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

=== 8.5.2 Mitigation Potential of Urban Subnational Actors ===

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A significant research question that has been paid more attention in both the scientific and policy communities is related to subnational actors’ role in and contribution to global climate mitigation. The 2018 UN Environment Programme’s (UNEP) annual Emissions Gap report in 2018 included for the first time a special chapter on subnational and non-state (i.e., businesses and private) actors and assessed the landscape of studies aiming to quantify their contributions to global climate mitigation. Non-state action on net-zero GHG or CO 2 emissions continues to be emphasised ( [[#UNEP--2021|UNEP 2021]] ) (Box 8.4). There has been an increase in the number of studies aiming to quantify the overall aggregate mitigation impact of subnational climate action globally. Estimates for the significance of their impact vary widely, from up to 30 MtCO 2 -eq from 25 cities in the United States in 2030 ( [[#Roelfsema--2017|Roelfsema 2017]] ), to a 2.3 GtCO 2 -eq reduction in 2030 compared to a current policy scenario from over 10,239 cities participating in GCoM ( [[#Hsu--2018|Hsu et al. 2018]] ; [[#GCoM--2019|GCoM 2019]] ). For regional governments, the Under 2 Coalition, which includes 260 governments pledging goals to keep global temperature rise below 2°C, is estimated to reduce emissions by 4.2 GtCO 2 -eq in 2030, compared to a current policy scenario ( [[#Kuramochi--2020|Kuramochi et al. 2020]] ).

Some studies suggest that subnational mitigation actions ( [[#Roelfsema--2017|Roelfsema 2017]] ; [[#Kuramochi--2020|Kuramochi et al. 2020]] ) are in addition to national government mitigation efforts and can therefore reduce emissions even beyond current national policies, helping to ‘bridge the gap’ between emissions trajectories consistent with least-cost scenarios for limiting temperature rise below 1.5°C or 2°C ( [[#Blok--2012|Blok et al. 2012]] ). In some countries, such as the United States, where national climate policies have been curtailed, the potential for cities’ and regions’ emissions reduction pledges to make up the country’s Nationally Determined Contribution under the Paris Agreement is assessed to be significant ( [[#Kuramochi--2020|Kuramochi et al. 2020]] ).

These estimates are also often contingent on assumptions that subnational actors fulfil their pledges and that these actions do not result in rollbacks in climate action (i.e., weakening of national climate legislation) from other actors or rebound in emissions growth elsewhere, but data tracking or quantifying the likelihood of their implementation remains rare ( [[#Chan--2018|Chan et al. 2018]] ; [[#Hsu--2019|Hsu et al. 2019]] ; [[#Hale--2020|Hale et al. 2020]] ; [[#Kuramochi--2020|Kuramochi et al. 2020]] ). Reporting networks may attract high-performing cities, suggesting an artificially high level of cities interested in taking climate action or piloting solutions that may not be effective elsewhere ( [[#van%20der%20Heijden--2018|van der Heijden 2018]] ). These studies could also present a conservative view of potential mitigation impact because they draw upon publicly reported mitigation actions and inventory data, excluding subnational actors that may be taking actions but not reporting them ( [[#Kuramochi--2020|Kuramochi et al. 2020]] ). The nuances of likelihood, and the drivers and obstacles of climate action across different contexts is a key source of uncertainty around subnational actors’ mitigation impacts.

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