Editing IPCC:AR6/WGIII/Chapter-8 (section)

==== 8.3.4.2 Scenarios of Future Urban Greenhouse Gas Emissions ====

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There remains little globally comprehensive literature on projections of future baseline GHG emissions from urban areas or scenarios deploying urban mitigation actions on the part of city or regional governments. This dearth of research rests on limited urban emissions data that are consistent and comparable across the globe, making review and synthesis challenging ( [[#Creutzig--2016b|Creutzig et al. 2016b]] ). Some research has presented urban emissions forecasts and related projections, including estimated urban energy use in 2050 ( [[#Creutzig--2015|Creutzig et al. 2015]] ), energy savings for low-carbon development ( [[#Creutzig--2016b|Creutzig et al. 2016b]] ), emission savings from existing and new infrastructure ( [[#Creutzig--2016a|Creutzig et al. 2016a]] ) (Figure 8.12), and urban emissions from buildings, transport, industry, and agriculture ( [[#IEA--2016a|IEA 2016a]] ).

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'''Figure 8.11: Reference scenario and mitigation potential for global urban areas in the residential and commercial building, transport, waste, and material production sectors.''' The top red line indicates the reference scenario where no further emissions reduction efforts are taken, while the bottom dark line indicates the combined potential of reducing emissions across the sectors displayed. Wedges are provided for potential emissions savings associated with decarbonising residential buildings, commercial buildings, transport, waste, and materials as indicated in the legend. The shaded areas that take place among the wedges with lines indicate contributions from decarbonisation of electricity supply. Source: Re-used with permission from [[#Coalition%20for%20Urban%20Transitions--2019|Coalition for Urban Transitions (2019)]] .

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'''Figure 8.12: Urban infrastructure-based''' '''CO''' 2 '''-eq''' '''emission mitigation wedges.''' Urban infrastructure-based CO 2 -eq emission mitigation wedges across categories of existing (yellow/green), new (blue), and construction (grey) of urban infrastructure. The wedges include low-carbon energy systems and infrastructure, modal shift, tolls/tax, or behavioural change, and reductions from construction materials. Source: re-used with permission from [[#Creutzig--2016a|Creutzig et al. (2016a)]] .

In its study of about 700 urban areas with a population of at least 750,000, the [[#Coalition%20for%20Urban%20Transitions--2019|Coalition for Urban Transitions (2019)]] , attempts to quantify the urban portion of global GHG emissions, including the residential and commercial building, transport, waste, and material production (focusing on cement, aluminium, and steel) sectors, along with mitigation wedges aimed at staying below a 2°C level of atmospheric warming (Figure 8.11). Starting in 2015 with a global urban emissions total of almost 14 GtCO 2 -eq, the study projects an increase to 17.3 GtCO 2 -eq by 2050 – but this reduces to 1.8 GtCO 2 -eq by 2050 with the inclusion of mitigation wedges: 58% from buildings, 21% from transport, 15% materials efficiency, and 5% waste, with decarbonisation of electricity supply as a cross-cutting strategy across the wedges.

[[#footnote-003|6]] [[#footnote-002|7]] ''[[#footnote-001|8]]''

'''[[#footnote-000|9]]'''

Similar analysis by the urban networks C40 and GCoM examine current and future GHG emissions on smaller subsets of global cities, offering further insight on the potential emissions impacts of urban mitigation options. However, this analysis is limited to just a sample of the global urban landscape and primarily focused on cities in the Global North ( [[#GCoM--2018|GCoM 2018]] , 2019; C40 Cities et al. 2019) with methods to project avoided emissions in development ( [[#Kovac--2020|Kovac et al. 2020]] ). Different scopes of analysis between sectors, as well as limited knowledge of the impact of existing and new urban infrastructure, limit the possibility of direct comparisons in emissions. Still, the shares of urban mitigation potential ranges between 77.7% and 78.9% for combined strategies that involve decarbonised buildings and transport in urban infrastructure, and the wedges approach the remaining emissions reductions also considering construction materials and waste. This data supports urban areas pursuing a package of multiple, integrated mitigation strategies in planning for decarbonisation (Sections 8.4 and 8.6, and Figure 8.21).

The most comprehensive approach to-date for quantifying urban emissions within the global context ( [[#Gurney--2021|Gurney et al. 2021]] , 2022) combines the per capita carbon footprint estimates for 13,000 cities from [[#Moran--2018|Moran et al. (2018)]] with projections of the share of urban population ( [[#Jiang--2017|Jiang and O’Neill 2017]] ) within the IPCC’s SSP-RCP framework ( [[#van%20Vuuren--2014|van Vuuren et al. 2014]] , 2017a; [[#Riahi--2017|Riahi et al. 2017]] ). Urban emissions in seven SSP-RCP scenarios are shown in Figure 8.13 along with an estimate of the global total CO 2 -eq for context.

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'''Figure 8.13: Carbon dioxide equivalent (''' '''CO''' 2 '''-eq''' ''') emissions from global urban areas in seven SSP-RCP variations spanning the 1990 to 2100 time period.''' Urban areas are aggregated to six regional domains based on the AR6 WGIII 6-region aggregation. Global total CO 2 -eq emissions (CO 2 and CH 4 (methane)) are also shown as marked by the dashed line. Future urban emissions in the context of SSP-RCP-Shared Policy Assumption (SPA) variations correspond to '''(a)''' SSP1-RCP1.9-SPA1, '''(b)''' SSP1-RCP2.6-SPA1, '''(c)''' SSP4-RCP3.4-SPA4, '''(d)''' SSP2-RCP4.5-SPA2, '''(e)''' SSP4-RCP6.0-SPA4, '''(f)''' SSP3-RCP7.0-SPA0 and '''(g)''' SSP5-RCP8.5 based on the marker scenario implementations. 6 The first three scenarios (a–c) with more stringent reduction pathways represent contexts where urban per capita emissions decline rapidly against various increases in urban population and are oriented to reach net-zero emissions within this century at different radiative forcing levels. SSP1 scenarios (a, b) represent contexts where urbanisation takes place rapidly while providing resource efficiency based on compact urban form ( [[#Jiang--2017|Jiang and O’Neill 2017]] ), with high levels of electrification ( [[#van%20Vuuren--2017b|van Vuuren et al. 2017b]] ; [[#Rogelj--2018|Rogelj et al. 2018]] ). The scenario context of SSP1-RCP1.9 represents a pathway in which there can be a transformative shift towards sustainability. Note that the scale of panels (f) and (g) is different from the other panels. 7 See Table 8.2 detailing the SSP-RCPs. Source: adapted from [[#Gurney--2022|Gurney et al. (2022)]] . 8

In 2020, total urban emissions (including CO 2 and CH 4 ) derived from consumption-based accounting were estimated to be 29 GtCO 2 -eq, representing between 67% and 72% of global CO 2 and CH 4 emissions, excluding aviation, shipping, and biogenic sources of emissions. By 2050, with moderate to low urban mitigation efforts, urban emissions are projected to rise to 34.0 GtCO 2 -eq (SSP2-RCP4.5) or 40.2 GtCO 2 -eq (SSP3-RCP7.0) – driven by growing urban population, infrastructure, and service demands. However, scenarios that involve rapid urbanisation can have different outcomes as seen in SSP1-RCP1.9 based on green growth, versus SSP5-RCP8.5 with the strongest carbon lock-in lacking any decarbonisation. Other scenarios involve mixed and/or low urbanisation, along with other differences, including the implementation of electrification, energy, and material efficiency, technology development and innovation, renewable energy preferences, and behavioural, lifestyle, and dietary responses (Table 8.2). With aggressive and immediate mitigation efforts to limit global warming to 1.5°C (&gt;50%) with no or limited overshoot, urban GHG emissions could approach net-zero and reach a maximum of 3.3 GtCO 2 -eq in 2050 (SSP1-RCP1.9). Under aggressive but not immediate urban mitigation efforts to limit global warming to 2°C (&gt;67%), urban emissions could reach 17.2 GtCO 2 -eq in 2050 (SSP1-RCP2.6).

'''Table 8.2: Synthesis of the urbanisation and scenario contexts of the urban emissions scenarios.''' Descriptions for urbanisation are adapted based on [[#Jiang--2017|Jiang and O’Neill (2017)]] while high, medium, low, or mixed levels in the scenario context are drawn from the marker model implementations of SSP1-SSP5 for IMAGE ( [[#van%20Vuuren--2017b|van Vuuren et al. 2017b]] ; [[#Rogelj--2018|Rogelj et al. 2018]] ), MESSAGE-GLOBIOM ( [[#Fricko--2017|Fricko et al. 2017]] ), AIM/CGE ( [[#Fujimori--2017|Fujimori et al. 2017]] ), GCAM ( [[#Calvin--2017|Calvin et al. 2017]] ), and REMIND-MAgPIE ( [[#Kriegler--2017|Kriegler et al. 2017]] ). The letters in parentheses refer to the panels in Figure 8.13. Energy and material efficiency relate to energy efficiency improvement and decrease in the intermediate input of materials, including steel and cement. Dietary responses include less meat-intensive diets. Implications for urban areas relate to the mitigation options in [[#8.4|Section 8.4]] . Source: adapted from [[#Gurney--2022|Gurney et al. (2022)]] .

{| class="wikitable"
|-
! rowspan="2"| '''SSP/RCP framework'''

! rowspan="2"| '''Urbanisation context'''

! colspan="6"| '''Scenario context'''

|-
! Electrification

! Energy and material efficiency

! Technology development/ innovation

! Renewable energy preferences

! Behavioural, lifestyle and dietary responses

! Afforestation and re-forestation

|-
| rowspan="2"| SSP1 RCP1.9 (a) RCP2.6 (b)

| rowspan="2"| Resource efficient, walkable and sustainable rapid urbanisation

| High

| High

| High

| High

| High

| High

|-
| colspan="6"| '''Implications for urban climate mitigation include:'''

– Electrification across the urban energy system while supporting flexibility in end-use

– Resource efficiency from a consumption-based perspective with cross-sector integration

– Knowledge and financial resources to promote urban experimentation and innovation

– Empowerment of urban inhabitants for reinforcing positive lock-in for decarbonisation

– Integration of sectors, strategies and innovations across different typologies and regions

|-
| '''SSP2''' RCP4.5 (d)

| Moderate progress

| Medium

| Medium

| Medium

| Medium

| Medium

| Medium

|-
| '''SSP3'''

RCP7.0 (f)

| Slow urbanisation, inadequate urban planning

| Medium

| Low

| Low

| Medium

| Low

| Low

|-
| '''SSP4''' RCP3.4 (c)

RCP6.0 (e)

| Pace of urbanisation differs with inequalities

| Mixed

| Mixed

| Mixed

| Mixed

| Mixed

| Mixed

|-
| '''SSP5''' RCP8.5 (g)

| Rapid urbanisation with carbon lock-in

| High

| Low

| High

| Low

| Low

| –

|}

When 2020 levels are compared to the values for the year 2030, urban areas that utilise multiple opportunities towards resource-efficient and walkable urbanisation are estimated to represent a savings potential of 9.8 GtCO 2 -eq of urban emissions, under SSP1-RCP1.9 scenario conditions, on the path towards net-zero CO 2 and CH 4 emissions. In contrast, urban emissions would increase by 3.4 GtCO 2 -eq from 2020 levels in 2030 under SSP2-RCP4.5 scenario conditions with moderate changes lacking ambitious mitigation action (Figure 8.14).

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'''Figure 8.14: Comparison of urban emissions under different urbanisation scenarios (Gt''' '''CO''' 2 '''-eq''' '''y''' '''r''' –1 ''') for the AR6 WGIII 6-region aggregation.''' The panels represent the estimated urban emissions change in two different scenarios for the time period 2020–2030. Panel '''(a)''' represents resource efficient and compact urbanisation while panel '''(b)''' represents urbanisation with moderate progress. The two scenarios are consistent with estimated urban emissions under the SSP1-RCP1.9-SPA1 and SSP2-RCP4.5-SPA2 scenarios, respectively (Figure 8.13). In both panels, urban emissions estimates for the year 2020 are marked by the lines for each region. In the resource efficient and compact scenario, various reductions in urban emissions that take place by 2030 are represented by the dashed areas within the bars. The remaining solid shaded areas represent the remaining urban emissions in 2030 for each region on the path towards net-zero emissions. The total reductions in urban emissions worldwide that are given by the last dashed grey bar in panel (a) is estimated to be 9.8 GtCO 2 -eq yr –1 between 2020 and 2030 in this scenario. In the scenario with moderate progress, there are no regions with reductions in urban emissions. Above the white lines that represent urban emissions in 2020, the grey shaded areas are the estimated increases for each region so that the total urban emissions would increase by 3.4 GtCO 2 -eq yr –1 from 2020 levels in 2030 under this scenario. The values are based on urban scenario analyses as given in Gurney et al. (2021, 2022) ''.'' Source: synthesised based on data from [[#Gurney--2022|Gurney et al. (2022)]] . 9

Among the 500 urban areas with the highest consumption-based urban emissions footprint in 2015 ( [[#Moran--2018|Moran et al. 2018]] ), urban-level emission scenarios under SSP1 conditions are constructed for 420 urban areas located across all regions of the world ( [[#Kılkış--2021a|Kılkış 2021a]] ). These scenarios are based on urban-level population projections by SSP ( [[#Kii--2021|Kii 2021]] ), trends in relevant CMIP6 scenarios ( [[#Gidden--2019|Gidden et al. 2019]] ), and a 100% renewable energy scenario ( [[#Bogdanov--2021|Bogdanov et al. 2021]] ). In the year 2020, the 420 urban areas are responsible for about 10.7 ± 0.32 GtCO 2 -eq, or 27% of the global total CO 2 and CH 4 emissions of about 40 GtCO 2 -eq, excluding aviation, shipping, and biogenic sources. Under three SSP1-based scenarios, the urban emissions of the 420 urban areas in 2030 is projected to be about 7.0 GtCO 2 -eq in SSP1-RCP1.9, 10.5 GtCO 2 -eq in SSP1-RCP2.6, and 5.2 GtCO 2 -eq in the SSP1 renewable energy scenario.

The Illustrative Mitigation Pathways (IMPs) represent different strategies for maintaining temperature goals that are compliant with the Paris Agreement, as well as their comparison with the continuation of current policies (Sections 1.5 and 3.2.5, and Table 8.3). The key characteristics that define the IMPs involve aspects of energy, land use, lifestyle, policy, and innovation. Urban areas provide cross-cutting contexts where each of these key characteristics can be enabled and have a particularly important role in the transformation pathways for renewable energy (IMP-Ren), low demand (IMP-LD), and shifting to sustainability (IMP-SP). Pathways that are compliant with the Paris Agreement include such urban implications as a reversal of decreasing land-use efficiency in urban areas to lower energy demand based on spatial planning for compact urban form ( [[#8.4.2|Section 8.4.2]] ), changes in urban infrastructure for supporting demand flexibility to handle variable energy supply ( [[#8.4.3|Section 8.4.3]] ), as well as policies and governance that are conducive to innovation in urban areas ( [[#8.5|Section 8.5]] ). Spatial planning for compact urban form can enable reduced energy demand and changes in service provisioning, including through walkable neighbourhoods and mixed land use, providing venues for socio-behavioural change towards active transport ( [[#8.4.5|Section 8.4.5]] ). Electrification and sector coupling in urban infrastructure can, for instance, be an important enabler of supporting higher penetrations of renewable energy in the energy system.

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