All options, not silver bullets, needed to limit global warming to 1.5 °C: a scenario appraisal

All of the emissions scenarios presented in the SR1.5 as 1.5 °C scenarios rely on deep mitigation portfolios comprising some combination of (a) far-reaching decarbonisation of the global energy, transport, industry and buildings and construction systems; (b) massive scaling up of technologies to remove carbon dioxide (CO₂) from the atmosphere; (c) the transformation of global agricultural and land-use practices; (d) major changes in global consumer behaviour; and (e) reduction of non-CO₂ greenhouse gases and other short-lived climate forcers (note that these can have heating and/or cooling effects, depending on the particular forcing agent and the time scale of interest). These purported solutions face many barriers and associated trade-offs (Smith et al 2016), not all of which are considered by the models with which the scenarios were developed.

Using different combinations of these mitigation levers (not including the non-CO₂ lever), the scenarios arrive at different levels of cumulative CO₂ emissions, known as the carbon budget, as per equation (1):

$$\begin{equation}\mathop \sum \limits_{t = 2018}^y {C_y}\, = \mathop \sum \limits_{t = 2018}^y \left[ {{\text{C}}{{\text{I}}_t} \times {E_t} + {C_{{\text{IP}},t}} + {C_{{\text{AFOLU}},t}} - {\text{CD}}{{\text{R}}_{{\text{geo}}}}} \right]\end{equation} \tag{ 1 }$$

where CI_t is the carbon intensity of final energy demand in a given year, t; E_t is the final energy demand; C_IP,t is the annual rate of CO₂ emissions from industrial processes; C_AFOLU,t is the annual rate of CO₂ emissions from agriculture, forestry and land-use change (AFOLU), which can be expressed as the difference between residual positive AFOLU emissions and CO₂ that is removed from the atmosphere through AFOLU (i.e. C_AFOLU,t = C_AFOLU,res − CDR_AFOLU); and CDR_geo,t is the annual rate at which CO₂ is removed from the atmosphere and stored geologically using human-made technologies.

The left-hand side of equation (1) describes cumulative CO₂ emissions. Peak cumulative CO₂ emissions, and to a lesser degree cumulative CO₂ emissions at the end of the century, provide a proxy for peak and end-of-century global mean temperature (GMT) rise (Zickfeld et al 2016, Tokarska et al 2019). Our selection of the '1.5 °C no overshoot' and '1.5 °C low overshoot' scenarios from the SR1.5 ensemble is analogous (but not exactly equivalent) to placing limits on peak and end-of-century cumulative CO₂ emissions.

The decomposition on the right-hand side of equation (1) therefore identifies a set of broad mitigation 'levers' for staying within carbon budgets, and by extrapolation temperature limits (e.g. 1.5 °C with no or low overshoot). Here we focus on four of the five levers described in equation (1). For each lever we define a parameter characterising the use of the lever in the scenarios in the year 2050. The premise of the analysis is that the attainability of a particular scenario decreases with increasing use of the mitigation levers. This allows us to decompose the overall assessment of scenario attainability into an assessment of to what extent available mitigation levers are used, which can be compared with information on the mitigation potential of those levers from the literature.

Although non-fossil fuel emissions from industrial processes (C_IP,t in equation (1), around 60% of which result from cement production) are a decarbonisation bottleneck in the industry sector, in the analysis that follows we have chosen not to turn it into an explicit mitigation lever to evaluate the attainability of mitigation measures associated with industrial process emissions. Firstly, the technological and institutional challenges associated with decarbonisation of industrial processes are very specific and so will be the solutions to overcome decarbonisation bottlenecks in this area. This does not lend itself to a broad brush assessment of mitigation bounds from the general literature. Secondly, unlike decarbonisation of the energy sector, which would result in emissions reductions across the board, industrial process emissions only represent a small amount of overall emissions. Hence, should some mitigation bottlenecks in this area arise, there is the potential to compensate this with only a modest increase in the use of other levers. Figure S1 (available online at stacks.iop.org/ERL/16/064037/mmedia) shows the emissions from industrial processes for the scenario ensemble used for this analysis. It can be seen that most 1.5 °C scenarios assume a strong reduction in industrial process emissions.

We also do not include mitigation levers associated with carbon dioxide removal (CDR) options associated with ocean fertilisation, carbon storage in industrial products (e.g. wooden building materials) and solar radiation management, owing to their absence from the scenarios studies.

The temperature impact of non-CO₂ greenhouse gases (GHGs) and short-lived climate forcers can modulate the relationship between warming and cumulative CO₂ emissions. In deep mitigation scenarios, non-CO₂ warming typically increases in the short term due to a rapid reduction of aerosol cooling and then peaks and declines when the increasing cooling impact of deep reductions in short-lived GHG emissions, particularly methane, overtake the decline in aerosol cooling (Rogelj et al 2018). Therefore, short-lived GHG emission reductions is an important additional mitigation lever to the mitigation levers highlighted in the right hand side of equation (1). We add it as fifth lever to our analysis. The resulting five mitigation levers are listed in table 1, schematically depicted in figures 1(a) and (b) and are the object of our analysis.

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We begin by applying upper bounds on the five aggregate mitigation levers to our ensemble of scenarios taken from the SR1.5. Of the 50 scenarios considered, none can achieve 1.5 °C with no or low overshoot whilst keeping all the mitigation levers at reasonable levels (i.e. hypothesis A is not true). This means that all SR1.5 scenarios with no or low overshoot of 1.5 °C use at least one mitigation lever beyond reasonable levels. The scenarios appear to be most excessive in their use of CDR_geo, with only 20% (10/50) of all scenarios using this lever at reasonable levels; at least 40% of the scenarios use all other levers at reasonable levels (see table S3; figure S2(a) provides an overview of how the scenarios are distributed within each of the five parameters and includes alternative bounds for the mitigation levers found in the literature and used in this analysis). Only six scenarios use all but one lever at reasonable levels. In three of these cases, CDR_geo is used at challenging levels, in two it is CDR_AFOLU and in one it is CI₂₀₅₀. This suggests that in particular CDR_geo is employed as the ultimate means, i.e. the 'silver bullet', for achieving 1.5 °C while using other mitigation levers at reasonable levels.

22 scenarios stay within all high upper bounds on the levers (we refer to these scenarios hereafter as the 'filtered ensemble'), leading to the conclusion that 1.5 °C could still be attainable, but will require that some mitigation levers be used at challenging levels (i.e. hypothesis A*). The scenario parameters of each of these 22 scenarios are depicted in figure 2 in increasing order of the coverage indicator, V (the same figure for all scenarios is available in the supplement), allowing for a deeper analysis of the use of the individual mitigation levers in these scenarios to stay below 1.5 °C (with no or low overshoot).

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A closer look at the filtered ensemble reveals the kind of trade-offs that result from respecting the upper bounds: the scenarios with no temperature overshoot make use of at least one lever at speculative levels. If a low temporary overshoot in limiting warming to 1.5 °C is permitted, carbon-dioxide removal technologies and measures can be exploited at challenging levels in order to limit the extent of the transformation of the energy system to within reasonable expectations. This approach is followed for example by REMIND-MAgPIE 1.7–3.0: SMP_1p5C_Def, REMIND-MAgPIE 1.7–3.0: SMP_1p5C_lifesty and WITCH-GLOBIOM 4.2: ADVANCE_2020_1.5C-2100. Even when combining large-scale CDR deployment with a relatively large reduction in energy demand (IMAGE 3.0.1: SSP1-19), or with a relatively rapid and large reduction in carbon intensity (REMIND-MAgPIE 1.7–3.0: PEP_1p5C_red_eff), a low overshoot of the 1.5 °C target some time in the 21st century appears inevitable.

Nevertheless, highly ambitious transformation of the energy system (both carbon intensity and energy demand) relaxes the pressure on CDR technologies to achieve the necessary mitigation. One scenario stays within all high upper bounds without any geologic-CDR (P1; MESSAGEix-GLOBIOM 1.0: LowEnergyDemand), accompanied by large reductions in energy demand by 2050 (40% relative to 2018) and considerable reduction in methane emissions (56% relative to 2018). This strong decoupling of emissions and energy production explains how large reductions in climate forcing are achieved, while staying within the high-potential upper bounds for CDR and methane reductions.

The characteristic pathways P1 and P3 from the SR1.5 belong to the filtered ensemble; P2 and P4 do not. P2 (based on a population scenario that stabilises at just over 7 billion in 2100) assumes reduction of methane emissions until 2050 at speculative levels (80% reduction compared to 67% reduction high upper bound). P4 more than doubles the high upper bound for CDR_geo (16.1 GtCO₂/yr compared to 7 GtCO₂/yr). Furthermore, P4 temporarily overshoots 1.5 °C by more than 0.1 °C due to high near-term CO₂ emissions and consequently high peak cumulative emissions, thus pushing up peak warming.

A common critique of ambitious mitigation scenarios is the over-reliance on a single mitigation strategy or technology, i.e. a 'silver bullet', that alone can achieve the 1.5 °C target with low overshoot, without making full use of the available mitigation options. Looking again at figure 2 (especially in comparison to the corresponding figure in the supplement) creates the qualitative impression that 1.5 °C can only be achieved within the high upper bounds if most levers are pulled at challenging levels (i.e. in contrast to reliance on just a few mitigation levers). We confirm this quantitatively by comparing the cumulative histograms of the coverage indicator, V for all scenarios and for the filtered ensemble (figure 3) and find that the filtered scenarios have statistically higher coverage than the entire ensemble, with an average V of 0.93 compared to 0.89. In fact, the lowest value of V, in the filtered ensemble is 0.76 compared to 0.49 for the entire ensemble. This confirms that most, but not necessarily all, levers need to be pulled at challenging levels to make 1.5 °C attainable. In fact, in most of the filtered ensemble, more than one lever must be pulled at challenging levels.

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There is therefore no 'silver bullet' that would allow decarbonisation of the energy system, reduction of energy use, reduction of non-CO₂ emissions, and deploying CDR measures to be dispensed with completely. Even for those scenarios that make substantial use of geologic-CDR (i.e. 3.6–7 GtCO₂/yr in 2050), other mitigation levers must still be pulled at challenging levels in order to stay within the carbon budgets.

We do not find evidence that relevant combinations of mitigation levers, i.e. a broad and extensive use of mitigation portfolios across all levers, are missing from the SR1.5 scenario ensemble. Nine of the 22 scenarios in the filtered ensemble have reasonable coverage of mitigation levers (i.e. V > 0.95), two scenarios have full coverage (V=1) and one uses all levers at or beyond the medium upper bounds (note that in the case of the AIM/CGE2.1: TERL_LowCarbonTransportPolicy scenario, V=1 even though E₂₀₅₀ is slightly below the medium upper bound, due to how the two energy-sector levers are combined when calculating V - see supplement for more information). We therefore find evidence in support of hypothesis B in table 2, suggesting that achieving the 1.5 °C targets is likely only possible when utilising some mitigation levers at challenging levels. This assessment would only change if there are relevant and significant mitigation levers that are not yet included in the scenarios (e.g. carbon storage in industrial products, ocean CDR and solar reduction management).

However, we do find some room for further investigation of portfolios that favour energy- and land-use-system transformation compared to geologic CDR technologies. Figure 4 offers insight into the 'overuse' or 'underuse' of different mitigation levers within the complete and filtered ensembles (i.e. the mitigation lever is above or below the medium upper bound). In this figure the two energy-sector levers (CI₂₀₅₀ and E₂₀₅₀) are combined, as in the calculation of the coverage indicator (for more information see the supplement). We find that the reduction in energy and CH₄ emissions in both the filtered and entire ensembles lie on average close to the medium upper bound, whereas CDR_geo tends to lie above it (i.e. it is overused). The spread in CDR_AFOLU is large, with a greater tendency to overuse in the filtered ensemble. Given that the reduction in energy-sector emissions contributes over two thirds of the total medium-potential mitigation (see the right-most column of table S2 in the supplement), this suggests there is still some room to increase CO₂ mitigation in the wider SR1.5 scenario ensemble, whilst remaining within the medium upper bounds for all levers. This would potentially reduce overall reliance on speculative CDR technologies and roll-out plans in the scenario ensemble.

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