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

== Frequently Asked Questions (FAQs) ==

<div id="FAQ 8.1 |  Why are urban areas important to global climate change mitigation?" class="h2-container"></div>

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=== FAQ 8.1 | Why are urban areas important to global climate change mitigation? ===

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Over half of the world’s population currently resides in urban areas – a number forecasted to increase to nearly 70% by 2050. Urban areas also account for a growing proportion of national and global emissions, depending on emissions scope and geographic boundary. These trends are projected to grow in the coming decades; in 2100, some scenarios show the urban share of global emissions above 80%, with 63% being the minimum for any scenario (with the shares being in different contexts of emissions reduction or increase) (Sections 8.3.3 and 8.3.4). As such, urban climate change mitigation considers the majority of the world’s population, as well as some of the key drivers of global emissions. In general, emissions scenarios with limited outward urban land expansion are also associated with a smaller rise in global temperature ( [[#8.3.4|Section 8.3.4]] ).

The urban share of global emissions and its projected growth stem in part from urban carbon lock-in – that is, the path dependency and inertia of committed emissions through the long lifespan of urban layout, infrastructures, and behaviour. As such, urban mitigation efforts that address lock-in can significantly reduce emissions ( [[#8.4.1|Section 8.4.1]] ). Electrification of urban energy systems, in tandem with implementing multiple urban-scale mitigation strategies, could reduce urban emissions by 90% by 2050 – thereby significantly reducing global emissions ( [[#8.3.4|Section 8.3.4]] ). Urban areas can also act as points of intervention to amplify synergies and co-benefits for accomplishing the Sustainable Development Goals ( [[#8.2|Section 8.2]] ).

<div id="FAQ 8.2 |  What are the most impactful options cities can take to mitigate urban emissions, and how can these be best implemented?" class="h2-container"></div>

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=== FAQ 8.2 | What are the most impactful options cities can take to mitigate urban emissions, and how can these be best implemented? ===

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The most impactful urban mitigation plans reduce urban GHG emissions by considering the long lifespan of urban layout and urban infrastructures (Sections 8.4.1 and 8.6). [https://www.ipcc.ch/report/ar6/wg3/chapter/chapter-8 Chapter 8] identifies three overarching mitigation strategies with the largest potential to decrease current, and avoid future, urban emissions: (i) reducing or changing urban energy and material use towards more sustainable production and consumption across all sectors including through spatial planning and infrastructure that supports compact, walkable urban form ( [[#8.4.2|Section 8.4.2]] ); (ii) decarbonise through electrification of the urban energy system, and switch to net-zero-emissions resources (i.e., low-carbon infrastructure) ( [[#8.4.3|Section 8.4.3]] ); and (iii) enhance carbon sequestration through urban green and blue infrastructure (e.g., green roofs, urban forests and street trees), which can also offer multiple co-benefits like reducing ground temperatures and supporting public health and well-being ( [[#8.4.4|Section 8.4.4]] ). Integrating these mitigation strategies across sectors, geographic scales, and levels of governance will yield the greatest emissions savings (Sections 8.4 and 8.5).

A city’s layout, patterns, and spatial arrangements of land use, transportation systems, and built environment (urban form), as well as its state and form(s) of development (urban growth typology), can inform the most impactful emissions savings ‘entry points’ and priorities for urban mitigation strategies (Sections 8.4.2 and 8.6). For rapidly growing and emerging urban areas, there is the opportunity to avoid carbon lock-in by focusing on urban form that promotes low-carbon infrastructure and enables low-impact behaviour facilitated by co-located medium to high densities of jobs and housing, walkability, and transit-oriented development (Sections 8.6.2 and 8.6.3). For established cities, strategies include electrification of the grid and transport, and implementing energy efficiency across sectors ( [[#8.6.1|Section 8.6.1]] ).

<div id="FAQ 8.3 |      How do we estimate global emissions from cities, and how reliable are the estimates?" class="h2-container"></div>

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=== FAQ 8.3 | How do we estimate global emissions from cities, and how reliable are the estimates? ===

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There are two different emissions estimation techniques applied, individually or in combination, to the four frameworks outlined in [[#8.1.6.2|Section 8.1.6.2]] to estimate urban GHG emissions: ‘top-down’ and ‘bottom-up’. The top-down technique uses atmospheric GHG concentrations and atmospheric modelling to estimate direct (scope 1) emissions (see Glossary). The bottom-up technique estimates emissions using local activity data or direct measurements such as in smokestacks, traffic data, energy consumption information, and building use. Bottom-up techniques will often include indirect emissions (see Glossary) from purchased electricity (scope 2) and the urban supply chain (scope 3). Inclusion of supply-chain emissions often requires additional data such as consumer purchasing data and supply chain emission factors. Some researchers also take a hybrid approach combining top-down and bottom-up estimation techniques to quantify territorial emissions. Individual self-reported urban inventories from cities have shown chronic underestimation when compared to estimates using combined top-down/bottom-up atmospherically calibrated estimation techniques.

No approach has been systematically applied to all cities worldwide. Rather, they have been applied individually or in combination to subsets of global cities. Considerable uncertainty remains in estimating urban emissions. However, top-down approaches have somewhat more objective techniques for uncertainty estimation in comparison to bottom-up approaches. Furthermore, supply chain estimation typically has more uncertainty than direct or territorial emission frameworks.

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