Editing IPCC:AR6/WGIII/Chapter-12 (section)

=== 12.2.1 Introduction ===

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The term ‘mitigation potential’ is used here to report the quantity of net greenhouse gas emissions reductions that can be achieved by a given mitigation option relative to a specified reference scenario. The net greenhouse gas emission reduction is the sum of reduced emissions and enhanced sinks. Several types of potential can be distinguished. The technical potential is the mitigation potential constrained by theoretical limits in addition to the availability of technology and practices. Quantification of technical potentials primarily takes into account technical considerations, but social, economic and/or environmental considerations are sometimes also considered, if these represent strong barriers to the deployment of an option. The economic potential, being the potential reported in this section, is the proportion of the technical potential for which the social benefits exceed the social costs, taking into account a social discount rate and the value of externalities (see Annex I: Glossary). In this section, only externalities related to greenhouse gas emissions are taken into account. They are represented by using different cost cut-off levels of options in terms of USD per tonne of avoided CO 2 -eq emissions. Other potentials, such as market potentials, could also be considered, but they are not included in this section.

The analysis presented here is based, as far as possible, on information contained in Chapters 6, 7, 9, 10 and 11, where costs and potentials, referred to here as ‘sectoral mitigation potentials’ have been discussed for each individual sector. In the past, these were designated as bottom-up potentials, in contrast to the top-down potentials that are obtained from integrated energy-economic models and IAMs. However, IAMs increasingly include ‘bottom-up’ elements, which makes the distinction less clear. Still, sectoral studies often have more technical and economic detail than IAMs. They may also provide more up-to-date information on technology options and associated costs. However, aggregation of results from sectoral studies is more complex, and although interactions and overlap are corrected for as far as possible in this analysis, it is recognised that such systemic effects are much more rigorously taken into account in IAMs. A comparison is made between the sectoral results and the outcomes of the IAMs in [[#12.2.3|Section 12.2.3]] .

Costs of mitigation options will change over time. For many technologies, costs will reduce as a result of technological learning. An attempt has been made to take into account the average, implementation-weighted costs until 2030. However, the underlying literature did not always allow such costs to be presented. For the year 2030, the results are presented similarly to AR4, with a breakdown of the potential in ‘cost bins’. For the year 2050, a more qualitative approach is provided. The origins of the cost data in this section are mostly based on studies carried out in the period 2015–2020. Given the wide range of the cost bins that are used in this section it is not meaningful (and often not possible) to convert to USD values for one specific year. This may lead to some extra uncertainty, but this is expected to be relatively small.

As indicated previously, net emission reduction potentials are presented based on comparison with a reference scenario. Unfortunately, not all costs and potentials found in the literature are determined against the same reference scenarios. In this assessment, reference scenarios are based on what were assumed current-policy scenarios in the period 2015–2019. Typical reference scenarios are the Shared Socio-economic Pathway (SSP2) scenarios ( [[#Fricko--2017|Fricko et al. 2017]] ) and the Current Policies scenario from the World Energy Outlook (WEO) 2019 (IEA 2019). They can both be considered scenarios with middle-of-the-road expectations on population growth and economic development, but there are still some differences between the two (Table 12.2). The net emissions reduction potentials reported here were generally based on analyses carried out before 2020, so the impact of the COVID-19 pandemic was not taken into account. For comparison, the Stated Policies scenario of the World Energy Outlook 2020 ( [[#IEA--2020a|IEA 2020a]] ) is also shown, one of the scenarios in which the impact of COVID-19 was considered. Variations of up to 10% between the different reference scenarios exist with respect to macro-variables such as total primary energy use and total GHG emissions. The potential estimates presented below should be interpreted against this background. The total emissions under the reference scenarios in 2030 are expected to be in the range of 54 to 68 GtCO 2 -eq yr –1 with a median of 60 GtCO 2 -eq yr –1 (Table 4.1).

For the energy sector the potentials are determined using the World Energy Outlook 2019 Current Policies Scenario as a reference (IEA 2019). However, for the economic assessment, more recent Levelised Costs of Electricity (LCOEs) for different electricity generating technologies were used ( [[#IEA--2020a|IEA 2020a]] ). For the AFOLU sector, the potentials were derived from a variety of studies. It may be expected that the best estimates, as averages, match with the reference in a middle-of-the-road scenario. For the buildings sector, the Current Policies scenario of World Energy Outlook 2019 (IEA 2019) was used as a reference. For the transport sector, the references of the underlying sources were used. For the industry sector, the scenarios used have emissions that are slightly higher than in the Current Policies scenario from the World Energy Outlook 2019 (IEA 2019).

'''Table 12.2 | Key characteristics of the scenarios used as a reference for determining costs and potentials.''' The values are for the year 2030.

{| class="wikitable"
|-
!
! SSP2 reference

(MESSAGE-GLOBIOM)

( [[#Fricko--2017|Fricko et al. 2017]] )

! All reference scenarios

median (25th–75th percentiles in parenthesis)

(AR6 scenarios database, [[#IIASA--2021|IIASA, 2021]] )

! WEO-2019

(Current Policies)

(IEA 2019)

! WEO-2020

(Stated Policies)

( [[#IEA--2020a|IEA 2020a]] )

! AR6 WG III Chapter 4

(Chapter 4, Table 4.1)

|-
| Real GDP (purchasing power parity, PPP) (10 12 USD)

| 158

(USD2010)

| 159

(154 '''–''' 171)

| 3.6% p.a. ↑

(2018 to 2030)

| 2.9% p.a. ↑

(2019 to 2030)

|
|-
| Population (billion)

| 8.30

| 8.30

(8.20–8.34)

| 8.60

|
|-
| Total primary energy use (EJ)

| 627

| 670 (635–718)

| 710

| 660

|
|-
| Total final energy use (EJ)

| 499

| 480

(457 '''–''' 508)

| 502

| 472

|
|-
| Energy-related CO 2 emissions (Gt)

| 33.0

| 37.9

(34.7 '''–''' 41.4)

| 37.4

| 33.2 a

| 37

(35 '''–''' 45)

|-
| CO 2 emissions energy and industry (Gt)

| 37.9

| 42.3

(39.0–45.8)

|
| 36.0

|
|-
| Total CO 2 emissions (Gt)

| 40.6

| 45.7

(41.8–49.4)

|
| 43

(38–51)

|-
| Total greenhouse gas emissions (GtCO 2 -eq)

| 52.7

| 59.7

(55.0–65.8)

|
| 60

(54–68)

|}

a The difference between WEO-2020 and WEO-2019 is partly explained by the fact that WEO-2019 had two different reference scenarios: Current Policies and Stated Policies. WEO-2020 has only one reference: the Stated Policies Scenario, which ‘is based on today’s policy settings’. The Stated Policies Scenario in WEO-2019 had energy-related emissions of 34.9 GtCO 2 . EJ, exajoules (1 x 1018 joules); p.a., per annum.

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