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

=== 10.7.2 Global Emissions Trajectories ===

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In 2018, transport emitted 8.5 GtCO 2 -eq, reaching a near doubling from 1990 levels after two decades of 2% per year emissions growth ( [[#10.1|Section 10.1]] ). Assessing future trajectories, Figure 10.17 provides an overview of direct CO 2 emissions estimates from the transport sector across IAMs (colour bars) and selected global transport models (grey bars). The results from the IAMs are grouped in bins by temperature goal. Global transport energy models are grouped into reference and policy bins, since the transport sector cannot by itself achieve fixed global temperature goals. The policy scenarios in GTEMs and NTEMs cover a wide range of ‘non-reference’ scenarios, which include, for example, assumptions based on the ‘fair share action’ principles. In these scenarios, transport emissions reach reductions consistent with the overall emissions trajectories aligning with warming levels of 2°C. These scenarios may also consider strengthening existing transport policies, such as increasing fuel economy standards or large-scale deployments of electric vehicles. In most cases, these Policy scenarios are not necessarily in line with the temperature goals explored by the IAMs.

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[[File:7e68c42701cd3beee0e92de7bac2caea IPCC_AR6_WGIII_Figure_10_17.png]]

'''Figure 10.17 | Direct CO''' 2 '''emissions from transport in 2030, 2050, and 2100 indexed to 2020 modelled year across R6 Regions and World.''' IAM results are grouped by temperature targets. Sectoral studies are grouped by referenceand policy categories. Plots show 5–95th percentile, 25–75th percentile, and median. Numbers above the bars indicate the number of scenarios. Data from the AR6 scenario database.

According to the collection of simulations from the IAM and GTEM models shown in Figure 10.17, global transport emissions could grow up to 2–47% (5–95th percentile) by 2030 and –6–130% by 2050 under the C7 scenarios that limit warming to 4°C (&gt;50%) throughout the 21st century and C8 scenarios that exceed 4°C (≥50%) during the 21st century. Population and GDP growth and the secondary effects, including higher travel service demand per capita and increased freight activities per GDP, drive the growth in emissions in these scenarios ( [[#10.7.3|Section 10.7.3]] ). Though transport efficiencies (energy use per pkm travelled and per tkm of goods delivered) are expected to continue to improve in line with the historical trends ( [[#10.7.4|Section 10.7.4]] ), total transport emissions would grow due to roughly constant carbon intensity ( [[#10.7.5|Section 10.7.5]] ) under the C7 and C8 scenarios that limit warming to 4°C (&gt;50%) throughout the 21st century or exceed 4°C (≥50%) during the 21st century. In these scenarios, Significant increases in emissions (&gt;150% for the medium values by 2050) would come from Asia and Pacific, the Middle East, and Africa. Compared to estimated 2020 levels, in 2050 Developed Countries would have median 25% decrease in transport emissions in the C7 scenarios that limit warming to 4°C (&gt;50%) throughout the 21st century or median 15% increase in transport emissions in the C8 scenarios that exceed warming of 4°C (≥50%) during the 21st century.

To meet temperature goals, by 2050 global transport emissions would need to decrease by 17% (+67% to –23% for the 5–95th percentile) below 2020Mod levels in the scenarios that limit warming to 2°C (&gt;67%), 2°C (&gt;50%) and 2.5 °C (&gt;50%) throughout the 21st century (C3-C5 scenarios – orange bars), and 47% (14–80% for the 5–95th percentile) in the scenarios that limit warming to 1.5°C (&gt;50%) during the 21st century with no or limited overshoot or return to 1.5°C (&gt;50%) during the 21st century after high overshoot during the 21st century (C1–C2 scenarios – green bars). However, transport-related emission reductions may not happen uniformly across regions. For example, transport emissions from the Developed Countries and Eastern Europe and West Central Asia would decrease from 2020 levels by 2050 across all C1–C2 scenarios, but could increase in Africa, Asia and Pacific, Latin America and Caribbean, and the Middle East, in some of these scenarios. In particular, the median transport emissions in India and Africa could increase by 2050 in C1–C2 scenarios, while the 95th percentile emissions in Asia and Pacific, Latin America and Caribbean, and the Middle East, could be higher in 2050 than in 2020.

The Reference scenario emission pathways from GTEMs described in Figure 10.17 have similar ranges to C7–C8 scenario groups in 2050. The Policy scenarios are roughly in line with C6–C7 scenarios for the world region. The results suggest that the majority of the Policy scenarios examined by the GTEMS reviewed here are in the range of the C3–C6 scenarios examined by the IAMs ( [[#Gota--2016|Gota et al. 2016]] ; [[#IEA--2017b|IEA 2017b]] ; [[#Yeh--2017|Yeh et al. 2017]] ; [[#Fisch-Romito--2019|Fisch-Romito and Guivarch 2019]] ). The NDCs in the transport sector include a mix of measures targeting efficiency improvements of vehicles and trucks; improving public transit services; decarbonising fuels with alternative fuels and technologies including biofuels, fossil- or bio-based natural gas, and electrification; intelligent transport systems; and vehicle restrictions ( [[#Gota--2016|Gota et al. 2016]] ). Because of the long lag-time for technology turnover, these measures are not expected to change 2030 emissions significantly. However, they could have greater impacts on 2050 emissions.

Several GTEMs not included in AR6 scenario database have examined ambitious CO 2 mitigation scenarios. For example, a meta-analysis of scenarios suggests that global transport emissions consistent with warming levels of 2°C, would peak in 2020 at around 7–8 GtCO 2 and decrease to 2.5–9.2 Gt for 2°C, with an average of 5.4 Gt by 2050 ( [[#Gota--2019|Gota et al. 2019]] ). For comparison, the IEA’s Sustainable Development Scenario suggests global transport emissions decrease to 3.3 Gt (or 55% reduction from 2020 level) by 2050 ( [[#IEA--2021f|IEA 2021f]] ). The latest IEA ''Net Zero by 2050'' report proposes transport emissions to be close to zero by 2050 ( [[#IEA--2021e|IEA 2021e]] ). The latter is lower than the interquartile ranges of the C1 group of scenarios from the AR6 database analysed here.

Low-carbon scenarios are also available from national models (Latin America, Brazil, Canada, China, France, Germany, Indonesia, India, Italy, Japan, Mexico, South Africa, UK, US) with a good representation of the transport sector. The low-carbon scenarios are either defined with respect to a global climate stabilisation level of, for example, 2°C/1.5°C Scenario ( [[#Dhar--2018|Dhar et al. 2018]] ), or a CO 2 target that is more stringent than what has been considered in the NDCs, such as the net-zero emissions pathways ( [[#Bataille--2020|Bataille et al. 2020]] ; [[#IEA--2021e|IEA 2021e]] ). These studies have generally used bottom-up models (see Annex III) for the analysis, but in some cases, they are run by national teams using global models (e.g., the Global Change Assessment Model (GCAM) for China and India). National studies show that transport CO 2 emissions could decline significantly in low-carbon scenarios in all the developed countries reviewed ( [[#Bataille--2015|Bataille et al. 2015]] ; [[#Kainuma--2015|Kainuma et al. 2015]] ; Hillebrandt et al. 2015; [[#Mathy--2015|Mathy et al. 2015]] ; [[#Pye--2015|Pye et al. 2015]] ; [[#Virdis--2015|Virdis et al. 2015]] ; [[#Williams--2015|Williams et al. 2015]] ; [[#Zhang--2016a|Zhang et al. 2016a]] ) in 2050 from the emissions in 2010 and reductions could vary from 65% to 95%. However, in developing countries reviewed ( [[#Di%20Sbroiavacca--2014|Di Sbroiavacca et al. 2014]] ; [[#Altieri--2015|Altieri et al. 2015]] ; [[#Buira--2015|Buira and Tovilla 2015]] ; [[#Rovere--2015|Rovere et al. 2015]] ; [[#Shukla--2015|Shukla et al. 2015]] ; [[#Siagian--2015|Siagian et al. 2015]] ; [[#Teng--2015|Teng et al. 2015]] ; [[#Dhar--2018|Dhar et al. 2018]] ), emissions could increase in 2050 in the range of 35% to 83% relative to 2010 levels. Transport CO 2 emissions per capita in the developing countries were much lower in 2010 (varying from 0.15 to 1.39 tCO 2 per capita) relative to developed countries (varying from 1.76 to 5.95 tCO 2 per capita). However, results from national modelling efforts suggest that, by 2050, the CO 2 emissions per capita in developed countries (varying from 0.19 to 1.04 tCO 2 per capita) could be much lower than in developing countries (varying from 0.21 to 1.7 tCO 2 per capita).

The transport scenario literature’s mean outcomes suggest that the transport sector may take a less steep emissions reduction trajectory than the cross-sectoral average and still be consistent with the 2°C goal. For example, most of the scenarios that limit or return warming to 1.5°C (&gt;50%) during the 21st century (C1–C2) reach zero emissions by 2060, whereas transport sector emissions are estimated in the range of 20% of the 2020Mod level (4–65% for the 10th to 90th percentiles) by 2100. This finding is in line with perspectives in the literature suggesting that transport is one of the most difficult sectors to decarbonise ( [[#Davis--2018|Davis et al. 2018]] ). There is, however, quite a spread in the results for 2050. Since temperature warming levels relate to global emissions from all sectors, modelling results from IAMs tend to suggest that in the short and medium term, there might be lower cost mitigation options outside the transport sector. On the other hand, compared with GTEMs/NTEMs, some IAMs may have limited mitigation options available, including technology, behavioural changes, and policy tools especially for aviation and shipping. The models therefore rely on other sectors and/or negative emissions elsewhere to achieve the overall desired warming levels. This potential shortcoming should be kept in mind when interpreting the sectoral results from IAMs.

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