capability’ and ‘equity’ (Höhne et al., 2017; Pan et al., 2017; Robiou du Pont et al., 2017) <sup>[[#fn:r954|954]]</sup> . There is an important methodological gap in the assessment of the NDCs’ fairness and equity implications, partly due to lack of information on countries’ own assessments (Winkler et al., 2017) <sup>[[#fn:r955|955]]</sup> . Implementation of Article 2.2 of the Paris Agreement could reflect equity and the principle of ‘common but differentiated responsibilities and respective capabilities’, due to different national circumstances and different interpretations of equity principles (Lahn and Sundqvist, 2017; Lahn, 2018) <sup>[[#fn:r956|956]]</sup> .

'''Adaptation'''

The Paris Agreement recognizes adaptation by establishing a global goal for adaptation (Kato and Ellis, 2016; Rajamani, 2016; Kinley, 2017; Lesnikowski et al., 2017; UNEP, 2017a) <sup>[[#fn:r957|957]]</sup> . This is assessed qualitatively, as achieving a temperature goal would determine the level of adaptation ambition required to deal with the consequent risks and impacts (Rajamani, 2016) <sup>[[#fn:r958|958]]</sup> . Countries can include domestic adaptation goals in their NDCs, which together with national adaptation plans (NAPs) give countries flexibility to design and adjust their adaptation trajectories as their needs evolve and as progress is evaluated over time. A challenge for assessing progress on adaptation globally is the aggregation of many national adaptation actions and approaches. Knowledge gaps still remain about how to design measurement frameworks that generate and integrate national adaptation data without placing undue burdens on countries (UNEP, 2017a) <sup>[[#fn:r959|959]]</sup> .

The Paris Agreement stipulates that adaptation communications shall be submitted as a component of or in conjunction with other communications, such as an NDC, a NAP, or a national communication. Of the 197 Parties to the UNFCCC, 140 NDCs have an adaptation component, almost exclusively from developing countries. NDC adaptation components could be an opportunity for enhancing adaptation planning and implementation by highlighting priorities and goals (Kato and Ellis, 2016) <sup>[[#fn:r960|960]]</sup> . At the national level they provide momentum for the development of NAPs and raise the profile of adaptation (Pauw et al., 2016b, 2018) <sup>[[#fn:r961|961]]</sup> . The Paris Agreement’s transparency framework includes adaptation, through which ‘adaptation communication’ and accelerated adaptation actions are submitted and reviewed every five years (Hermwille, 2016; Kato and Ellis, 2016) <sup>[[#fn:r962|962]]</sup> . This framework, unlike others used in the past, is applicable to all countries taking into account differing capacities amongst Parties (Rajamani, 2016) <sup>[[#fn:r963|963]]</sup> .

Adaptation measures presented in qualitative terms include sectors, risks and vulnerabilities that are seen as priorities by the Parties. Sectoral coverage of adaptation actions identified in NDCs is uneven, with adaptation primarily reported to focus on the water sector (71% of NDCs with adaptation component), agriculture (63%), health (54%), and biodiversity/ecosystems (50%) (Pauw et al., 2016b, 2018) <sup>[[#fn:r964|964]]</sup> .

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