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

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

* Mikiko Kainuma (Japan)
* Kristie L. Ebi (United States)
* Sabine Fuss (Germany)
* Elmar Kriegler (Germany)
* Keywan Riahi (Austria)
* Joeri Rogelj (Austria, Belgium)
* Petra Tschakert (Australia, Austria)
* Rachel Warren (United Kingdom)

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Climate change scenarios have been used in IPCC assessments since the First Assessment Report (Leggett et al., 1992) <sup>[[#fn:r124|124]]</sup> . The '''SRES scenarios''' (named after the IPCC Special Report on Emissions Scenarios published in 2000; IPCC, 2000) <sup>[[#fn:r125|125]]</sup> , consist of four scenarios that do not take into account any future measures to limit greenhouse gas (GHG) emissions. Subsequently, many policy scenarios have been developed based upon them (Morita et al., 2001) <sup>[[#fn:r126|126]]</sup> . The SRES scenarios are superseded by a set of scenarios based on the Representative Concentration Pathways (RCPs) and Shared Socio-Economic Pathways (SSPs) (Riahi et al., 2017) <sup>[[#fn:r127|127]]</sup> . The RCPs comprise a set of four GHG concentration trajectories that jointly span a large range of plausible human-caused climate forcing ranging from 2.6 W m <sup>−2</sup> (RCP2.6) to 8.5 W m <sup>−2</sup> (RCP8.5) by the end of the 21st century (van Vuuren et al., 2011) <sup>[[#fn:r128|128]]</sup> . They were used to develop climate projections in the Coupled Model Intercomparison Project Phase 5 (CMIP5; Taylor et al., 2012) <sup>[[#fn:r129|129]]</sup> and were assessed in the IPCC Fifth Assessment Report (AR5). Based on the CMIP5 ensemble, RCP2.6, provides a better than two-in-three chance of staying below 2°C and a median warming of 1.6°C relative to 1850–1900 in 2100 (Collins et al., 2013) <sup>[[#fn:r130|130]]</sup> .

The SSPs were developed to complement the RCPs with varying socio-economic challenges to adaptation and mitigation. SSP-based scenarios were developed for a range of climate forcing levels, including the end-of-century forcing levels of the RCPs (Riahi et al., 2017) <sup>[[#fn:r131|131]]</sup> and a level below RCP2.6 to explore pathways limiting warming to 1.5°C above pre-industrial levels (Rogelj et al., 2018) <sup>[[#fn:r132|132]]</sup> . The SSP-based 1.5°C pathways are assessed in Chapter 2 of this report. These scenarios offer an integrated perspective on socio-economic, energy-system (Bauer et al., 2017) <sup>[[#fn:r133|133]]</sup> , land use (Popp et al., 2017) <sup>[[#fn:r134|134]]</sup> , air pollution (Rao et al., 2017) <sup>[[#fn:r135|135]]</sup> and, GHG emissions developments (Riahi et al., 2017) <sup>[[#fn:r136|136]]</sup> . Because of their harmonised assumptions, scenarios developed with the SSPs facilitate the integrated analysis of future climate impacts, vulnerabilities, adaptation and mitigation.

'''Scenarios and Pathways in this Report'''

This report focuses on pathways that could limit the increase of global mean surface temperature (GMST) to 1.5°C above pre-industrial levels and pathways that align with the goals of sustainable development and poverty eradication. The pace and scale of mitigation and adaptation are assessed in the context of historical evidence to determine where unprecedented change is required (see Chapter 4). Other scenarios are also assessed, primarily as benchmarks for comparison of mitigation, impacts, and/or adaptation requirements. These include baseline scenarios that assume no climate policy; scenarios that assume some kind of continuation of current climate policy trends and plans, many of which are used to assess the implications of the nationally determined contributions (NDCs); and scenarios holding warming below 2°C above pre-industrial levels. This report assesses the spectrum from global mitigation scenarios to local adaptation choices – complemented by a bottom-up assessment of individual mitigation and adaptation options, and their implementation (policies, finance, institutions, and governance, see Chapter 4). Regional, national, and local scenarios, as well as decision-making processes involving values and difficult trade-offs are important for understanding the challenges of limiting GMST increase to 1.5°C and are thus indispensable when assessing implementation.

Different climate policies result in different temperature pathways, which result in different levels of climate risks and actual climate impacts with associated long-term implications. Temperature pathways are classified into continued warming pathways (in the cases of baseline and reference scenarios), pathways that keep the temperature increase below a specific limit (like 1.5°C or 2°C), and pathways that temporarily exceed and later fall to a specific limit (overshoot pathways). In the case of a temperature overshoot, net negative CO <sub>2</sub> emissions are required to remove excess CO <sub>2</sub> from the atmosphere (Section 1.2.3).

In a ‘prospective’ mitigation pathway, emissions (or sometimes concentrations) are prescribed, giving a range of GMST outcomes because of uncertainty in the climate response. Prospective pathways are considered ‘1.5°C pathways’ in this report if, based on current knowledge, the majority of available approaches assign an approximate probability of one-in-two to two-in-three to temperatures either remaining below 1.5°C or returning to 1.5°C either before or around 2100. Most pathways assessed in Chapter 2 are prospective pathways, and therefore even ‘1.5°C pathways’ are also associated with risks of warming higher than 1.5°C, noting that many risks increase non-linearly with increasing GMST. In contrast, the ‘risks of warming of 1.5°C’ assessed in Chapter 3 refer to risks in a world in which GMST is either passing through (transient) or stabilized at 1.5°C, without considering probabilities of different GMST levels (unless otherwise qualified). To stay below any desired temperature limit, mitigation measures and strategies would need to be adjusted as knowledge of the climate response is updated (Millar et al., 2017b; Emori et al., 2018) <sup>[[#fn:r137|137]]</sup> . Such pathways can be called ‘adaptive’ mitigation pathways. Given there is always a possibility of a greater-than-expected climate response (Xu and Ramanathan, 2017) <sup>[[#fn:r138|138]]</sup> , adaptive mitigation pathways are important to minimise climate risks, but need also to consider the risks and feasibility (see Cross-Chapter Box 3 in this chapter) of faster-than-expected emission reductions. Chapter 5 includes assessments of two related topics: aligning mitigation and adaptation pathways with sustainable development pathways, and transformative visions for the future that would support avoiding negative impacts on the poorest and most disadvantaged populations and vulnerable sectors.

'''Definitions of Scenarios and Pathways'''

Climate scenarios and pathways are terms that are sometimes used interchangeably, with a wide range of overlapping definitions (Rosenbloom, 2017) <sup>[[#fn:r139|139]]</sup> .

A ‘ '''scenario’''' is an internally consistent, plausible, and integrated description of a possible future of the human–environment system, including a narrative with qualitative trends and quantitative projections (IPCC, 2000) <sup>[[#fn:r140|140]]</sup> . Climate change scenarios provide a framework for developing and integrating projections of emissions, climate change, and climate impacts, including an assessment of their inherent uncertainties. The long-term and multi-faceted nature of climate change requires climate scenarios to describe how socio-economic trends in the 21st century could influence future energy and land use, resulting emissions and the evolution of human vulnerability and exposure. Such driving forces include population, GDP, technological innovation, governance and lifestyles. Climate change scenarios are used for analysing and contrasting climate policy choices.

The notion of a '''‘pathway’''' can have multiple meanings in the climate literature. It is often used to describe the temporal evolution of a set of scenario features, such as GHG emissions and socio-economic development. As such, it can describe individual scenario components or sometimes be used interchangeably with the word ‘scenario’. For example, the RCPs describe GHG concentration trajectories (van Vuuren et al., 2011) <sup>[[#fn:r141|141]]</sup> and the SSPs are a set of narratives of societal futures augmented by quantitative projections of socio-economic determinants such as population, GDP and urbanization (Kriegler et al., 2012; O’Neill et al., 2014) <sup>[[#fn:r142|142]]</sup> . Socio-economic driving forces consistent with any of the SSPs can be combined with a set of climate policy assumptions (Kriegler et al., 2014) <sup>[[#fn:r143|143]]</sup> that together would lead to emissions and concentration outcomes consistent with the RCPs (Riahi et al., 2017) <sup>[[#fn:r144|144]]</sup> . This is at the core of the scenario framework for climate change research that aims to facilitate creating scenarios integrating emissions and development pathways dimensions (Ebi et al., 2014; van Vuuren et al., 2014) <sup>[[#fn:r145|145]]</sup> .

In other parts of the literature, ‘pathway’ implies a solution-oriented trajectory describing a pathway from today’s world to achieving a set of future goals. '''Sustainable Development Pathways''' describe national and global pathways where climate policy becomes part of a larger sustainability transformation (Shukla and Chaturvedi, 2013; Fleurbaey et al., 2014; van Vuuren et al., 2015) <sup>[[#fn:r146|146]]</sup> . The AR5 presented '''c''' '''limate-''' '''r''' '''esilient pathways''' as sustainable development pathways that combine the goals of adaptation and mitigation (Denton et al., 2014) <sup>[[#fn:r147|147]]</sup> , more broadly defined as iterative processes for managing change within complex systems in order to reduce disruptions and enhance opportunities associated with climate change (IPCC, 2014a) <sup>[[#fn:r148|148]]</sup> . The AR5 also introduced the notion of '''climate-resilient development pathways,''' with a more explicit focus on dynamic livelihoods, multi-dimensional poverty, structural inequalities, and equity among poor and non-poor people (Olsson et al., 2014) <sup>[[#fn:r149|149]]</sup> . '''A''' '''daptation pathways''' are understood as a series of adaptation choices involving trade-offs between short-term and long-term goals and values (Reisinger et al., 2014) <sup>[[#fn:r150|150]]</sup> . They are decision-making processes sequenced over time with the purpose of deliberating and identifying socially salient solutions in specific places (Barnett et al., 2014; Wise et al., 2014; Fazey et al., 2016) <sup>[[#fn:r151|151]]</sup> . There is a range of possible pathways for transformational change, often negotiated through iterative and inclusive processes (Harris et al., 2017; Fazey et al., 2018; Tàbara et al., 2018) <sup>[[#fn:r152|152]]</sup> .

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