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

== Cross-Chapter Box 1: Scenarios and Pathways ==

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====== 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 The SRES scenarios are superseded by a set of scenarios based on the Representative Concentration Pathways (RCPs) and Shared Socio-Economic Pathways (SSPs) (Riahi et al., 2017) 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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=== 1.2.4 Geophysical Warming Commitment ===

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It is frequently asked whether limiting warming to 1.5°C is ‘feasible’ (Cross-Chapter Box 3 in this chapter). There are many dimensions to this question, including the warming ‘commitment’ from past emissions of greenhouse gases and aerosol precursors. Quantifying commitment from past emissions is complicated by the very different behaviour of different climate forcers affected by human activity: emissions of long-lived greenhouse gases such as CO <sub>2</sub> and nitrous oxide (N <sub>2</sub> O) have a very persistent impact on radiative forcing (Myhre et al., 2013) <sup>[[#fn:r153|153]]</sup> , lasting from over a century (in the case of N <sub>2</sub> O) to hundreds of thousands of years (for CO <sub>2</sub> ). The radiative forcing impact of short-lived climate forcers (SLCFs) such as methane (CH <sub>4</sub> ) and aerosols, in contrast, persists for at most about a decade (in the case of methane) down to only a few days. These different behaviours must be taken into account in assessing the implications of any approach to calculating aggregate emissions (Cross-Chapter Box 2 in this chapter).

Geophysical warming commitment is defined as the unavoidable future warming resulting from physical Earth system inertia. Different variants are discussed in the literature, including (i) the ‘constant composition commitment’ (CCC), defined by Meehl et al. (2007) <sup>[[#fn:r154|154]]</sup> as the further warming that would result if atmospheric concentrations of GHGs and other climate forcers were stabilised at the current level; and (ii) and the ‘zero emissions commitment’ (ZEC), defined as the further warming that would still occur if all future anthropogenic emissions of greenhouse gases and aerosol precursors were eliminated instantaneously (Meehl et al., 2007; Collins et al., 2013) <sup>[[#fn:r155|155]]</sup> .

The CCC is primarily associated with thermal inertia of the ocean (Hansen et al., 2005) <sup>[[#fn:r156|156]]</sup> , and has led to the misconception that substantial future warming is inevitable (Matthews and Solomon, 2013) <sup>[[#fn:r157|157]]</sup> . The CCC takes into account the warming from past emissions, but also includes warming from future emissions (declining but still non-zero) that are required to maintain a constant atmospheric composition. It is therefore not relevant to the warming commitment from past emissions alone.

The ZEC, although based on equally idealised assumptions, allows for a clear separation of the response to past emissions from the effects of future emissions. The magnitude and sign of the ZEC depend on the mix of GHGs and aerosols considered. For CO <sub>2</sub> , which takes hundreds of thousands of years to be fully removed from the atmosphere by natural processes following its emission (Eby et al., 2009; Ciais et al., 2013) <sup>[[#fn:r158|158]]</sup> , the multi-century warming commitment from emissions to date in addition to warming already observed is estimated to range from slightly negative (i.e., a slight cooling relative to present-day) to slightly positive (Matthews and Caldeira, 2008; Lowe et al., 2009; Gillett et al., 2011; Collins et al., 2013) <sup>[[#fn:r159|159]]</sup> . Some studies estimate a larger ZEC from CO <sub>2</sub> , but for cumulative emissions much higher than those up to present day (Frölicher et al., 2014; Ehlert and Zickfeld, 2017) <sup>[[#fn:r160|160]]</sup> . The ZEC from past CO <sub>2</sub> emissions is small because the continued warming effect from ocean thermal inertia is approximately balanced by declining radiative forcing due to CO <sub>2</sub> uptake by the ocean (Solomon et al., 2009; Goodwin et al., 2015; Williams et al., 2017) <sup>[[#fn:r161|161]]</sup> . Thus, although present-day CO <sub>2</sub> -induced warming is irreversible on millennial time scales (without human intervention such as active carbon dioxide removal or solar radiation modification; Section 1.4.1), past CO <sub>2</sub> emissions do not commit to substantial further warming (Matthews and Solomon, 2013) <sup>[[#fn:r162|162]]</sup> .

Sustained net zero anthropogenic emissions of CO <sub>2</sub> and declining net anthropogenic non-CO <sub>2</sub> radiative forcing over a multi-decade period would halt anthropogenic global warming over that period, although it would not halt sea level rise or many other aspects of climate system adjustment. The rate of decline of non-CO <sub>2</sub> radiative forcing must be sufficient to compensate for the ongoing adjustment of the climate system to this forcing (assuming it remains positive) due to ocean thermal inertia. It therefore depends on deep ocean response time scales, which are uncertain but of order centuries, corresponding to decline rates of non-CO <sub>2</sub> radiative forcing of less than 1% per year. In the longer term, Earth system feedbacks such as the release of carbon from melting permafrost may require net negative CO <sub>2</sub> emissions to maintain stable temperatures (Lowe and Bernie, 2018) <sup>[[#fn:r163|163]]</sup> .

For warming SLCFs, meaning those associated with positive radiative forcing such as methane, the ZEC is negative. Eliminating emissions of these substances results in an immediate cooling relative to the present (Figure 1.5, magenta lines) (Frölicher and Joos, 2010; Matthews and Zickfeld, 2012; Mauritsen and Pincus, 2017) <sup>[[#fn:r164|164]]</sup> . Cooling SLCFs (those associated with negative radiative forcing) such as sulphate aerosols create a positive ZEC, as elimination of these forcers results in rapid increase in radiative forcing and warming (Figure 1.5, green lines) (Matthews and Zickfeld, 2012; Mauritsen and Pincus, 2017; Samset et al., 2018) <sup>[[#fn:r165|165]]</sup> . Estimates of the warming commitment from eliminating aerosol emissions are affected by large uncertainties in net aerosol radiative forcing (Myhre et al., 2013, 2017) <sup>[[#fn:r166|166]]</sup> and the impact of other measures affecting aerosol loading (e.g., Fernández et al., 2017) <sup>[[#fn:r167|167]]</sup> . If present-day emissions of all GHGs (short- and long-lived) and aerosols (including sulphate, nitrate and carbonaceous aerosols) are eliminated (Figure 1.5, yellow lines) GMST rises over the following decade, driven by the removal of negative aerosol radiative forcing. This initial warming is followed by a gradual cooling driven by the decline in radiative forcing of short-lived greenhouse gases (Matthews and Zickfeld, 2012; Collins et al., 2013) <sup>[[#fn:r168|168]]</sup> . Peak warming following elimination of all emissions was assessed at a few tenths of a degree in AR5, and century-scale warming was assessed to change only slightly relative to the time emissions are reduced to zero (Collins et al., 2013) <sup>[[#fn:r169|169]]</sup> . New evidence since AR5 suggests a larger methane forcing (Etminan et al., 2016) <sup>[[#fn:r170|170]]</sup> but no revision in the range of aerosol forcing (although this remains an active field of research, e.g., Myhre et al., 2017) <sup>[[#fn:r171|171]]</sup> . This revised methane forcing estimate results in a smaller peak warming and a faster temperature decline than assessed in AR5 (Figure 1.5, yellow line).

Expert judgement based on the available evidence (including model simulations, radiative forcing and climate sensitivity) suggests that if all anthropogenic emissions were reduced to zero immediately, any further warming beyond the 1°C already experienced would ''likely'' be less than 0.5°C over the next two to three decades, and also ''likely'' less than 0.5°C on a century time scale.

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'''Warming commitment from past emissions of greenhouse gases and aerosols.'''
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Radiative forcing (top) and global mean surface temperature change (bottom) for scenarios with different combinations of greenhouse gas and aerosol precursor emissions reduced to zero in 2020. Variables were calculated using a simple climate–carbon cycle model (Millar et al., 2017a) <sup>[[#fn:r172|172]]</sup> with a simple representation of atmospheric chemistry (Smith et al., 2018) <sup>[[#fn:r173|173]]</sup> . The bars on the right-hand side indicate the median warming in 2100 and 5–95% uncertainty ranges (also indicated by the plume around the yellow line) taking into account one estimate of uncertainty in climate response, effective radiative forcing and carbon cycle sensitivity, and constraining simple model parameters with response ranges from AR5 combined with historical climate observations (Smith et al., 2018) <sup>[[#fn:r174|174]]</sup> . Temperatures continue to increase slightly after elimination of CO <sub>2</sub> emissions (blue line) in response to constant non-CO <sub>2</sub> forcing. The dashed blue line extrapolates one estimate of the current rate of warming, while dotted blue lines show a case where CO <sub>2</sub> emissions are reduced linearly to zero assuming constant non-CO <sub>2</sub> forcing after 2020. Under these highly idealized assumptions, the time to stabilize temperatures at 1.5°C is approximately double the time remaining to reach 1.5°C at the current warming rate.

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Since most sources of emissions cannot, in reality, be brought to zero instantaneously due to techno-economic inertia, the current rate of emissions also constitutes a conditional commitment to future emissions and consequent warming depending on achievable rates of emission reductions. The current level and rate of human-induced warming determines both the time left before a temperature threshold is exceeded if warming continues (dashed blue line in Figure 1.5) and the time over which the warming rate must be reduced to avoid exceeding that threshold (approximately indicated by the dotted blue line in Figure 1.5). Leach et al. (2018) <sup>[[#fn:r175|175]]</sup> use a central estimate of human-induced warming of 1.02°C in 2017, increasing at 0.215°C per decade (Haustein et al., 2017) <sup>[[#fn:r176|176]]</sup> , to argue that it will take 13–32 years (one-standard-error range) to reach 1.5°C if the current warming rate continues, allowing 25–64 years to stabilise temperatures at 1.5°C if the warming rate is reduced at a constant rate of deceleration starting immediately. Applying a similar approach to the multi-dataset average GMST used in this report gives an assessed ''likely'' range for the date at which warming reaches 1.5°C of 2030 to 2052. The lower bound on this range, 2030, is supported by multiple lines of evidence, including the AR5 assessment for the ''likely'' range of warming (0.3°C–0.7°C) for the period 2016–2035 relative to 1986–2005. The upper bound, 2052, is supported by fewer lines of evidence, so we have used the upper bound of the 5–95% confidence interval given by the Leach et al. (2018) <sup>[[#fn:r177|177]]</sup> method applied to the multi-dataset average GMST, expressed as the upper limit of the ''likely'' range, to reflect the reliance on a single approach. Results are sensitive both to the confidence level chosen and the number of years used to estimate the current rate of anthropogenic warming (5 years used here, to capture the recent acceleration due to rising non-CO <sub>2</sub> forcing). Since the rate of human-induced warming is proportional to the rate of CO <sub>2</sub> emissions (Matthews et al., 2009; Zickfeld et al., 2009) <sup>[[#fn:r178|178]]</sup> plus a term approximately proportional to the rate of increase in non-CO <sub>2</sub> radiative forcing (Gregory and Forster, 2008; Allen et al., 2018 <sup>[[#fn:r179|179]]</sup> ; Cross-Chapter Box 2 in this chapter), these time scales also provide an indication of minimum emission reduction rates required if a warming greater than 1.5°C is to be avoided (see Figure 1.5, Supplementary Material 1.SM.6 and FAQ 1.2).

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