Editing IPCC:AR6/WGIII/Chapter-10 (section)

=== 10.7.5 Energy and Carbon Efficiency Trajectories ===

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This section explores what vehicle energy efficiencies and fuel carbon intensity trajectories, from the data available in the AR6 database from IAMs and GTEMs, could be compatible with different temperature goals. Figure 10.20 shows passenger and freight energy intensity, and fuel carbon intensity, indexed relative to 2020Mod values. The top panel shows passenger energy intensity across all modes. LDVs constitute a major share of this segment. [[#Yeh--2017|Yeh et al. (2017)]] report 2.5–2.75 MJ vkm –1 in 2020 across models for the LDV segment, which is very close to the IEA estimate of 2.5 MJ vkm –1 for the global average fuel consumption for LDVs in 2017 ( [[#IEA--2020d|IEA 2020d]] ). For reference, these numbers correspond to 1.6–1.7 MJ pkm –1 for an occupancy rate of 1.5. The following results of the AR6 database are conditional on the corresponding reductions in fuel carbon intensity. Figure 10.20 shows that the scenarios suggest that passenger transport’s energy intensity drops to between 10–23% (interquartile ranges across C1–C4) in 2030 for scenarios in line with warming levels below 2°C. In 2050, the medians across the group of scenarios that limit or return warming to 1.5°C (&gt;50%) during the 21st century (C1–C2), and scenarios that limit warming to 2°C (&gt;67% or &gt;50%) throughout the 2st century (C3–C4) suggest energy intensity reductions of 51% and 45–46% respectively. These values correspond to annual average energy efficiency improvement rates of 2.3–2.4% and 2.0–2.1%, respectively, from 2020 to 2050. For reference, the IEA reports an annual energy efficiency improvement rate of 1.85% per year in 2005–16 ( [[#IEA--2020d|IEA 2020d]] ). In contrast, the results from GTEMs suggest lower energy efficiency improvement, with median values for policy scenarios of 39% reduction in 2050, corresponding to annual energy efficiency improvement rates close to 1.6%. The IAM scenarios suggest median energy intensity reductions of passenger transport of 57–61% by the end of the century would align with warming levels of both 1.5°C and 2°C (C1–C4) given the corresponding decarbonisation of the fuels.

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'''Figure 10.20 | Energy efficiency and carbon intensity of transport in 2030, 2050, and 2100 indexed to 2020 modelled year across scenarios.''' Plots show 5th/95th percentile, 25th/75th percentile, and median. Numbers above the bars indicate the number of scenarios. Data from the AR6 scenario database.

The scenarios in line with warming levels of 1.5°C or 2°C goals (C1 to C4) show different trends for freight’s energy intensity. The amount of overshoot and differences in demand for freight services and, to some extent, fuel carbon intensities contribute to these differences. For the two scenarios aligning with the warming levels of 1.5°C, the trajectories in 2030 and 2050 are quite different. The median C2 scenario that returns warming to 1.5°C (&gt;50%) during the 21st century after high overshoot takes a trajectory with lower energy intensity improvements in the first half of the century. In contrast, the C1 scenario that limits warming to 1.5°C (&gt;50%) during the 21st century with no or limited overshoot take on a more steadily declining trajectory across the means. The IAMs provide a less clear picture of required energy intensity improvements for freight than for passenger transport associated with different temperature targets. As for the carbon intensity of direct energy used across both passenger and freight, the modelling scenarios suggest very moderate reductions by 2030. The interquartile ranges for the C1 scenarios suggest global average reductions in carbon intensity of 5–10%. Across the other scenarios compatible with warming levels of 1.5°C or 2°C (C2–C4), the interquartile ranges span from 1–6% reductions in carbon intensity of direct energy used for transport. For 2050, the scenarios suggest that dependence on fuel decarbonisation increases with more stringent temperature targets. For the scenarios that limits warming to 1.5°C (&gt;50%) during the 21st century with no or limited overshoot (C1), global carbon intensity of energy used for transport decreases by 37–60% (interquartile range) by 2050 with a mean of 50% reduction. The IAM scenarios in the AR6 database do not suggest full decarbonisation of transport fuels by 2100. The interquartile ranges across the C1–C4 set of scenarios, compatible with warming levels of 2°C and less, span from 61–91% reduction from 2020Mod levels.

Increasing the occupancy rate of passenger transport ( [[#Grubler--2018|Grubler et al. 2018]] ) and reducing empty miles or increasing payload in freight deliveries ( [[#Gucwa--2013|Gucwa and Schäfer 2013]] ; [[#McKinnon--2018|McKinnon 2018]] ) via improved logistics efficiency or streamlined supply chains ( [[#Mulholland--2018|Mulholland et al. 2018]] ), can present significant opportunities to effectively improve energy efficiency and decrease GHG emissions in transport. However, the recent trends of consumer behaviours have shown a declining occupancy rate of light-duty vehicles in industrialised countries ( [[#Schäfer--2020|Schäfer and Yeh 2020]] ), and the accelerating growing preference for SUVs challenges emissions reductions in the passenger car market ( [[#IEA--2019d|IEA 2019d]] ). These trends motivate a strong focus on demand-side options.

Based on the scenario literature, a 51% reduction in median energy intensity of passenger transport and a corresponding 38–50% reduction in median carbon intensity by 2050 would be aligned with transition trajectories yielding warming levels below 1.5°C by the end of the century. For comparison, the LCA literature suggests a switch from current ICEs to current BEVs would yield a reduction in energy intensity well beyond 45% and up to 70%, for a mid-sized vehicle ( [[#10.4|Section 10.4]] ). Correspondingly, a switch from diesel or gasoline to low-carbon electricity or low-carbon hydrogen would yield carbon intensity reduction beyond the median scenario value. Thus, the LCA literature suggests technologies exist today that would already match and exceed the median energy and carbon intensities values that might be needed by 2050 for low warming levels.

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