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

== 1.5 Emissions Scenarios and Illustrative Mitigation Pathways (IMPs) ==

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Scenarios are a powerful tool for exploring an uncertain future world against the background of alternative choices and development. Scenarios can be constructed using both narrative and quantitative methods. When these two methods are combined they provide complementary information and insights. Quantitative and narrative models are frequently used to represent scenarios to explore choices and challenges. The IPCC has a long history of assessing scenarios ( [[#Nakicenovic--2000|Nakicenovic et al. 2000]] ; [[#van%20Vuuren--2011|van Vuuren et al. 2011]] , 2014) (see also AR6 WGI [[#1.6|Section 1.6]] for a history of scenarios within the IPCC). This WGIII assessment employs a wide range of qualitative and quantitative scenarios including quantitative scenarios developed through a wide and heterogeneous set of tools ranging from spreadsheets to complex computational models (Annex III: Scenarios and Modelling Methods provides further discussion and examples of computational models).

The concept of an '''illustrative pathway (IP)''' was introduced in the IPCC Special Report on Global Warming of 1.5ºC ( [[#IPCC--2018b|IPCC 2018b]] ) to highlight a subset of the quantitative scenarios, drawn from a larger pool of published literature, with specific characteristics that would help represent some of the key findings emerging from the assessment in terms of different strategies, ambitions and options available to achieve the Paris goals.

'''Integrated assessment models (IAMs)''' are the primary tools for quantitatively evaluating the technological and macroeconomic implications of decarbonisation, particularly for global long-term pathways. They broadly divide into ‘stylised aggregate benefit-cost models’, and more complex ‘detailed process’ IAMs ( [[#Weyant--2017|Weyant 2017]] ), often mirroring the benefit-cost and cost-effective approaches outlined in [[#1.7.1|Section 1.7.1]] , with more detailed classification in, for example, [[#Nikas--2019|Nikas et al. (2019)]] . IAMs embody a number of structural and socio-demographic assumptions and include multiple modelling approaches, ranging from economic optimising behaviour to simulation (see Annex III). Detailed process models can include energy system models used to analyse decarbonisation and ‘net zero’ scenarios by international agencies (e.g., [[#IEA--2020a|IEA 2020a]] ).

Calculating cost-effective trajectories towards given goals typically involves detailed process IAMs. Often these calculate the dynamic portfolio of technologies consistent with a given climate target. Some track records of technology forecasting in IAMs are outlined in [[IPCC:Wg3:Chapter:Chapter-2#2.5.4|Section 2.5.4]] , and Box 16.1. Climate targets may be imposed in models in a variety of ways that include, but are not limited to, constraints on emissions or cumulated emissions (carbon budgets), and the pricing of emissions. The time-path of mitigation costs calculated through these models may be translated into ‘shadow prices’ that (like the social cost of carbon; SCC) offer a benchmark to assess the cost-effectiveness of investments, as used by some governments and companies ( [[#1.8.2|Section 1.8.2]] ).

'''Scenarios in the IPCC and AR6.''' For AR6, WGIII received submissions of more than 2500 model-based scenarios published in the scientific literature. Such scenarios, which explore different possible evolutions of future energy and land use (with or without climate policy) and associated emissions, are made available through an interactive AR6 scenario database. The main characteristics of pathways in relation to ‘net zero’ emissions and remaining ‘carbon budgets’ are summarised in Box 3.5 in Chapter 3. The warming contribution of CO 2 is very closely related to cumulative CO 2 emissions, but the remaining ‘carbon budget’ for a given warming depends strongly ''inter alia'' on emissions of other GHGs; for targets below 2°C this may affect the corresponding ‘carbon budget’ by about ±220 GtCO 2 , compared to central estimates of around 500 GtCO 2 (for 1.5°C) and 1350 GtCO 2 (for 2°C) (AR6 WGI, Table SPM.2) (Cross-Working Group Box 1 in Chapter 3).

'''Pathways and ‘net zero’.''' The date at which the world needs aggregate emissions to reach net zero for Paris-consistent temperature goals depends both on progress in reducing non-CO 2 GHG emissions and near-term progress in reducing CO 2 emissions. Faster progress in the near term extends the date at which net zero must be reached, while conversely, slower near-term progress brings the date even closer to the present. Some of the modelled 1.5°C pathways with limited overshoot cut global CO 2 emissions in half until 2030, which allows for a more gradual decline thereafter, reaching net zero CO 2 after 2050; also, net zero GHGs occurs later, with remaining emissions of some non-CO 2 GHGs compensated by ‘net negative’ CO 2 (see Glossary and FAQ 1.3, and Cross-Chapter Box 3 in Chapter 3).

Drawing from the scenarios database, five '''Illustrative Mitigation Pathways (IMPs)''' were defined for this report (Figure 3.5 and Table 1.1). These are introduced here, with a more complete description and discussion provided in [[IPCC:Wg3:Chapter:Chapter-3#3.2.5|Section 3.2.5]] . These IMPs were chosen to illustrate key themes with respect to mitigation strategies across the entire WGIII assessment. The IMPs embody both a storyline, which describes in narrative form the key socio-economic characteristics of that scenario, and a quantitative illustration providing numerical values that are internally consistent and comparable across chapters of this report. Quantitative IMPs can be associated directly with specific human activities and provide a quantitative point of reference that links activities in different parts of socio-economic systems. Some parts of the report draw on these quantitative scenarios, whilst others use only the narratives. No assessment of the likelihood of each IMP has been made (as they reflect both human choice and deep uncertainty).

The IMPs are organised around two dimensions: the ''level of ambition'' consistent with meeting Paris goals, and the scenario features (Figure 1.4). The IMPs explore different pathways potentially consistent with meeting the long-term temperature goals of the Paris Agreement. As detailed in [[IPCC:Wg3:Chapter:Chapter-3#3.2.5|Section 3.2.5]] and in Chapter 4, a pathway of Gradual Strengthening of current policies ( '''IMP-''' GS ) to 2030, if followed by very fast reductions, may stay below 2°C. The '''IMP''' -NEG pathway, with somewhat deeper emission cutbacks to 2030, might enable 1.5°C to be reached but only after significant overshoot, through the subsequent extensive use of CDR in the energy and the industry sectors to achieve net negative global emissions, as discussed in Chapters 3, 6, 7, 10 and 12.

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[[File:d0ef991b6766f04463f3d36ca07254a8 IPCC_AR6_WGIII_Figure_1_4.png]]

'''Figure 1.4 | Illustrative Mitigation Pathways (IMPs) used in AR6: illustration of key features and leve''' '''ls of ambition.'''

Three other IMPs illustrate different features of technology scenarios with more short-term rapid emission reductions, which could deliver outcomes compatible with the temperature range in the Paris Agreement without large overshoot. Based on the assessment in [[IPCC:Wg3:Chapter:Chapter-5#5.3.3|Section 5.3.3]] , one key mitigation strategy would be to rely on the opportunities for reducing demand ( '''IMP-''' LD ). Chapters 6 and 7–11 show how energy systems based on accelerated deep renewable energy penetration and electrification can also provide a pathway to deep mitigation ( '''IMP-''' REN ). Chapters 3, 4 and 17 provide insights into how shifting development pathways can lead to deep emission reductions and achieve sustainable development goals ( '''IMP-''' SP ).

These pathways can be implemented with different levels of ambition, that can be measured through the classes (C) of temperature levels from the scenarios database, see [[IPCC:Wg3:Chapter:Chapter-3|Chapter 3]] (Table 3.2). In the IMP framework, [[IPCC:Wg3:Chapter:Chapter-3#3.2.5|Section 3.2.5]] presents and explores quantitative scenarios that can limit warming to 1.5°C (with a probability of 50% or greater, i.e., C1 for the illustrated quantification of LD, SP and REN, and C2 for NEG scenario), along with other GS pathways which keep warming below 2°C with a probability of 67% or greater (C3). In addition to these primary IMPs, the full scenario database contains sensitivity cases that explore alternative warming levels.

In addition to the IMPs two additional scenarios were selected, which illustrate the consequences of current policies and pledges. Current Policies ( '''CurPol''' ) explores the consequences of continuing along the path of implemented climate policies in 2020 and only a Gradual Strengthening after that, drawing on numerous such scenarios in the literature. Moderate Action ( '''ModAct''' ) explores the impact of implementing NDCs to 2030, but without further strengthening: both result in global mean temperature above 2°C. They provide benchmarks against which to compare the IMPs.

Table 1.1 summarises the main storyline elements of the reference scenarios and each IMP.

'''Table 1.1 | Illustrative Mitigation Pathways used in AR6.'''

{| class="wikitable"
|-
! colspan="3"| '''Scenarios'''

! '''Full name'''

! '''Main policy characteristics'''

|-
| colspan="3"| '''CurPol'''

| Current Policies

| Implementation of current climate ''policies'' (mostly as reported in Nationally Determined Contributions (NDCs)), neglecting stated subsequent goals and objectives (e.g., for 2030); only Gradual Strengthening after 2030; grey COVID recovery.

|-
| colspan="3"| '''ModAct'''

| Moderate Action

| Implementation of current policies ''and'' achievement of 2030 NDCs, with further strengthening post-2030. Similarly to the situation implied by the diversity of NDCs (both policies and pledges), a fragmented policy landscape remains; mixed COVID recovery.

|-
| rowspan="5"| '''IMPs'''

| rowspan="5"| '''1.5°C/ &lt;2°C'''

| '''GS'''

| Gradual Strengthening

| Until 2030, primarily current NDCs are implemented; after that a strong universal regime leads to coordinated and rapid decarbonisation actions.

|-
| '''Neg'''

| Net Negative Emissions

| Successful international climate policy regime reduces emissions below ModAct or GS to 2030, but with a focus on the long-term temperature goal, negative emissions kick in at growing scales thereafter, so that mitigation in all sectors also includes a growing and ultimately large reliance on negative emissions, with large ‘net global negative’ after 2050 to meet 1.5˚C after significant overshoot.

|-
| '''Ren'''

| Renewables

| Successful international climate policy regime with immediate action, particularly policies and incentives (including international finance) favouring renewable energy; less emphasis on negative-emission technologies. Rapid deployment and innovation of renewables and systems; electrification of all end use.

|-
| '''LD'''

| Low Demand

| Successful international climate policy regime with immediate action on the demand side; policies and financial incentives favouring reduced demand that in turn leads to early emission reductions; this reduces the decarbonisation effort on the supply side.

|-
| '''SP'''

| Shifting Pathways

| Successful international climate policy regime with a focus on additional SDG policies aiming, for example, at poverty reduction and broader environmental protection. Major transformations shift development towards sustainability and reduced inequality, including deep GHG emissions reduction.

|}

'''What the IMPs do and don’t do.''' The IMPs are, as their name implies, a set of scenarios meant to illustrate some important themes that run through the entire WGIII assessment. They illustrate that the climate outcomes that individuals and society will face in the century ahead depend on individual and societal choices. In addition, they illustrate that there are multiple ways to successful achievement of Paris long-term temperature goals.

IMPs are not intended to be comprehensive. They are not intended to illustrate all possible themes in this report. They do not, for example, attempt to illustrate the range of alternative socio-economic pathways against which efforts to implement Paris goals may be set, or to reflect variations in potential regional development pathways. They do not explore issues around income distribution or environmental justice, but assume implicitly that ''where'' and ''how'' action occurs can be separated from who pays, in ways to adequately address such issues. They are essentially pathways of technological evolution and demand shifts reflecting broad global trends in social choice. The IMPs do not directly assess issues of realisation linked to the ‘drivers and constraints’ summarised in our previous section, and the quantifications use, for the most part, models that are grounded mainly in the Aggregate Economics Frameworks ( [[IPCC:Wg3:Chapter:Chapter-7#7.1|Section 7.1]] ). As such they reflect primarily the geophysical, economic and technological Dimensions of Assessment, but can be assessed in relation to the full set of Feasibility criteria ( [[#1.8.1|Section 1.8.1]] ).

Together the IMPs provide illustrations of potential future developments that can be shaped by human choices, including: Where are current policies and pledges leading? What is needed to reach specific temperature goals under varying assumptions? What are the consequences of different strategies to meet climate targets (i.e., demand-side strategy, a renewable energy strategy or a strategy with a role for net negative emissions)? What are the consequences of delay? What are the implications for other SDGs of various climate mitigation pathways?

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