Cities and regions tackle climate change mitigation but often focus on less effective solutions

The study investigates how effective subnational (city and regional) climate mitigation strategies are at reducing greenhouse gas emissions and whether current policy and research priorities align with the most impactful strategies. Subnational actors control key emitting sectors such as energy, waste, land use, buildings, and transportation and have pledged ambitious action, yet comparative, quantitative evidence on realized or potential emissions reductions across strategies is limited. By synthesizing quantitative case studies across multiple jurisdictional scales, the authors aim to estimate expected per-capita emissions reductions for major strategy categories and assess whether cities and regions are prioritizing the most effective approaches. The work addresses critical needs identified by the IPCC and prior literature for robust, cross-context evidence to inform planning and coordination across levels of government.

Prior work recognizes the substantial mitigation potential of subnational actors and applies systematic and meta-analytic methods across climate domains. However, most subnational studies emphasize targets and planned actions rather than quantified impacts, and aggregate cross-city or cross-region comparisons are scarce due to heterogeneous contexts and methods. Some meta-analyses focus on specific sectors or qualitative landscapes of policy, with only one identified study comparing relative reductions across a range of urban strategies. The literature also highlights the promise of cross-sectoral interventions and the need for ex-post evaluations. This study extends prior research by comparing absolute (not relative) emissions reductions across multiple sectors and cross-sectoral categories, and by including both cities and regions.

The authors conducted a systematic review and meta-analysis. Keyword searches in Scopus and Web of Science (2010 onward) combined subnational and mitigation terms, yielding 299,502 deduplicated records. Using the ClimActor dataset to identify articles naming specific subnational actors, and automated rules-based screening with n-gram analysis, abstracts and full texts were screened against eligibility criteria: (1) numeric GHG emissions impacts; (2) evaluation of a mitigation strategy; (3) impacts within a city or regional jurisdiction; (4) primary research. Manual screening produced a final set of 234 eligible case studies (full dataset), from which 1,413 emissions reduction impacts and metadata were extracted. Impacts were standardized to annualized, per-capita metric tons of CO2e (tCO2e capita−1 year−1) when possible, retaining observations where strategies applied to the entire subnational context or a defined share of it. Standardization was feasible for 779 impacts from 137 studies (synthesis dataset); the remainder lacked sufficient information for conversion. A separate actions dataset captured 134 specific government actions from 49 studies analyzing implemented or planned policies. For synthesis, the authors used non-parametric, cluster bootstrapping (10,000 resamples, clustered by study) to estimate mean emissions reductions and 95% confidence intervals for 12 strategy categories (nine sector-specific and three cross-sectoral). Coefficients of variation (CV) were calculated to compare relative uncertainty. Sensitivity analyses with alternative weights and specifications confirmed robustness of category rankings.

- Dataset and coverage: 234 case studies, 1,413 impacts; 137 studies and 779 standardized impacts used for synthesis; actions dataset includes 134 government actions from 49 studies and 50 unique actors (39 cities, 11 regions). 242 unique subnational actors overall; ~55% cities, 45% regions. Geographically overrepresented in North America (34%), East Asia & Pacific (27%), and Europe (15%); ~37% of impacts from the Global South. - Overall expected impact: Mean expected emissions reduction across the synthesis dataset is 0.26 ± 0.05 tCO2e capita−1 year−1. - Sector-specific categories (means ± SD, tCO2e capita−1 year−1): • Electricity and heat: Clean energy generation 0.44 ± 0.18; Energy system operations 0.37 ± 0.14. CVs ~41% and 38%, respectively. • Transport: Transportation system management 0.17 ± 0.06 (under-studied, 8 studies; predominantly travel demand management). Alternative transportation modes 0.03 ± 0.01 (lowest reductions, high certainty). Clean vehicle transportation had greater average reductions than alternative modes but the highest uncertainty (CV ~72%). • Buildings: Building construction and improvement had smaller mean impacts but lower uncertainty (CV ~21%) than building energy and heat systems (CV ~43%). • Industry: Industrial facility improvements had too few standardized impacts (only one study) due to heterogeneity in units and contexts. - Cross-sectoral categories (means ± SD, tCO2e capita−1 year−1): • Land use and development: 0.80 ± 0.30 (nearly 90% afforestation/greening; variation linked to available land and implementation scale). • Market-based mechanisms: 0.71 ± 0.37 (evaluated in Canada, China, Japan, US; high uncertainty, CV ~52%). • Circular economy: 0.30 ± 0.10 (industrial symbiosis, recycled materials), similar in magnitude to electricity/heat sector categories and higher than most sector-specific categories. - Category ranking (equal weight on effectiveness and uncertainty): Land use and development and circular economy rank highly on both; waste and water treatment practices rank third due to low uncertainty. Building construction and improvement ranks low in effectiveness but high in certainty; market-based mechanisms the reverse; rankings shift with different weighting schemes. - Actions dataset insights: Transportation strategies are most frequently implemented (40/100 strategies identified), followed by cross-sectoral and waste. Instrument use varies by sector—transport and waste emphasize policy support and direct investment (also the only sectors with education/information instruments). Buildings rely on financial incentives and regulatory instruments (e.g., codes, standards). Cities (78% of actions) used more direct investment and policy support (66% of city actions), while regions used more economic and regulatory instruments (63% of regional actions). Geographic patterns show transport dominance in North America and Europe, cross-sectoral strategies more common in Asia. - Misalignment: No significant correlation between strategy prevalence in actions/studies and the categories with highest expected impacts or lowest uncertainty, indicating subnational actors often prioritize strategies offering co-benefits, feasibility, or political viability rather than maximum mitigation impact.

The analysis directly addresses the research question by quantifying and comparing expected absolute per-capita emissions reductions across a broad set of subnational mitigation strategies and by examining the strategies governments actually implement. Findings show cross-sectoral strategies—particularly land use and development and circular economy—tend to yield larger expected reductions than single-sector strategies, aligning with IPCC guidance favoring integrated, cross-sectoral approaches. Electricity and heat strategies also show substantial mitigation potential among sector-specific options. Yet the strategies most pursued by cities and regions (e.g., transport measures like alternative modes and clean vehicles) often deliver smaller or more uncertain reductions, suggesting other considerations (co-benefits such as health, air quality, adaptation synergies; governance capacity; jurisdictional authority; resources; and political feasibility) guide choices. Differences from prior rankings are attributable to this study’s focus on multiple subnational scales, inclusion criteria emphasizing subnational-wide interventions, and the use of absolute rather than relative reductions. Policymakers can use these expected ranges and uncertainty profiles to benchmark options, tailor decisions to local contexts and authorities, and weigh trade-offs between magnitude and certainty, while recognizing that co-benefits and implementation constraints are critical determinants of strategy selection.

This study provides a comparative, quantitative assessment of absolute per-capita emissions reductions from subnational climate mitigation strategies across cities and regions. Using standardized metrics and cluster bootstrapping, it ranks 12 strategy categories by effectiveness and uncertainty and documents real-world subnational actions and policy instruments. Key contributions include evidence that land use and development, circular economy, and waste and water treatment practices offer high expected or reliable reductions, while commonly implemented transportation strategies tend to yield smaller or more uncertain impacts. The work highlights a misalignment between policy focus and mitigation potential and underscores the need for more ex-post evaluations. Future research should: (1) incorporate costs and cost-effectiveness; (2) examine interactions, synergies, and trade-offs among strategies; (3) develop measures of governance authority, implementation scale, and existing actions to explain within-category variation; and (4) expand coverage of the Global South and disaggregate analyses by actor type and region as sample sizes permit.

- Limited reporting of error metrics in primary studies constrained traditional quality weighting; heterogeneous methodologies and baseline/counterfactual definitions introduce variability. Sensitivity checks with alternative weights/specifications yielded consistent rankings but residual uncertainty remains. - Only 55% of impacts (779 of 1,413) and 59% of studies (137 of 234) could be standardized to annualized, per-capita tCO2e due to missing information and incompatible units; buildings and industry were particularly hard to standardize. - Strategies treated independently; potential synergies or trade-offs between strategies were not modeled. Costs and cost-effectiveness were not assessed. - Sample sizes precluded disaggregation of synthesis results by actor type (city vs region) or global region; industrial facility improvements lacked comparable observations. - Most studies report potential rather than realized (ex-post) reductions; ex-post evaluations remain scarce. - Focus on absolute per-capita reductions may not capture all context-specific considerations (e.g., sectoral emissions structure, scope definitions, or jurisdictional authority).