Editing IPCC:AR6/SR15/Chapter-2 (section)

=== 2.4.3 Energy End-Use Sectors ===

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Since the power sector is almost decarbonized by mid-century in both 1.5°C and 2°C pathways, major differences come from CO <sub>2</sub> emission reductions in end-use sectors. Energy-demand reductions are key and common features in 1.5˚C pathways, and they can be achieved by efficiency improvements and various specific demand-reduction measures. Another important feature is end-use decarbonization including by electrification, although the potential and challenges in each end-use sector vary significantly.

In the following sections, the potential and challenges of CO <sub>2</sub> emission reductions towards 1.5°C and 2°C- consistent pathways are discussed for each end-use energy sector (industry, buildings, and transport). For this purpose, two types of pathways are analysed and compared: IAM (integrated assessment modelling) studies and sectoral (detailed) studies. IAM data are extracted from the database that was compiled for this assessment (see Supplementary Material 2.SM.1.3), and the sectoral data are taken from a recent series of publications; ‘Energy Technology Perspectives’ (ETP) (IEA, 2014, 2015b, 2016a, 2017a) <sup>[[#fn:r395|395]]</sup> , the IEA/IRENA report (OECD/IEA and IRENA, 2017) <sup>[[#fn:r396|396]]</sup> , and the Shell Sky report (Shell International B.V., 2018) <sup>[[#fn:r397|397]]</sup> . The IAM pathways are categorized according to their temperature rise in 2100 and the overshoot of temperature during the century (see Table 2.1 in Section 2.1). Since the number of Below-1.5°C pathways is small, the following analyses focus only on the features of the 1.5°C-low-OS and 1.5°C-high-OS pathways (hereafter denoted together as 1.5°C overshoot pathways or IAM-1.5DS-OS) and 2°C-consistent pathways (IAM-2DS). In order to show the diversity of IAM pathways, we again show specific data from the four illustrative pathways archetypes used throughout this chapter (see Sections 2.1 and 2.3).

IEA ETP-B2DS (‘Beyond 2 Degrees’) and ETP-2DS are pathways with a 50% chance of limiting temperature rise below 1.75°C and 2°C by 2100, respectively (IEA, 2017a) <sup>[[#fn:r398|398]]</sup> . The IEA-66%2DS pathway keeps global mean temperature rise below 2°C, not just in 2100 but also over the course of the 21st century, with a 66% chance of being below 2°C by 2100 (OECD/IEA and IRENA, 2017) <sup>[[#fn:r399|399]]</sup> . The comparison of CO <sub>2</sub> emission trajectories between ETP-B2DS and IAM-1.5DS-OS show that these are consistent up to 2060 (Figure 2.18). IEA scenarios assume that only a very low level of BECCS is deployed to help offset emissions in difficult-to-decarbonize sectors, and that global energy-related CO <sub>2</sub> emissions do not turn net negative at any time but stay at zero from 2060 to 2100 (IEA, 2017a) <sup>[[#fn:r400|400]]</sup> . Therefore, although its temperature rise in 2100 is below 1.75°C rather than below 1.5°C, this scenario can give information related to a 1.5°C overshoot pathway up to 2050. The trajectory of IEA-66%2DS (also referred to in other publications as IEA’s ‘Faster Transition Scenario’) lies between IAM-1.5DS-OS and IAM-2DS pathway ranges, and IEA-2DS stays in the range of 2°C-consistent IAM pathways. The Shell-Sky scenario aims to hold the temperature rise to well below 2°C, but it is a delayed action pathway relative to others, as can be seen in Figure 2.18.

Energy-demand reduction measures are key to reducing CO <sub>2</sub> emissions from end-use sectors for low-carbon pathways. The upstream energy reductions can be from several times to an order of magnitude larger than the initial end-use demand reduction. There are interdependencies among the end-use sectors and between energy-supply and end-use sectors, which elevate the importance of a wide, systematic approach. As shown in Figure 2.19, global final energy consumption grows by 30% and 10% from 2010 to 2050 for 2°C-consistent and 1.5°C overshoot pathways from IAMs, respectively, while much higher growth of 75% is projected for reference scenarios. The ranges within a specific pathway class are due to a variety of factors as introduced in Section 2.3.1, as well as differences between modelling frameworks. The important energy efficiency and conservation improvements that facilitate many of the 1.5°C pathways raise the issue of potential rebound effects (Saunders, 2015) <sup>[[#fn:r401|401]]</sup> , which, while promoting development, can make the achievement of low-energy demand futures more difficult than modelling studies anticipate (see Sections 2.5 and 2.6).

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<span id="figure-2.18"></span>
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'''Figure 2.18'''

<span id="comparison-of-co-2-emission-trajectories-of-sectoral-pathways-iea-etp-b2ds-etp-2ds-iea-662ds-shell-sky-with-the-ranges-of-iam-pathway-2ds-are-2c-consistent-pathways-and-1.5ds-os-are1.5c-overshoot-pathways.-the-co-2-emissions-shown-here-are-the-energy-related-emissions-including-industrial-process-emissions."></span>
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'''Comparison of CO <sub>2</sub> emission trajectories of sectoral pathways (IEA ETP-B2DS, ETP-2DS, IEA-66%2DS, Shell-Sky) with the ranges of IAM pathway (2DS are 2°C-consistent pathways and 1.5DS-OS are1.5°C overshoot pathways). The CO <sub>2</sub> emissions shown here are the energy-related emissions, including industrial process emissions.'''
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[[File:e322b16bc219b14e094fab9d8fa9e92b Figure-2.18-1024x788.jpg]]

Original Creation for this Report using IAMC 1.5°C Scenario Data hosted by IIASA

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<span id="figure-2.19"></span>
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'''Figure 2.19'''

<span id="a-global-final-energy-b-direct-co-2-emissions-from-the-all-energy-demand-sectors-c-carbon-intensity-and-d-structure-of-final-energy-electricity-liquid-fuel-coal-and-biomass."></span>
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'''(a) Global final energy, (b) direct CO <sub>2</sub> emissions from the all energy demand sectors, (c) carbon intensity, and (d) structure of final energy (electricity, liquid fuel, coal, and biomass).'''
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[[File:7d01fdeeec2dad8a304824fa0bfaedae Figure-2.19-1024x1024.jpg]]

The squares and circles indicate the IAM archetype pathways and diamonds indicate the data of sectoral scenarios. The red dotted line indicates the 2010 level. H2DS = Higher-2°C, L2DS = Lower-2°C, 1.5DS-H = 1.5°C-high-OS, 1.5DS-L = 1.5°C-low-OS. The label 1.5DS combines both high and low overshoot 1.5°C-consistent pathway. See Section 2.1 for descriptions.

Original Creation for this Report using IAMC 1.5°C Scenario Data hosted by IIASA

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Final energy demand is driven by demand in energy services for mobility, residential and commercial activities (buildings), and manufacturing. Projections of final energy demand depend heavily on assumptions about socio-economic futures as represented by the SSPs (Bauer et al., 2017) <sup>[[#fn:r402|402]]</sup> (see Sections 2.1, 2.3 and 2.5). The structure of this demand drives the composition of final energy use in terms of energy carriers (electricity, liquids, gases, solids, hydrogen etc.).

Figure 2.19 shows the structure of global final energy demand in 2030 and 2050, indicating the trend toward electrification and fossil fuel usage reduction. This trend is more significant in 1.5°C pathways than 2°C pathways. Electrification continues throughout the second half of the century, leading to a 3.5- to 6-fold increase in electricity demand (interquartile range; median 4.5) by the end of the century relative to today (Grubler et al., 2018; Luderer et al., 2018) <sup>[[#fn:r403|403]]</sup> . Since the electricity sector is completely decarbonized by mid-century in 1.5°C pathways (see Figure 2.20), electrification is the primary means to decarbonize energy end-use sectors.

The CO <sub>2</sub> emissions <sup>[[#fn:6|6]]</sup> of end-use sectors and carbon intensity are shown in Figure 2.20. The projections of IAMs and IEA studies show rather different trends, especially in the carbon intensity. These differences come from various factors, including the deployment of CCS, the level of fuel switching and efficiency improvements, and the effect of structural and behavioural changes. IAM projections are generally optimistic for the industry sectors, but not for buildings and transport sectors. Although GDP increases by a factor of 3.4 from 2010 to 2050, the total energy consumption of end-use sectors grows by only about 30% and 20% in 1.5°C overshoot and 2°C-consistent pathways, respectively. However, CO <sub>2</sub> emissions would need to be reduced further to achieve the stringent temperature limits. Figure 2.20 shows that the reduction in CO <sub>2</sub> emissions of end-use sectors is larger and more rapid in 1.5°C overshoot than 2°C-consistent pathways, while emissions from the power sector are already almost zero in 2050 in both sets of pathways, indicating that supply-side emissions reductions are almost fully exploited already in 2°C-consistent pathways (see Figure 2.20) (Rogelj et al., 2015b, 2018; Luderer et al., 2016b) <sup>[[#fn:r404|404]]</sup> . The emission reductions in end-use sectors are largely made possible by efficiency improvements, demand reduction measures and electrification, but the level of emissions reductions varies across end-use sectors. While the carbon intensity of the industry and buildings sectors decreases to a very low level of around 10 gCO <sub>2</sub> MJ <sup>-1</sup> , the carbon intensity of transport becomes the highest of any sector by 2040 due to its higher reliance on oil-based fuels. In the following subsections, the potential and challenges of CO <sub>2</sub> emission reduction in each end-use sector are discussed in detail.

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<span id="figure-2.20"></span>
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'''Figure 2.20'''

<span id="comparison-of-a-direct-co-2-emissions-and-b-carbon-intensity-of-the-power-and-energy-end-use-sectors-industry-buildings-and-transport-sectors-between-iams-and-sectoral-studies-iea-etp-and-ieairena."></span>
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'''Comparison of (a) direct CO <sub>2</sub> emissions and (b) carbon intensity of the power and energy end-use sectors (industry, buildings, and transport sectors) between IAMs and sectoral studies (IEA-ETP and IEA/IRENA).'''
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[[File:332935fa58dd4bbe9b8418e4636e94e1 Figure-2.20-1024x719.jpg]]

Diamond markers in panel (b) show data for IEA-ETP scenarios (2DS and B2DS), and IEA/IRENA scenario (66%2DS). Note: for the data from IAM studies, there is rather large variation of projections for each indicator. Please see the details in the following figures in each end-use sector section.

Original Creation for this Report using IAMC 1.5°C Scenario Data hosted by IIASA

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<span id="industry"></span>
==== 2.4.3.1 Industry ====

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The industry sector is the largest end-use sector, both in terms of final energy demand and GHG emissions. Its direct CO <sub>2</sub> emissions currently account for about 25% of total energy-related and process CO <sub>2</sub> emissions, and emissions have increased at an average annual rate of 3.4% between 2000 and 2014, significantly faster than total CO <sub>2</sub> emissions (Hoesly et al., 2018) <sup>[[#fn:r405|405]]</sup> . In addition to emissions from the combustion of fossil fuels, non-energy uses of fossil fuels in the petrochemical industry and metal smelting, as well as non-fossil fuel process emissions (e.g., from cement production) contribute a small amount (~5%) to the sector’s CO <sub>2</sub> emissions inventory. Material industries are particularly energy and emissions intensive: together, the steel, non-ferrous metals, chemicals, non-metallic minerals, and pulp and paper industries accounted for close to 66% of final energy demand and 72% of direct industry-sector emissions in 2014 (IEA, 2017a) <sup>[[#fn:r406|406]]</sup> . In terms of end-uses, the bulk of energy in manufacturing industries is required for process heating and steam generation, while most electricity (but smaller shares of total final energy) is used for mechanical work (Banerjee et al., 2012; IEA, 2017a) <sup>[[#fn:r407|407]]</sup> .

As shown in Figure 2.21, a major share of the additional emission reductions required for 1.5°C-overshoot pathways compared to those in 2°C-consistent pathways comes from industry. Final energy, CO <sub>2</sub> emissions, and carbon intensity are consistent in IAM and sectoral studies, but in IAM-1.5°C-overshoot pathways the share of electricity is higher than IEA-B2DS (40% vs. 25%) and hydrogen is also considered to have a share of about 5% versus 0%. In 2050, final energy is increased by 30% and 5% compared with the 2010 level (red dotted line) for 1.5°C-overshoot and 2°C-consistent pathways, respectively, but CO <sub>2</sub> emissions are decreased by 80% and 50% and carbon intensity by 80% and 60%, respectively. This additional decarbonization is brought by switching to low-carbon fuels and CCS deployment.

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'''Figure 2.21'''

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Comparison of (a) final energy, (b) direct CO <sub>2 </sub> emissions, (c) carbon intensity, (d) electricity and biomass consumption in the industry sector between IAM and sectoral studies.
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[[File:cbcf8b9105f0dd74c21060f4c715586f Figure-2.21-1024x953.jpg]]

The label 1.5DS combines both high and low overshoot 1.5°C-consistent pathways. Section 2.1 for descriptions. The squares and circles indicate the IAM archetype pathways and diamonds the data of sectoral scenarios. The red dotted line indicates the 2010 level. H2DS = Higher-2°C, L2DS = Lower-2°C, 1.5DS-H = 1.5°C-high-OS, 1.5DS-L = 1.5°C-low-OS. The label 1.5DS combines both high and low overshoot 1.5°C-consistent pathways. Section 2.1 for descriptions.

Original Creation for this Report using IAMC 1.5°C Scenario Data hosted by IIASA

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Broadly speaking, the industry sector’s mitigation measures can be categorized in terms of the following five strategies: (i) reducing demand, (ii) energy efficiency, (iii) increasing electrification of energy demand, (iv) reducing the carbon content of non-electric fuels, and (v) deploying innovative processes and application of CCS. IEA ETP estimates the relative contribution of different measures for CO <sub>2</sub> emission reduction in their B2DS scenario compared with their reference scenario in 2050 as follows: energy efficiency 42%, innovative process and CCS 37%, switching to low-carbon fuels and feedstocks 13% and material efficiency (include efficient production and use to contribute to demand reduction) 8%. The remainder of this section delves more deeply into the potential mitigation contributions of these strategies as well as their limitations.

Reduction in the use of industrial materials, while delivering similar services, or improving the quality of products could help to reduce energy demand and overall system-level CO <sub>2</sub> emissions. Strategies include using materials more intensively, extending product lifetimes, increasing recycling, and increasing inter-industry material synergies, such as clinker substitution in cement production (Allwood et al., 2013; IEA, 2017a) <sup>[[#fn:r408|408]]</sup> . Related to material efficiency, use of fossil-fuel feedstocks could shift to lower-carbon feedstocks, such as from oil to natural gas and biomass, and end-uses could shift to more sustainable materials, such as biomass-based materials, reducing the demand for energy-intensive materials (IEA, 2017a) <sup>[[#fn:r409|409]]</sup> .

Reaping energy efficiency potentials hinges critically on advanced management practices, such as energy management systems, in industrial facilities as well as targeted policies to accelerate adoption of the best available technology (see Section 2.5). Although excess energy, usually as waste heat, is inevitable, recovering and reusing this waste heat under economically and technically viable conditions benefits the overall energy system. Furthermore, demand-side management strategies could modulate the level of industrial activity in line with the availability of resources in the power system. This could imply a shift away from peak demand and as power supply decarbonizes, this demand-shaping potential could shift some load to times with high portions of low-carbon electricity generation (IEA, 2017a) <sup>[[#fn:r410|410]]</sup> .

In the industry sector, energy demand increases more than 40% between 2010 and 2050 in baseline scenarios. However, in the 1.5°C-overshoot and 2°C-consistent pathways from IAMs, the increase is only 30% and 5%, respectively (Figure 2.21). These energy-demand reductions encompass both efficiency improvements in production and reductions in material demand, as most IAMs do not discern these two factors.

CO <sub>2</sub> emissions from industry increase by 30% in 2050 compared to 2010 in baseline scenarios. By contrast, these emissions are reduced by 80% and 50% relative to 2010 levels in 1.5°C-overshoot and 2°C-consistent pathways from IAMs, respectively (Figure 2.21). By mid-century, CO <sub>2</sub> emissions per unit of electricity are projected to decrease to near zero in both sets of pathways (see Figure 2.20). An accelerated electrification of the industry sector thus becomes an increasingly powerful mitigation option. In the IAM pathways, the share of electricity increases up to 30% by 2050 in 1.5°C-overshoot pathways (Figure 2.21) from 20% in 2010. Some industrial fuel uses are substantially more difficult to electrify than others, and electrification would have other effects on the process, including impacts on plant design, cost and available process integration options (IEA, 2017a) <sup>[[#fn:r411|411]]</sup> . <sup>[[#fn:7|7]]</sup>

In 1.5°C-overshoot pathways, the carbon intensity of non-electric fuels consumed by industry decreases to 16 gCO <sub>2</sub> MJ <sup>−1</sup> by 2050, compared to 25 gCO <sub>2</sub> MJ <sup>−1</sup> in 2°C-consistent pathways. Considerable carbon intensity reductions are already achieved by 2030, largely via a rapid phase-out of coal. Biomass becomes an increasingly important energy carrier in the industry sector in deep-decarbonization pathways, but primarily in the longer term (in 2050, biomass accounts for only 10% of final energy consumption even in 1.5°C-overshoot pathways). In addition, hydrogen plays a considerable role as a substitute for fossil-based non-electric energy demands in some pathways.

Without major deployment of new sustainability-oriented low-carbon industrial processes, the 1.5°C-overshoot target is difficult to achieve. Bringing such technologies and processes to commercial deployment requires significant investment in research and development. Some examples of innovative low-carbon process routes include: new steelmaking processes such as upgraded smelt reduction and upgraded direct reduced iron, inert anodes for aluminium smelting, and full oxy-fuelling kilns for clinker production in cement manufacturing (IEA, 2017a) <sup>[[#fn:r412|412]]</sup> .

CCS plays a major role in decarbonizing the industry sector in the context of 1.5°C and 2°C pathways, especially in industries with higher process emissions, such as cement, iron and steel industries. In 1.5°C-overshoot pathways, CCS in industry reaches 3 GtCO <sub>2</sub> yr <sup>−1</sup> by 2050, albeit with strong variations across pathways. Given the projected long-lead times and need for technological innovation, early scale-up of industry-sector CCS is essential to achieving the stringent temperature target. Development and demonstration of such projects has been slow, however. Currently, only two large-scale industrial CCS projects outside of oil and gas processing are in operation (Global CCS Institute, 2016) <sup>[[#fn:r413|413]]</sup> . The estimated current cost <sup>[[#fn:8|8]]</sup> of CO <sub>2</sub> avoided (in USD2015) ranges from $20–27 tCO <sub>2</sub> <sup>−1</sup> for gas processing and bio-ethanol production, and $60–138 tCO <sub>2</sub> <sup>−1</sup> for fossil fuel-fired power generation up to $104–188 tCO <sub>2</sub> <sup>−1</sup> for cement production (Irlam, 2017) <sup>[[#fn:r414|414]]</sup> .

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<span id="buildings"></span>
==== 2.4.3.2 Buildings ====

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In 2014, the buildings sector accounted for 31% of total global final energy use, 54% of final electricity demand, and 8% of energy-related CO <sub>2</sub> emissions (excluding indirect emissions due to electricity). When upstream electricity generation is taken into account, buildings were responsible for 23% of global energy-related CO <sub>2</sub> emissions, with one-third of those from direct fossil fuel consumption (IEA, 2017a) <sup>[[#fn:r415|415]]</sup> .

Past growth of energy consumption has been mainly driven by population and economic growth, with improved access to electricity, and higher use of electrical appliances and space cooling resulting from increasing living standards, especially in developing countries (Lucon et al., 2014) <sup>[[#fn:r416|416]]</sup> . These trends will continue in the future and in 2050, energy consumption is projected to increase by 20% and 50% compared to 2010 in the IAM-1.5°C-overshoot and 2°C-consistent pathways, respectively (Figure 2.22). However, sectoral studies (IEA-ETP scenarios) show different trends. Energy consumption in 2050 decreases compared to 2010 in ETP-B2DS, and the reduction rate of CO <sub>2</sub> emissions is higher than in IAM pathways (Figure 2.22). Mitigation options are often more widely covered in sectoral studies (Lucon et al., 2014) <sup>[[#fn:r417|417]]</sup> , leading to greater reductions in energy consumption and CO <sub>2</sub> emissions.

Emissions reductions are driven by a clear tempering of energy demand and a strong electrification of the buildings sector. The share of electricity in 2050 is 60% in 1.5°C-overshoot pathways, compared with 50% in 2°C-consistent pathways (Figure 2.22). Electrification contributes to the reduction of direct CO <sub>2</sub> emissions by replacing carbon-intensive fuels, like oil and coal. Furthermore, when combined with a rapid decarbonization of the power system (see Section 2.4.1) it also enables further reduction of indirect CO <sub>2</sub> emissions from electricity. Sectoral bottom-up models generally estimate lower electrification potentials for the buildings sector in comparison to global IAMs (see Figure 2.22). Besides CO <sub>2</sub> emissions, increasing global demand for air conditioning in buildings may also lead to increased emissions of HFCs in this sector over the next few decades. Although these gases are currently a relatively small proportion of annual GHG emissions, their use in the air conditioning sector is expected to grow rapidly over the next few decades if alternatives are not adopted. However, their projected future impact can be significantly mitigated through better servicing and maintenance of equipment and switching of cooling gases (Shah et al., 2015; Purohit and Höglund-Isaksson, 2017) <sup>[[#fn:r418|418]]</sup> .

IEA-ETP (IEA, 2017a) <sup>[[#fn:r419|419]]</sup> analysed the relative importance of various technology measures toward the reduction of energy and CO <sub>2</sub> emissions in the buildings sector. The largest energy savings potential is in heating and cooling demand, largely due to building envelope improvements and high efficiency and renewable equipment. In the ETP-B2DS, energy demand for space heating and cooling is 33% lower in 2050 than in the reference scenario, and these reductions account for 54% of total reductions from the reference scenario. Energy savings from shifts to high-performance lighting, appliances, and water heating equipment account for a further 24% of the total reduction. The long-term, strategic shift away from fossil-fuel use in buildings, alongside the rapid uptake of energy efficient, integrated and renewable energy technologies (with clean power generation), leads to a drastic reduction of CO <sub>2</sub> emissions. In ETP-B2DS, the direct CO <sub>2</sub> emissions are 79% lower than the reference scenario in 2050, and the remaining emissions come mainly from the continued use of natural gas.

The buildings sector is characterized by very long-living infrastructure, and immediate steps are hence important to avoid lock-in of inefficient carbon and energy-intensive buildings. This applies both to new buildings in developing countries where substantial new construction is expected in the near future and to retrofits of existing building stock in developed regions. This represents both a significant risk and opportunity for mitigation. <sup>[[#fn:9|9]]</sup> A recent study highlights the benefits of deploying the most advanced renovation technologies, which would avoid lock-in into less efficient measures (Güneralp et al., 2017) <sup>[[#fn:r420|420]]</sup> . Aside from the effect of building envelope measures, adoption of energy-efficient technologies such as heat pumps and, more recently, light-emitting diodes is also important for the reduction of energy and CO <sub>2</sub> emissions (IEA, 2017a) <sup>[[#fn:r421|421]]</sup> . Consumer choices, behaviour and building operation can also significantly affect energy consumption (see Chapter 4, Section 4.3).

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'''Figure 2.22'''

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Comparison of (a) final energy, (b) direct  CO <sub>2</sub>  emissions, (c) carbon intensity, (d) electricity and biomass consumption in the buildings sector between IAM and sectoral studies.
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[[File:c3d2c55eae9c83d5be4c7e3ad1c1799a Figure-2.22-1024x968.jpg]]

The squares and circles indicate the IAM archetype pathways and diamonds the data of sectoral scenarios. The red dotted line indicates the 2010 level. H2DS = Higher-2°C, L2DS = Lower-2°C, 1.5DS-H = 1.5°C-high-OS, 1.5DS-L = 1.5°C-low-OS. The label 1.5DS combines both high and low overshoot 1.5°C-consistent pathways. Section 2.1 for descriptions.

Original Creation for this Report using IAMC 1.5°C Scenario Data hosted by IIASA

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<span id="transport"></span>
==== 2.4.3.3 Transport ====

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Transport accounted for 28% of global final energy demand and 23% of global energy-related CO <sub>2</sub> emissions in 2014. Emissions increased by 2.5% annually between 2010 and 2015, and over the past half century the sector has witnessed faster emissions growth than any other. The transport sector is the least diversified energy end-use sector; the sector consumed 65% of global oil final energy demand, with 92% of transport final energy demand consisting of oil products (IEA, 2017a) <sup>[[#fn:r422|422]]</sup> , suggesting major challenges for deep decarbonization.

Final energy, CO <sub>2</sub> emissions, and carbon intensity for the transport sector are shown in Figure 2.23. The projections of IAMs are more pessimistic than IEA-ETP scenarios, though both clearly project deep cuts in energy consumption and CO <sub>2</sub> emissions by 2050. For example, 1.5°C-overshoot pathways from IAMs project a reduction of 15% in energy consumption between 2015 and 2050, while ETP-B2DS projects a reduction of 30% (Figure 2.23). Furthermore, IAM pathways are generally more pessimistic in the projections of CO <sub>2</sub> emissions and carbon intensity reductions. In AR5 (Clarke et al., 2014; Sims et al., 2014) <sup>[[#fn:r423|423]]</sup> , similar comparisons between IAMs and sectoral studies were performed and these were in good agreement with each other. Since the AR5, two important changes can be identified: rapid growth of electric vehicle sales in passenger cars, and more attention towards structural changes in this sector. The former contributes to reduction of CO <sub>2</sub> emissions and the latter to reduction of energy consumption.

Deep emissions reductions in the transport sector would be achieved by several means. Technology-focused measures such as energy efficiency and fuel-switching are two of these. Structural changes that avoid or shift transport activity are also important. While the former solutions (technologies) always tend to figure into deep decarbonization pathways in a major way, this is not always the case with the latter, especially in IAM pathways. Comparing different types of global transport models, Yeh et al. (2016) <sup>[[#fn:r424|424]]</sup> find that sectoral (intensive) studies generally envision greater mitigation potential from structural changes in transport activity and modal choice. Though, even there, it is primarily the switching of passengers and freight from less- to more-efficient travel modes (e.g., cars, trucks and airplanes to buses and trains) that is the main strategy; other actions, such as increasing vehicle load factors (occupancy rates) and outright reductions in travel demand (e.g., as a result of integrated transport, land-use and urban planning), figure much less prominently. Whether these dynamics accurately reflect the actual mitigation potential of structural changes in transport activity and modal choice is a point of investigation. According to the recent IEA-ETP scenarios, the share of avoid (reduction of mobility demand) and shift (shifting to more efficient modes) measures in the reduction of CO <sub>2</sub> emissions from the reference to B2DS scenarios in 2050 amounts to 20% (IEA, 2017a) <sup>[[#fn:r425|425]]</sup> .

The potential and strategies to reduce energy consumption and CO <sub>2</sub> emissions differ significantly among transport modes. In ETP-B2DS, the shares of energy consumption and CO <sub>2</sub> emissions in 2050 for each mode are rather different (see Table 2.8), indicating the challenge of decarbonizing heavy-duty vehicles (HDV, trucks), aviation, and shipping. The reduction of CO <sub>2</sub> emissions in the whole sector from the reference scenario to ETP-B2DS is 60% in 2050, with varying contributions per mode (Table 2.8). Since there is no silver bullet for this deep decarbonization, every possible measure would be required to achieve this stringent emissions outcome. The contribution of various measures for the CO <sub>2</sub> emission reduction from the reference scenario to the IEA-B2DS in 2050 can be decomposed to efficiency improvement (29%), biofuels (36%), electrification (15%), and avoid/shift (20%) (IEA, 2017a) <sup>[[#fn:r426|426]]</sup> . It is noted that the share of electrification becomes larger compared with older studies, reflected by the recent growth of electric vehicle sales worldwide. Another new trend is the allocation of biofuels to each mode of transport. In IEA-B2DS, the total amount of biofuels consumed in the transport sector is 24EJ <sup>[[#fn:10|10]]</sup> in 2060, and allocated to LDV (light-duty vehicles, 17%), HDV (35%), aviation (28%), and shipping (21%), that is, more biofuels is allocated to the difficult-to-decarbonize modes (see Table 2.8).

<div id="section-2-4-3-3-block-2"></div>

<span id="table-2.8"></span>
<!-- START TABLE -->
'''Table 2.8'''

<span id="transport-sector-indicators-by-mode-in-2050-iea-2017a."></span>
'''Transport sector indicators by mode in 2050 (IEA, 2017a).'''

Share of energy consumption, biofuel consumption, CO <sub>2</sub> emissions, and reduction of energy consumption and CO <sub>2</sub> emissions from 2014. (CO <sub>2</sub> emissions are well-to-wheel emissions, including the emission during the fuel production.), LDV: light duty vehicle, HDV: heavy duty vehicle.
<!-- TABLE -->
{| class="wikitable"
|-
! rowspan="2"|
! colspan="3"| Share of Each Mode (%)
! colspan="2"| Reduction from 2014 (%)
|-
! Energy
! Biofuel
! CO <sub>2</sub>
! Energy
! CO <sub>2</sub>
|-
| LDV
| 36
| 17
| 30
| 51
| 81
|-
| HDV
| 33
| 35
| 36
| 8
| 56
|-
| Rail
| 6
| –
| –1
| –136
| 107
|-
| Aviation
| 12
| 28
| 14
| 56
|-
| Shipping
| 17
| 21
| 26
| 29
|}
<!-- END TABLE -->

<div id="section-2-4-3-3-block-3"></div>

In road transport, incremental vehicle improvements (including engines) are relevant, especially in the short to medium term. Hybrid electric vehicles are also instrumental to enabling the transition from internal combustion engine vehicles to electric vehicles, especially plug-in hybrid electric vehicles. Electrification is a powerful measure to decarbonize short-distance vehicles (passenger cars and two and three wheelers) and the rail sector. In road freight transport (trucks), systemic improvements (e.g., in supply chains, logistics, and routing) would be effective measures in conjunction with efficiency improvement of vehicles. Shipping and aviation are more challenging to decarbonize, while their demand growth is projected to be higher than other transport modes. Both modes would need to pursue highly ambitious efficiency improvements and use of low-carbon fuels. In the near and medium term, this would be advanced biofuels while in the long term it could be hydrogen as direct use for shipping or an intermediate product for synthetic fuels for both modes (IEA, 2017a) <sup>[[#fn:r428|428]]</sup> .

The share of low-carbon fuels in the total transport fuel mix increases to 10% and 16% by 2030 and to 40% and 58% by 2050 in 1.5°C-overshoot pathways from IAMs and the IEA-B2DS pathway, respectively. The IEA-B2DS scenario is on the more ambitious side, especially in the share of electricity. Hence, there is wide variation among scenarios, including the IAM pathways, regarding changes in the transport fuel mix over the first half of the century. As seen in Figure 2.23, the projections of energy consumption, CO <sub>2</sub> emissions and carbon intensity are quite different between IAM and ETP scenarios. These differences can be explained by more weight on efficiency improvements and avoid/shift decreasing energy consumption, and the higher share of biofuels and electricity accelerating the speed of decarbonization in ETP scenarios. Although biofuel consumption and electric vehicle sales have increased significantly in recent years, the growth rates projected in these pathways would be unprecedented and far higher than has been experienced to date.

<div id="section-2-4-3-3-block-4"></div>

<span id="figure-2.23"></span>
<!-- START IMG -->
<!-- IMG TITLE -->
'''Figure 2.23'''

<span id="section-13"></span>
<!-- IMG CAPTION -->
Comparison of (a) final energy, (b) direct  CO <sub>2 </sub> emissions, (c) carbon intensity, (d) electricity and biofuel consumption in the transport sector between IAM and sectoral studies.
<!-- IMG FILE -->
[[File:5ee0b9397c5c07aa4ab8858cfa9aebca Figure-2.23-1024x982.jpg]]

The squares and circles indicate the IAM archetype pathways and diamonds the data of sectoral scenarios. The red dotted line indicates the 2010 level. H2DS = Higher-2°C, L2DS = Lower-2°C, 1.5DS-H = 1.5°C-high-OS, 1.5DS-L = 1.5°C-low-OS. The label 1.5DS combines both high and low overshoot 1.5°C-consistent pathways. Section 2.1 for descriptions.

Original Creation for this Report using IAMC 1.5°C Scenario Data hosted by IIASA

<!-- END IMG -->
<div id="section-2-4-3-3-block-5"></div>

The 1.5°C pathways require an acceleration of the mitigation solutions already featured in 2°C-consistent pathways (e.g., more efficient vehicle technologies operating on lower-carbon fuels), as well as those having received lesser attention in most global transport decarbonization pathways up to now (e.g., mode-shifting and travel demand management). Current-generation, global pathways generally do not include these newer transport sector developments, whereby technological solutions are related to shifts in traveller’s behaviour.

<span id="land-use-transitions-and-changes-in-the-agricultural-sector"></span>