Chapter 2. Mitigation pathways compatible with 1.5°C in the context of sustainable development. IPCC Special Report on global warming of 1.5°C2018
This chapter examines mitigation pathways consistent with limiting warming to 1.5°C above preindustrial levels. In doing so, it explores the following key questions: What is the remaining budget of CO2 emissions to stay below 1.5°C? To what extent do 1.5°C scenarios involve overshooting and returning to below 1.5°C by 2100? {2.2, 2.6} How is the carbon budget affected by non-CO2 emissions? {2.2, 2.3, 2.4, 2.6} What do 1.5°C pathways imply about transitions in energy, land use and sustainable development? {2.3, 2.4} How do policies in the near term affect the ability to limit warming to 1.5°C? {2.3, 2.5} What are the strengths and limitations of current modelling tools? {2.6} There is very high risk that under current emission trajectories or current national pledges the Earth will warm more than 1.5°C above preindustrial levels. Limiting warming to 1.5°C would require a rapid phase out of net global carbon dioxide (CO2) emissions and deep reductions in non-CO2 drivers of climate change such as methane. Such ambitious mitigation pathways are put at risk by high population growth, low economic development, and limited efforts to reduce energy demand. In comparison to a 2°C limit, required transformations are qualitatively similar but more pronounced and rapid over the next decades (high confidence) {2.3.1, 2.3.5, 2.5.1}. It is possible to define consistency with limiting warming to 1.5°C in different ways, including pathways that keep global average temperature below 1.5°C and those that overshoot 1.5°C and return later in the century. These different types of pathways come with very different implications and risks, including for sustainable development. For the purposes of this chapter, any scenario (non-overshoot and overshoot) with a greater than 50% probability of limiting warming to 1.5°C in 2100 is referred to as a "1.5°C scenario", with variations highlighted where appropriate. {2.3.1, 2.2.3, 2.5.3} This assessment evaluates the temperature outcome from quantitative model descriptions of emissions associated with the energy system, land use and the economy. While such model results provide insight into the consequences of policy options and their interplay with socioeconomic and technological development, the models are constrained by multiple underlying assumptions. For this reason, their results are complemented in this assessment with other types of studies and evidence. {2.1.3, 2.2.1, 2.6.1, 2.6.2} Remaining Carbon Budgets of 1.5°C pathways This assessment explores two types of remaining carbon budgets. The first is the Threshold Peak Budget (TPB), defined as cumulative CO2 emissions from 1 January 2016 until global mean temperature peaks. The second is the Threshold Return Budget (TRB), defined as cumulative CO2 emissions until global mean temperature returns to 1.5 or 2°C after a temporary temperature overshoot. Budgets are computed assuming that warming is limited to 1.5 or 2°C with either 50% likelihood or 66% likelihood, and accounting for non-CO2 drivers. Current emissions are ~40 GtCO2 yr-1 , which means budgets from 2019 onwards will be ~120 GtCO2 lower than counting from the start of 2016. The range accompanying budget calculations are based on available scenarios and cover physical uncertainty as well as variations in non-CO2 emissions. Values are presented in Table ES1. {2.2.2} Do Not Cite, Quote or Distribute 2-5 Total pages: 143 long-lived greenhouse gases (predominantly nitrous oxide from agriculture). Such pathways also reduce emissions of short-lived climate forcers (particularly methane) as much as possible. {2.2.2} Remaining uncertainties in the Earth system, including feedbacks and radiative forcings, primarily increase rather than decrease the risk of exceeding 1.5°C of warming (medium confidence). Uncertainties in radiative forcing and revisions in methane forcing allow only medium confidence in the assessed likely range. Most uncertainties in the Earth system, including permafrost feedbacks and the saturation of carbon uptake by the biosphere, are expected to reduce available carbon budgets and, therefore, increase the risk of exceeding 1.5°C of warming. In addition, budgets are sensitive to uncertainties in estimating temperature change since preindustrial times, current land-use emissions, climate sensitivity, and the impact of non-CO2 forcers (especially aerosols). {2.2.2, 2.6.2} The risk of passing 1.5°C and the requirements for urgent action Even with emissions reductions in line with countries' pledges under the Paris Agreement, known as Nationally-Determined Contributions (NDCs), a large share of the TPB would be exhausted by 2030 (median confidence). This means there is high risk that warming will exceed 1.5°C during the 21 st century and remain above it by 2100 if emissions are reduced only to the level of current commitments, or remain above them. Current NDCs are estimated to result in greenhouse gas emissions of ~49-56 GtCO2-eq yr-1 in 2030. In contrast, 1.5°C scenarios available to this assessment show an interquartile range of 14 to 48 GtCO2-eq yr-1 in 2030. If current pledges are followed to 2030, there are no model scenarios in which average warming is kept below 1.5°C. The large majority of models also fail to return warming to below 1.5°C by the end of the 21 st century if global emissions reduce in line with NDCs but no further. There is a high risk, therefore, that even if current NDCs are met, the post-2030 transformations that would be required to limit warming to 1.5°C are too steep and abrupt to be achieved even by the large portfolio of mitigation options that is considered in models (high confidence). {2.3.1.1, 2.3.5, Table 2.7, Cross-chapter Box 4.1} Delayed action or weak near-term policies increase the risk of exceeding 1.5°C and stranded investment in fossil-based capacity, leading to higher long-term mitigation challenges (high confidence). Historical emissions and policies already mean that pathways with at least a 66% likelihood of holding global warming below 1.5°C are out of the reach of models (medium confidence; Table ES1). Failure to achieve near-term emissions reductions would mean faster rates of change afterwards to stay consistent with 1.5°C, as well as generally higher cumulative CO2 emissions until carbon neutrality is reached (global net zero CO2 emissions). This, in turn, implies a larger requirement for carbon dioxide removal (CDR), and a higher and longer exceedance of the 1.5°C temperature limit. A lack of near-term policy commitment and regulatory credibility hinders mitigation investments and increases abatement costs. (high confidence) {2.1.3, 2.3.2, 2.5.1, 2.5.2} Strong carbon pricing mechanisms are necessary in 1.5°C scenarios to achieve the most cost-effective emissions reductions (high confidence). Discounted carbon prices for limiting warming to 1.5°C are three to seven times higher compared to 2°C, depending on models and socioeconomic assumptions (medium confidence). Carbon pricing can be usefully complemented by other policy instruments in the real world. For example, technology policies can also have an important role in the near term. {2.5.1, 2.5.2} Adopting a 1.5°C rather than 2°C pathway implies faster socio-technical transitions and deployment of mitigation measures. The shift from 2°C to 1.5°C also implies more ambitious, internationally cooperative and transformative policy environments in the short term that target both supply and demand (very high confidence). To keep the target of limiting warming to 1.5°C within reach, the stringency and effectiveness of policy portfolios is critical, as well as their diversity beyond carbon pricing. Pathways that assume stringent demand-side policies, and thus lower energy intensity and limited energy demand, reduce the risks of exceeding 1.5°C. {2.5, 2.5.1, 2.5.2} Limiting warming to 1.5°C requires a marked shift in investment patterns (high confidence), implying a financial system aligned with mitigation challenges. Studies reveal a gap between current investment patterns and those compatible with 1.5°C (or 2°C) scenarios. Whereas uncertainties exist regarding the extent of required investments (1.4-3.8 trillion USD annually on the supply side for 2016-2050), studies Do Not Cite, Quote or Distribute 2-7 Total pages: 143 confidence). Since some non-CO2 warming agents are emitted alongside CO2, particularly in the energy and transport sectors, non-CO2 emissions can be addressed through CO2 mitigation as well as through specific measures, for example to target agricultural methane, black carbon from kerosene lamps or HFCs (such as the Kigali Amendment). (high confidence) Every tenth of a degree of warming that comes from non-CO2 emissions reduces the remaining carbon budget for 1.5°C by ~150 GtCO2, increasing the risk of exceeding 1.5°C (medium confidence). Mitigating non-CO2 emissions can carry large benefits for public health and sustainable development, particularly through improved air quality. (high confidence) {2.2.2, 2.3.1, 2.4.2, 2.5.1} Properties of transitions in mitigation pathways before mid-century In 1.5°C scenarios, mitigation options are deployed more rapidly, at greater scale, and with a greater portfolio of options than in 2°C scenarios. Key technical and behavioural options are sector and region specific but generally include efficiency improvements, reduction in demand and switching to lower-carbon sources of energy (including renewables and/or nuclear) (high confidence). End-use electrification replacing fossil fuels plays a major role in the buildings, industry and transportation sectors. {2.3.4, 2.4.1, 2.4.2, 2.4.3} 1.5°C scenarios include rapid electrification of energy end use (about two thirds of final energy by 2100 alongside rapid decreases in the carbon intensity of electricity and of the residual fuel mix (high confidence). The electricity sector is fully decarbonized by mid-century in 1.5°C pathways, a feature shared with 2°C pathways....
Joeri Rogelj
