The 1.5°C Target, Political Implications, and the Role of BECCS
Summary and Keywords
The 2°C target for global warming had been under severe scrutiny in the run-up to the climate negotiations in Paris in 2015 (COP21). Clearly, with a remaining carbon budget of 470–1,020 GtCO2eq from 2015 onwards for a 66% probability of stabilizing at concentration levels consistent with remaining below 2°C warming at the end of the 21st century and yearly emissions of about 40 GtCO2 per year, not much room is left for further postponing action. Many of the low stabilization pathways actually resort to the extraction of CO2 from the atmosphere (known as negative emissions or Carbon Dioxide Removal [CDR]), mostly by means of Bioenergy with Carbon Capture and Storage (BECCS): if the biomass feedstock is produced sustainably, the emissions would be low or even carbon-neutral, as the additional planting of biomass would sequester about as much CO2 as is generated during energy generation. If additionally carbon capture and storage is applied, then the emissions balance would be negative. Large BECCS deployment thus facilitates reaching the 2°C target, also allowing for some flexibility in other sectors that are difficult to decarbonize rapidly, such as the agricultural sector. However, the large reliance on BECCS has raised uneasiness among policymakers, the public, and even scientists, with risks to sustainability being voiced as the prime concern. For example, the large-scale deployment of BECCS would require vast areas of land to be set aside for the cultivation of biomass, which is feared to conflict with conservation of ecosystem services and with ensuring food security in the face of a still growing population.
While the progress that has been made in Paris leading to an agreement on stabilizing “well below 2°C above pre-industrial levels” and “pursuing efforts to limit the temperature increase to 1.5°C” was mainly motivated by the extent of the impacts, which are perceived to be unacceptably high for some regions already at lower temperature increases, it has to be taken with a grain of salt: moving to 1.5°C will further shrink the time frame to act and BECCS will play an even bigger role. In fact, aiming at 1.5°C will substantially reduce the remaining carbon budget previously indicated for reaching 2°C. Recent research on the biophysical limits to BECCS and also other negative emissions options such as Direct Air Capture indicates that they all run into their respective bottlenecks—BECCS with respect to land requirements, but on the upside producing bioenergy as a side product, while Direct Air Capture does not need much land, but is more energy-intensive. In order to provide for the negative emissions needed for achieving the 1.5°C target in a sustainable way, a portfolio of negative emissions options needs to minimize unwanted effects on non–climate policy goals.
Before the Conference of the Parties in Paris in 2015 (COP21), the previously endorsed 2°C target had come under quite some scrutiny (Victor & Kennel, 2014; Geden & Beck, 2014; Knutti, Rogelj, Sedláček, & Fischer, 2016). The reason for this was that the IPCC’s Fifth Assessment Report in 2014 (Intergovernmental Panel on Climate Change [IPCC], 2014) had shown that the window of opportunity for early action was closing. Drastic emission reductions would be necessary to maintain larger than even chances of limiting warming to 2°C above pre-industrial levels. This would in many cases be achieved by a removal of CO2 from the atmosphere, which in the scenarios is mostly achieved by combining low-carbon Bioenergy with Carbon Capture and Storage (BECCS)—a highly contentious technology in the debate. In fact, not all models could find feasible pathways to 2°C if key technologies such as Carbon Capture and Storage (CCS) were excluded (Clarke et al., 2014). Rather than promoting specific technologies, however, the scenarios expressed the consequences of a simple relationship: with global warming being proportional to the cumulative amount of long-lived greenhouse gas emissions (GHGs) in the atmosphere1 (Allen et al., 2009; Matthews, Gillett, Stott, & Zickfeld, 2009), ongoing disposal of about 40 Gt CO2 per year into the atmosphere (Friedlingstein et al., 2014, Le Quéré et al., 2015), and an average remaining budget of less than 800 Gt CO2 from 2015 onward2 (IPCC, 2014), simple arithmetic tells us that rapid decarbonization is needed to maintain a likely chance of limiting global warming to 2°C. Still, there is some flexibility with respect to strategy (e.g., lower energy demand can make room for coping without carbon removal from the atmosphere), as the analyses by Luderer, Pietzcker et al. (2013) and Rogelj et al. (2015) show.
Despite these concerns and the open debate with respect to the feasibility of the 2°C target before the negotiations, what eventually emerged from Paris was an agreement to stabilize “well below 2°C above pre-industrial levels” and to “[pursue] efforts to limit the temperature increase to 1.5°C” (United Nations Framework Convention on Climate Change [UNFCCC], 2015a). The motivation for this development comes from the fact that a substantial fraction of the parties considered climate impacts at 2°C already to be beyond their coping capacity. More specifically, the proposal for Article 3 of the Paris Agreement had been put forth by the Alliance of Small Island States (AOSIS), comprising 38 states, and the Group of Least Developed Countries (LDCs), which counts 48 member countries (UNFCCC, 2015b). A then new study by Schleussner et al. (2016)3 added to the synthesis of risks and climate impacts by the IPCC’s WGII from the previous year, showing that the incremental differences in climate impacts between 1.5°C and 2°C are most significant and pronounced for regions with limited adaptive capacity and high exposure, which applies to both AOSIS and LDCs (UNFCCC, 2015b, 2015c). More specifically, the authors’ analysis of precipitation-related impacts of half a degree more warming reveals distinct regional differences. For example, the projected decline in median water availability for the Mediterranean is found to increase from 9% to 17% and the projected lengthening of regional dry spells rises from 7% to 11%. Also, there are large differences between projections for agricultural yields depending on crop type and region. Whereas high-latitude regions may benefit, tropical regions will be more exposed to local yield reductions, especially for wheat and maize (Schleussner et al., 2016).
Clearly, the move from 2°C to 1.5°C narrows the window of opportunity dramatically, as in the worst case, the remaining budget (as of 2015) to achieve the 1.5°C target has already been almost exhausted (IPCC, 2014). That means that there is not much room to maneuver without carbon removal—or so-called negative emissions—as will be shown in the synthesis of current scenario work in Pathways to 1.5°C. In fact, the work that has been done on the 1.5°C target, for example, in terms of the number of scenarios, is currently still less than the more comprehensive assessment of 2°C pathways. Yet, there is some literature (e.g., Luderer, Pietzcker et al., 2013; Rogelj et al., 2015) that can help to explain the characteristics of a 1.5°C stabilization pathway and how this differs from what we know about the 2°C ones. In addition, recently much research has been conducted on negative emissions and what the implications would be of large-scale BECCS deployment, in particular. So some light can be shed on the challenges to mitigation under the 1.5°C target and its implications for other policy goals.
A synthesis of the knowledge on 1.5°C pathways will be provided with a focus on the required sectoral emissions reductions over time, factors of uncertainty including the behavior of the carbon cycle under overshoot, and the role of negative emissions. The latter discussion will be focused on BECCS, as BECCS is the single largest carbon removal option deployed in current scenarios. Due to the heated debate around the implications of large-scale BECCS deployment, recent research highlighting other options will be presented as well. Finally, the political implications will be covered.
Pathways to 1.5°C
As previously argued, the wealth of information on 1.5°C pathways is still less than what we have at our disposal on 2°C from the IPCC’s 5th Assessment Report (AR5). However, Rogelj et al. (2015) identify a few studies offering scenarios consistent with 1.5°C (Luderer, Pietzcker et al., 2013; Rogelj, McCollum, Reisinger, Meinshausen, & Riahi, 2013; Rogelj, McCollum, O’Neill, & Riahi, 2013; Azar, Johansson, & Mattsson, 2013; Ranger et al., 2012) and build a set of more than 200 scenarios (from the Integrated Assessment Models employed in Luderer, Pietzcker et al., 2013; Rogelj, McCollum, Reisinger et al., 2013; Rogelj, McCollum, O’Neill et al., 2013) in order to study the characteristics of 1.5°C pathways compared to the ones targeting 2°C.4 This analysis is currently the most comprehensive assessment available. Surely, more research will emerge with the IPCC’s preparation of the special report in 2018 on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways (UNFCCC, 2015a). However, the resulting challenges can already be distilled in a more systematic way from this existing work.
Within the sample of Integrated Assessment Modeling (IAM) scenarios, one feature systematically distinguishes the 1.5°C from the 2°C challenge: there is not a single pathway with a high probability of achieving the target without surpassing it during the course of the century (Rogelj et al., 2015). That implies that there will at some point be an overshoot in terms of temperature increase. As emissions peak and then decline, the temperatures respond by declining toward the target value toward the end of the century.
However, it is by no means clear to which extent climate change is reversible. Zickfeld et al. (2013) present results from a large intercomparison of Earth System Models, concluding that it is very difficult to revert from a given level of warming at timescales relevant to human activity, even if emissions are completely eliminated. Their findings indicate that significant negative emissions would be needed to reverse global warming at such timescales. Jones et al. (2013) also present results from a model intercomparison project, where state-of-the-art climate models quantified carbon emissions compatible with the IAM results for the four different Representative Concentration Pathways (RCPs). The latter correspond to the four (escalating) GHG concentration trajectories adopted by the IPCC for the AR5 (van Vuuren, Edmonds et al., 2011). Looking at the most ambitious trajectory, which is RCP2.6 (van Vuuren, Stehfest et al. 2011), the authors find that an average reduction of 50% in emissions by 2050 from 1990 levels would be required. They emphasize that there is a very large model spread (14%–96%), however, and that the models disagree on the amount of negative emissions to achieve the RCP2.6 CO2 concentration and the probability of restricting global warming to below 2°C.
Furthermore, there is much uncertainty to what extent unknown thresholds could be surpassed during an overshoot, which would trigger irreversible damage (e.g., by releasing even more GHGs; van Vuuren, Stehfest et al. 2011) and how ocean and terrestrial sinks would respond to large carbon removals discussed in the following subsection (Ciais et al., 2013, section 6.5; Jones et al., 2016; Zickfeld, MacDougall, & Matthews, 2016).
Related to the characteristic of overshoot, there are no feasible 1.5°C scenarios to date that can cope without carbon dioxide removal. In the analyzed sample, cumulative negative emissions amount to between 450 and 1,000 Gt CO2 until 2100, which is substantially more than in likely 2°C scenarios (Fuss et al., 2014). It is important to note that Rogelj et al. (2015) detect likely 2°C scenarios, for which negative emissions are not only considerably lower, but there are also a few cases for which they are zero. For example, Luderer, Pietzcker et al. (2013) point to unprecedented improvements in energy efficiency as a possibility to expand the mitigation options space and potentially decrease overall policy costs. Turning to current 1.5°C pathways, there is to date not a single one that achieves stabilization without removals of carbon, however.
The Role of BECCS
Negative emissions in the scenarios currently available in the literature are mostly achieved through BECCS. In fact, turning to the 2°C scenarios in the AR5 Database, there are 116 scenarios that are consistent with a larger than 66% probability of limiting warming below 2°C (corresponding to concentrations of 430–480 ppm CO2eq in 2100), of which almost 90% extract CO2 from the atmosphere in the second half of this century (Fuss et al., 2014).
The main mechanism at work here is that the biomass grown as feedstock for bioenergy sequesters about as much carbon as is eventually released in the energy generation process, including all emissions from the supply chain such as harvest and transport (see below for references to techno-economic and life cycle analyses on this). If the emissions from the generation process are in addition captured and stored in geological reservoirs, this results in a net removal of carbon dioxide from the atmosphere.
BECCS has been subject to research for a long time (Williams, 1998; Obersteiner et al., 2001; Kraxner et al., 2003; Read & Lermit, 2005; Edenhofer et al., 2010; Azar et al., 2010; Gough & Upham, 2011; Tavoni & Socolow, 2013;5 Kriegler, Edenhofer, Reuster, Luderer, & Klein, 2013; van Vuuren et al., 2013; Minx et al., 2017) but appears to have ignited public concern only in the most recent years and especially since AR5 (Tollefson, 2015). Recent publications have looked more closely into the uncertainties (Fuss et al., 2014),6 synergies and trade-offs with other (non-climate) policy goals (von Stechow et al., 2016),7 and implications of realizing the potentials indicated by AR5 scenarios and whether alternative negative emission options—currently not included in the scenarios—could live up to these “demands” (Smith et al., 2016; Smith, 2016).
The concerns arising around large-scale deployment of BECCS in the low-stabilization scenarios are mostly related to what they imply for other priorities in the policy sphere. Most important, there is an ongoing discussion with respect to the vast areas of land that would be needed for the cultivation of biomass as a feedstock while still feeding a growing population. Furthermore, as with any land-intensive practice, the worry is that other ecosystem services will suffer losses as BECCS expands, in particular in terms of habitat loss for biodiversity. Another resource implication frequently put forth is the increased demand for fertilizers and water, even though it is clear that this will vary depending on the technology chosen, feedstock type, management practices, and many other factors. More fundamentally, it is often doubted whether bioenergy can indeed be produced in a more or less carbon-neutral way so that it will be possible to go carbon-negative. In particular, scientific assessments feature wide ranges and little disagreement about the amount of bioenergy that could be sustainably produced by mid-century (Creutzig et al., 2015). In the literature, there definitely exist a number of studies—including lifecycle analyses,8 techno-economic mitigation cost estimates (Uddin & Barreto, 2007; Hetland, Yowargana, Leduc, & Kraxner, 2016), and modeling studies (Kraxner, Nilsson, & Obersteiner, 2003; Repo, Tuovinen, & Liski, 2015), which demonstrate how carbon-neutral biomass could be sourced. Repo et al. (2015) show that increased carbon sequestration can compensate for the carbon loss from forest residue removal, so that zero net CO2 emissions occur over a forest rotation period. However, they still find that a warming climate impact occurs because of the time lag between the carbon loss (instantaneously) and the increased carbon sequestration (over a time span of decades). Other studies raising concern with respect to the carbon-neutrality assumption can be found in the literature on indirect land use change, site-specific barriers, and problems to achieve scale without impacts on the environment (Plevin, O’Hare, Jones, Torn, & Gibbs, 2010; Fargione, Hill, Tilman, Polasky, & Hawthorne, 2008; Havlík et al., 2011; Haberl et al., 2012; Popp et al., 2013). In addition, lifecycle analyses should be carefully interpreted, as methods differ widely, and it is often misleading to take their results at face value (see Plevin, Delucchi, & Creutzig, 2013 for an assessment of this vast strand of literature).
One way forward would be to develop more site-specific studies on local BECCS potentials, which can then complement the global scenarios, while at the same time being more explicit about local conditions (Kraxner et al., 2012). Another avenue is the development of country- and sector-specific road maps, which can align the findings from such detailed bottom-up studies with the “top-down demands” from the more aggregated stabilization pathways discussed in Overshoot. Creutzig et al. (2012) assess the discrepancy between top-down and bottom-up studies systematically and find that the former tend to be more optimistic because they cover longer planning horizons and technical progress, while bottom-up studies are often less optimistic with respect to new technologies and local problems, which are a lot less prominent in aggregate analyses. Furthermore, results are often influenced by the framework of analysis, where ecologists have a tendency to be more grounded in biophysical processes and today’s observations, while integrated assessment models emphasize economic dynamics and long timescales (Creutzig et al., 2015). The most important distinguishing factor here is probably what is implicitly or explicitly assumed about future improvement in biomass yields, leading to large ranges in potentials estimates.
Finally, as concerns related to BECCS might be difficult to address in the near term, it is also important to look into alternative options for carbon removal, which can help to alleviate some of the pressures that BECCS has been criticized for. On the latter, there has been some progress in the recent years, which is briefly reviewed here.
Alternatives to BECCS
BECCS is the one carbon removal option that has been deployed most widely in the low-stabilization pathways so far, reaching a median of about 12 Gt CO2 removal per year in 2100 for 2°C and about 18 Gt CO2 per year for 1.5°C (Rogelj et al., 2015). Alternatively, or as a complement, there is also literature that encompasses afforestation as a negative emissions option. Afforestation, reforestation, and forest management work through enhancing terrestrial sinks. That is, the planting of additional trees will sequester CO2 during their growth phase, thereby removing CO2 from the atmosphere and storing it in the newly cultivated biomass. Humpenöder et al. (2014), for example, employ a global land use model to estimate the negative emissions potential of afforestation, which they find to be similar to that of BECCS (600–700 Gt CO2 in 2100). However, while the concerns for BECCS are mostly related to its land footprint, it is interesting to see that afforestation would require even more land to generate the same amount of negative emissions. The authors also investigate the drivers of future mitigation potentials through BECCS and afforestation. For the former, the decisive factors are the development of future bioenergy yields and the availability of safe geological carbon storage; for the latter it is mostly the length of the crediting period for CO2 stored in new forests.
An option to remove carbon dioxide with a negligible land footprint is Direct Air Capture (DAC). DAC absorbs CO2 from the ambient air by means of chemical reaction and subsequently stores the absorbed CO2 in geological formations in the same way as CCS. The same concerns with respect to availability of safe storage mentioned in the context of BECCS apply here as well, therefore. In addition, the process of absorbing CO2 from ambient air is still energy-intensive, so even though the technical mitigation potential is as high as for BECCS, a low-carbon or ideally carbon-neutral source of energy first has to be established to guarantee the ability to go carbon-negative.
Similarly land-saving, but also relatively energy-intensive is the process of enhanced weathering, which involves grinding minerals that absorb CO2 and spreading them over agricultural or forest area (or even on the surface of the ocean). The mitigation potential is much lower compared to BECCS, afforestation, and DAC. This is mainly due to the limited supply of suitable minerals and eventually also the area on which the grinded material can be spread. Yet, enhanced weathering has been shown to be a viable complement to BECCS (Strefler, Bauer, Amann, Kriegler, & Hartmann, 2015), which in some cases even enhances agricultural fertility.
Other negative emission options, which have a relatively low potential, but partially economically attractive co-benefits, include soil carbon sequestration and biochar. The first option involves enhancing soil carbon, for example, through changes in management practices. Biochar is based on the pyrolysis of biomass, which is subsequently added to the soil to store the embedded carbon. The literature on these options is very diverse, and Smith (2016) provides a comprehensive synthesis with respect to the potential for negative emissions. The combined potential emerging from the study amounts to 5 Gt CO2 per year in 2100.9
Smith et al. (2016) offer the most up-to-date synthesis of the different options, comparing them on the basis of their mitigation potentials in 210010 and their requirements in terms of land, water, nutrients, cost, energy, and potential impacts on the albedo.11 Table 1 summarizes the main results of this study and Smith (2016) to give a short-cut overview here without going into the details of the calculations.
Negative Emission Option
2100 potential (Gt CO2/year)
Most important trade-offs
High land requirement, but produces energy, medium water requirement, variable on nutrients and albedo
High land and water requirement, very low energy demand, 16.8 kt nitrogen per year to achieve 12.1 Gt CO2 removals in 2100, but much lower investment costs than BECCS, negative albedo impact
No impact on nutrients and albedo and virtually no land footprint, but high energy requirement and much higher investment costs than BECCS
Similar to DAC, but less energy-intensive, less expensive (though more expensive than BECCS), and lower water requirement
Soil carbon storage
No water, land, or energy requirement, no effect on albedo, significant nutrient retention, negative cost options.
Similar to soil carbon storage, but higher land requirement (though small compared to BECCS), produces energy, might affect albedo, positive costs
While much of the research to date has been conducted for 2°C, the 1.5°C target will obviously intensify the reliance on negative emissions in as far as the mitigation potential for non-CO2 GHGs is mostly already exhausted for keeping warming below 2°C.
As will become clear in The Time and Sector Profile of Emissions Reductions, negative emissions in any ambitious climate change mitigation strategy cannot be relied upon as a fallback option or an alternative to near-term climate action. In fact, they accompany strong decarbonization efforts in the short and medium run. Embarking on a business-as-usual emissions pathway and removing much more CO2 later on will not be feasible and would also be accompanied by the otherwise prevalent side effects of higher concentrations such as ocean acidification.
The Time and Sector Profile of Emissions Reductions
Although the features of overshoot and negative emissions in the form of BECCS are outstanding in the characterization of 1.5°C scenarios, there are also more challenges emerging when looking more closely into the timeline and the sectoral decomposition of the required mitigation effort. With respect to the sectors, Rogelj et al. (2015) find that by mid-century emissions in the industrial, buildings, and transport sectors will need to be 25%, 40%, and 50%, respectively, lower compared to scenarios with a likely chance of meeting the 2°C target. This again underlines the loss of flexibility for decarbonization when moving toward a 1.5°C target. As the authors rightly point out, this implies that barriers will need to be removed to unfold the full mitigation potential in sectors like transport, which are more difficult and costly to decarbonize. In addition, more of the burden will be shifted to the end-use side.
Geographically speaking, more mitigation will have to occur in non–Annex 1 countries, simply because their baseline emissions are estimated to be higher due to ongoing population and economic growth and because their mitigation potentials at a given carbon price will be higher (Rogelj et al., 2015). This will be an additional distributional consequence to consider in further negotiations, as discussed in Distributional Consequences of Climate Change Mitigation and Political Feasibility.
While the IPCC (IPCC AR5 WGIII) has been quite clear that emissions reductions need to start in 2020 for the 2°C target, the emissions reductions observed for the set of 1.5°C scenarios analyzed in Rogelj et al. (2015) feature more rapid and profound reductions. These would require a large-scale reversion to low-carbon investments in this and the coming decade.12 In addition, energy demand is substantially constrained in those scenarios, with energy efficiency improvements being key to enable the achievement of the 1.5°C target.
In the light of the current knowledge on 1.5°C pathways just reviewed, political challenges to live up to this new ambition are vast. Four implications that will be important to be addressed in the short-to-medium-run will be highlighted, without claiming this to be an exhaustive analysis for the longer term and across all governance levels: (1) the larger context of the Paris Agreement and what reliance on the emission reduction pledges made by countries, the so-called Intended Nationally Determined Contributions (INDCs) would imply; (2) the lack of public acceptance that can limit political feasibility of many stabilization pathways; (3) the missing discussion on policy instruments, hampering fast action; (4) the need to address distributional consequences of ambitious climate change mitigation in order to improve political feasibility.
After Paris: How Far Do We Get with the INDCs?
The Paris Agreement has been celebrated for its move from a top-down approach allocating emissions reductions to nation-states. The new approach is a hybrid one, which builds on the INDCs. These are national emission reduction plans with a diversity of measures, which countries have pledged to carry out until 2030 and which will be subject to a process of global stock-taking (and possible ratcheting-up) in between. However, even though they are definitely a political breakthrough and clearly signal the parties’ increasing interest in an enhanced, decentralized approach to achieve climate change goals, there is currently no legal binding mechanism of enforcement or sanctioning of unfulfilled pledges. Even though the parties have started to work on implementation at COP22 in Marrakech in 2016, many questions will still have to be resolved. (See Climate Change Policy in the European Union.)
In addition, even though the estimated global emissions under fulfillment of the INDCs result in a clear reduction of emission growth until 2030 (Spencer, Pierfederici, Waisman, & Colombier, 2015), the remaining budget will still largely have been exhausted by that time (UNFCCC, 2016),13 in the worst case resulting in a carbon debt already at this stage. These pledges can therefore not be relied upon for an achievement of the 2°C or even 1.5°C targets. In fact, these INDC trajectories are very similar to the pathways with delayed action (Luderer, Pietzcker et al., 2013), where a delay of another 15 years is found to push even the 2°C target out of reach.
The first panel in Figure 1 adds the INDC emissions range in 2030 (red) to the GHG emissions pathways that are grouped in classes staying below 50 Gt CO2eq (dark green) and above 55 Gt CO2eq (light green). The latter category is consistent with the Cancún Pledges and shows the largest overlap with the INDC emissions range in 2030. Therefore, similarly ambitious CO2 emission reductions (second panel) will be needed after 2030, further underlining the political challenge.
Public Acceptance as a Major Bottleneck
In recent years a large debate on feasibility and sustainability has emerged around deep decarbonization with a particular focus on negative emissions as previously discussed. As these are mostly contributed in the form of BECCS, public acceptance is actually a problem on two fronts: on the one hand, CCS is unpopular due to concerns about it being a strategy in favor of prolonging the profitability of the fossil fuel industry (Shackley et al., 2009; Upham & Roberts, 2011; and indirectly also Wallquist, L’Orange Seigo, Visschers, & Siegrist, 2012). Further factors lowering acceptance relate to safety and environmental issues (e.g., De Best-Waldhober, Daamen, & Faaij, 2009; Ha-Duong, Nadai, & Campos, 2009; Reiner et al., 2006). On the other hand, bioenergy has come under scrutiny in the aftermath of the food price hikes in 2007 and 2008 (e.g. Trostle, 2010). Three points can be made on the basis of what the current state of knowledge tells us.
First, the current pathways to 2°C require that most of the fossil resources have to stay under ground (Bauer et al., 2013; McGlade & Ekins, 2015; Jakob & Hilaire, 2015). McGlade and Ekins (2015) estimate that about 80%, 50%, and 30% of coal, gas, and oil reserves, respectively, would need to remain untapped in order to achieve the 2°C target. Looking at the IPCC scenarios (IPCC AR DB, 2014), where still much of this needs to remain unused to keep chances to reach 2°C within reach and given the arithmetic of the 1.5°C target, this will be even more pronounced. Thus, CCS is an important element in the mitigation portfolio not because it enables continued use of fossil fuels, but because it enables the removal of CO2 in combination with low-carbon bioenergy. In the 2°C scenarios, the potential of mitigating non-CO2 GHGs is almost exhausted (Rogelj et al., 2015). In order to achieve 1.5°C, further reductions will have to come from CO2, and this is incompatible with continued use of fossil fuels. However, it also needs to be kept in mind that not having CCS also means that mitigation will cost substantially more (Luderer, Pietzcker et al., 2013).
Second, there is no conclusive empirical evidence that links the food price hikes in the late 2000s unambiguously and exclusively to the increased use of biomass for energy purposes (Myers, Johnson, Helmar, & Baumes, 2014; Algieri, Kalkuhl, & Koch, 2015). While it might have been a contributing factor, and land use competition of the extent projected in low-stabilization scenarios will definitely add to pressure on prices (Popp et al., 2013), other factors such as droughts in key supply regions are found to be equally important price drivers. These could be magnified under ongoing climate change. Without trying to argue in favor of bioenergy, the state of the empirical evidence should remind us that we are faced with delicate trade-offs here. These need to be better understood to inform decision-making rather than painted black and white to further shrink the narrowing options space.
Third, to the extent that bioenergy is found to come at unacceptable risks to other policy goals (cf. discussion in Negative Emissions), other options to remove CO2 and to reduce the reliance on negative emissions have to be considered. It has been shown in Negative Emissions that each of the options currently known runs into different sets of risks, so these have to be actively investigated in order to be able to maximize mitigation potential while minimizing risks. Being transparent about this and working on the communication of these results also in the phases of demonstration and deployment can further help to improve public acceptance and thereby revert attention to early action.
Finally, BECCS has also been criticized on the grounds of being an unproven technology. While bioenergy and CCS are both known, it is true that CCS deployment has been lagging behind oadmaps in line with a 2°C target (International Energy Agency, 2016; Peters et al. 2017), so would also fall short of requirements under 1.5°C.
Policy Instruments to Get Started
With the equivalent of more than 15,000 Gt CO2 still underground in the form of fossil reserves and the remaining repository in the atmosphere dwindling, there is a strong argument for pricing carbon (Edenhofer, Flachsland, Jakob, & Lessmann, 2013). This could take different forms and does not have to be tax per se, but the incentive to abandon fossil fuel use needs to be given to maintain the window of opportunity for meeting a 2°C or even 1.5°C target. However, despite the move toward a more ambitious climate target and the urgency to completely decarbonize in the face of a rapidly decreasing budget, little has been moving at the political front that could be called transformative (Peters, 2016; Anderson & Peters, 2016). Definitely, no incentives are being offered to bring CCS to the scale required for low stabilization pathways (Van Noorden, 2013), which casts further doubt on the possibility to go carbon-negative with BECCS.
Edenhofer et al. (2015) show that even in the absence of a global carbon pricing regime, there are important incentives for countries to put a price on their domestic emissions. Such unilateral motivations include public finance considerations, that is, using the revenues from carbon pricing to finance other policy goals, internalizing the climate impacts of their own emissions, and co-benefits such as clean air. Capitalizing on these domestic incentives for unilateral carbon pricing, progress could be made on the way to more international carbon pricing. Research using multi-criteria optimization further underlines that such an approach will not only provide an inroad to early action but also decrease the policy costs compared to an approach where different policy goals are pursued in isolation (McCollum et al., 2013).
At the international level, we have to learn from the few carbon pricing examples we have, such as the European Union’s Emission Trading Scheme (EU ETS). However, the EU ETS has been feared to underperform for a long time, thus risking both carbon lock-in and fragmentation (Knopf et al., 2014; Edenhofer, 2014). Understanding what instruments have worked and will work in which context is a prerequisite to formulate comprehensive, yet politically feasible, mitigation strategies.
Finally, Knopf et al. (2017) argue that the development of transformative policy instruments needs to be urgently prioritized, highlighting also the importance of hitherto-neglected areas such as demand-side measures, which have to date not been systematically assessed.
Distributional Consequences of Climate Change Mitigation and Political Feasibility
The most important political implication of the 1.5°C target is probably that the stringency it implies for climate policy can further raise resistance against its implementation. There is a significant literature showing that further delay and the unavailability of certain technologies will lower the chances of maintaining high probabilities of reaching ambitious stabilization targets (Edenhofer et al., 2010; Luderer et al., 2012; Luderer, Bertram, Calvin, De Cian, & Kriegler, 2013; Riahi et al., 2015; Kriegler et al., 2014). In addition, distributional consequences will also be more pronounced (Lüken, Edenhofer, Knopf, Luderer, & Bauer, 2011; Luderer, Pietzcker et al., 2013), thus making it difficult for politicians, who also worry about the competitiveness of their national industries, to remain credibly committed. Approaching the problem from a different angle, Luderer, Pietzcker et al. (2013) estimate that economic mitigation challenges become prohibitively high for targets below 1.7°C, which would thus include the 1.5°C target. Furthermore, costs would not be distributed homogenously.
In addition, further delay will lead to investments into long-lived infrastructure and thus high emissions, that is, there will be more fossil-based assets, which would be devalued by stringent climate policy. Davis and Socolow (2014) warn that one has to consider emissions committed in current and planned infrastructure. They find that total committed emissions related to the power sector have been increasing at about 4% per year after having reached an estimated 307 Gt CO2 in 2012. In the absence of a credible commitment to ambitious climate goals and the corresponding signals to investors, this implies a risky lock-in, which gets reinforced the longer emission reductions are delayed. Pfeiffer, Millar, Hepburn, and Beinhocker (2016) present an analysis defining what they call the “2°C capital stock” as the global stock of infrastructure that would lead to an even chance of global mean temperature increases of 2°C or more. Based on the IPCC scenarios (IPCC AR5 WGIII), they estimate that the 2°C capital stock for electricity will be reached by 2017 if other sectors move along emission reduction trajectories in line with a 2°C target. If one considers that many coal-fired power plants are already being built or at least approved and thus in the pipeline (Shearer, Ghio, Myllyvirta, Yu, & Nace, 2016), this demonstrates very clearly that a stringent climate policy consistent with 2°C or even 1.5°C targets implies a substantial devaluation not only of fossil reserves, but also of the assets related to their use, such as infrastructure (Carbon Tracker Initiative & Grantham Research Institute on Climate Change and the Environment, 2013). Any climate policy that can hope to be politically feasible thus needs to carefully take into account the distributional consequences. This counts for sub-national and national policy, as much as for the regional and global level, where the move to 1.5°C adds further challenges to discussions on effort sharing.
While at the international level, technology and financial transfers from industrialized to developing and emerging countries are often discussed as possible mechanisms to alleviate the burden of climate change mitigation to developing countries, distributional impacts at the national level—for example, differences between sectors, between industry and households, or between different income groups—can be addressed with well-designed revenue recycling programs and targeted redistribution (Parry, 1995; Pezzey & Jotzo, 2012; Edenhofer et al., 2015). Taking this idea to the international level again, Jakob et al. (2016) show that global carbon pricing consistent with the 2°C target—allowing for redistribution between countries—could raise sufficient finance to provide universal access to basic infrastructure like water, sanitation, and electricity, for example.
The currently limited sample of existing scenarios on 1.5°C differs from the more comprehensively assessed 2°C scenarios in a number of ways. First, 2°C scenarios still feature a limited degree of flexibility in terms of the ability to avoid a temperature overshoot during the 21st century and to constrain the reliance on negative emissions. The analyzed 1.5°C pathways, on the contrary, are all characterized by negative emissions and a temperature overshoot. Second, in the face of the much lower carbon budget to ensure a likely chance of reaching the 1.5°C target, the emissions reductions are also more rapid and pronounced for those pathways. More precisely, those profound reductions will need to start within the next decade. Third, substantially lower energy demand than in a business-as-usual world and improvements in energy efficiency will be essential. Fourth, the mitigation pathways to 1.5°C are more expensive than those to 2°C.
Negative emissions have been assessed in more detail in the light of their pronounced role in the 2°C scenarios. A recent synthesis shows that there are different options to go carbon-negative, which go beyond the currently mostly deployed strategies of BECCS and afforestation in scenarios. Comparing their potentials; land, water, and energy requirements; costs and possible side effects; the state of knowledge so far indicates that all options run into their respective bottlenecks. This means that more research will be needed with respect to the technologies and their potential side effects and risks for other policy goals (Fuss et al., 2016). This knowledge is needed for a more comprehensive assessment of the 1.5°C options space, where the optimal technology portfolio needs to maximize mitigation potentials given what is deemed to be acceptable levels of the identified risks or minimize risks given the required amount of negative emissions.
Political implications of what is currently known about pathways to 1.5°C involve realizing that efforts are now needed that go substantially beyond the ambitions of the INDCs—not only to achieve the technological transformation per se, but also to keep the option of reaching ambitious stabilization targets open at all. Yet, without a thorough understanding of the distributional consequences, public acceptance, and a context-specific “tool box” of policy instruments, political feasibility will be limited, even if technical and economic feasibility are given. Significant questions remain in this area, which need to be urgently addressed in order to keep 1.5°C and even 2°C within reach.
Many strands of literature on the low stabilization pathways with a focus on the 1.5°C target have been synthesized. The author is therefore grateful for comments from experts in the different areas covered, feedback on an earlier version, and continued discussions on the 1.5°C target. Thanks are given to Brigitte Knopf, Gunnar Luderer, Florian Kraxner, Pete Smith, Felix Creutzig, Jan Minx, Kerstin Burghaus, Ottmar Edenhofer, and further colleagues at the Mercator Research Institute on Global Commons and Climate Change and the Global Carbon Project, specifically the research initiative on Managing Global Negative Emission Technologies (MaGNET).
—Alliance of Small Island States
—the IPCC’s Fifth Assessment Report
—Bioenergy with Carbon Capture and Storage
—Carbon Capture and Storage
—Carbon Dioxide Removal
—the 21st Conference of the Parties
—Direct Air Capture
—European Union’s Emissions Trading Scheme
—gigatons of carbon dioxide
—Intended Nationally Determined Contribution
—Integrated Assessment Modeling
—Intergovernmental Panel on Climate Change
—Group of Least Developed Countries
—Representative Concentration Pathway
—United Nations Framework Convention on Climate Change
WG I, II, III
—the IPCC’s Working Groups I, II and III
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(1.) At least as long as temperatures are rising—after the peak, the behavior might be different.
(2.) More precisely, 470–1,020 GtCO2eq is the 10%–90% range over scenarios in IPCC WGIII’s scenario category 1, that is, the range to limit warming in 2100 to below 2°C since 1850–1900 with a larger than 66% probability accounting for the non-CO2 forcing (IPCC AR5 DB).
(4.) Note that the scenarios in this set are associated with a 50% chance of achieving the 1.5°C target.
(5.) This article heads a whole special issue on different aspects of large-scale carbon removals.
(6.) These uncertainties pertain to techno-economic and biophysical aspects of negative emission potentials and their sustainability, behavior of the carbon cycle under large-scale carbon removal, policy to achieve such drastic transformation, and questions on governance and public acceptance, discussed in “Political Implications.”
(8.) See, for example, Chum et al (2011) for an overview of corresponding attributional lifecycle analysis (ALCA) results on bioenergy targeting policymakers. Plevin et al. (2013) use this study as an example to demonstrate that ALCA is not designed to answer the question of whether a change in energy system use results in climate change mitigation benefits.
(10.) The potentials calculations take the median carbon removal through BECCS of 12.1 Gt CO2 per year in 2100 (observed for scenarios consistent with the 2°C target [430–480 ppm]) as the benchmark for estimating the implications of similar deployment rates for the other options. Lower potentials indicate that the benchmark potential cannot be achieved. In that case the maximum is given in brackets.
(11.) Note that large-scale land use changes—depending on what region they happen in—can alter the amount of radiation reflected back from the Earth’s surface. This could offset some of the temperature stabilization gains.
(12.) Note that these scenarios start in 2010, which also implies that any projected emission reduction projected in the first five years of the modeling horizon basically already lies in the past.
(13.) More precisely, the updated synthesis report on the aggregate effect of the INDCs by the UNFCCC (2016) estimates that until 2030 another 738.8 (703.6 to 770.9) Gt CO2 will be added to cumulative emissions. This exceeds the remaining budget for 1.5°C by far and comes close to the 800 Gt CO2 for more than even chances of stabilizing temperature increases below 2°C.