With Hong Kong recently recording record-breaking high temperatures, extreme weather has once again raised public awareness of global warming and emission reduction measures. A large-scale international study led by The Hong Kong University of Science and Technology (HKUST) has revealed that inter-city collaboration on emission reduction can improve overall sustainability performance by more than 10%. This comprehensive analysis covers 4,116 cities and spans 11 years of data, shedding new light on regional and global strategies towards achieving the United Nations Sustainable Development Goals (SDGs).
Led by Prof. LU Mengqian, Professor of the Department of Civil and Environmental Engineering at HKUST, the research titled "Spatiotemporal changes in inter-city sustainability impacts linked to emission challenges worldwide" has been published in the prestigious journal Nature Communications.
A Broader Approach to Emissions Reduction
Prof. Lu noted that conventional environmental governance typically views urban emission reduction as a purely local issue. Cities often pursue industrial upgrading, energy transition, or environmental policies for emission cuts, reaping only parochial benefits. However, the impact of atmospheric pollutants and greenhouse gas emissions transcends borders. The scientific community's long-standing inability to systematically understand the spatiotemporal heterogeneity of these inter-city interactions has hindered the formulation of globally coordinated emission reduction strategies.
To fill this research gap, the team employed an innovative spatial econometric model. The assessment covered inter-city interactions among emission-related SDG indicators across 4,116 cities in 103 countries, including Hong Kong, Paris, and Los Angeles, between 2010 and 2020. These cities accounted for over 90% of global Gross Domestic Product (GDP) during this period. Pollutant and greenhouse gas data were sourced from a range of satellite products.
Interdependence among cities
The findings further illuminate a significant ripple effect: the vast majority of cities (93.51%) are demonstrably influenced by the emission-related SDG indicators of others. Specifically, a 10% enhancement in particulate matter pollution (PM2.5 and PM10, under SDG 11.6 - reducing the adverse per capita environmental impact of cities, including by paying special attention to air quality), gaseous pollutants (SO2 and NOX, under SDG 12.4 - achieving the environmentally sound management of chemicals and all wastes throughout their life cycle), and greenhouse gases (CO2 and CH4, under SDG 13.2 - integrating climate change measures into national policies, strategies and planning to reduce greenhouse gas emissions) in other cities can lead to an average improvement of 0.2% to 0.45% in the focal city's corresponding SDG performance.
However, the inter-city interaction may either be positive, known as the synergistic effect, or negative, known as the trade-off effect. For example, Hong Kong benefited from inter-city synergistic effects in advancing SDG 13.2 (CO2 emissions) in 2020, possibly attributable to industrial upgrading by its neighbors in the Greater Bay Area. Also, Copenhagen's performance on SDG 13.2 (CO2 emissions) was positively correlated with that of its connected cities. This pattern reflects a synergistic effect, potentially stemming from cross-city cooperation in climate governance, integrated transport and energy systems, and the diffusion of low-carbon technologies and practices across the region.
In contrast, Beijing's performance on SDG 11.6 (PM2.5) and SDG 11.6 (PM10) was negatively correlated with that of its neighboring cities. This pattern reflect a trade-off effect, potentially stemming from the relocation of pollution-intensive activities and the resulting transfer of air pollution.
The team observed that over the entire 11-year study period, the number of cities benefiting from synergistic effects was about twice that of cities experiencing trade-off effect. The study also revealed that less developed cities, such as some African and Asian cities, undergoing rapid industrial expansion, are more susceptible to cross-city influences. These cities exhibit greater dependence on cross-city resource flows, industrial collaboration, and external economic linkages. Furthermore, a temporal analysis indicated a declining trend in the overall intensity of inter-city influences. Specifically, approximately 75.66% of inter-city influence intensities decreased over time, while only about 21.31% increased.
Calling for Global Collaboration in Sustainable Urban Development
To gain a deeper understanding of inter-city interactions, the team constructed five interaction scenarios, including baseline, isolation, cooperation, deep cooperation, and non-cooperation. Simulation results demonstrated that inter-city collaborations can improve overall SDG performance by 10.81%, outperforming scenarios where cities acted independently.
Prof. Lu concluded, "Emission governance is no longer a purely local affair, but a global issue requiring cross-regional, inter-city coordinated advancement. We hope these findings will empower governments and international organizations to formulate more coordinated and evidence-based strategies for emission reduction and sustainable urban development."
This research project is part of the UNESCO International Decade of Sciences for Sustainable Development (IDSSD, 2024-2033) and contributes to the Seamless Prediction and Services for Sustainable Natural and Built Environment (SEPRESS) Program (2025-2032).
It is a collaborative effort involving HKUST, Hong Kong Baptist University (HKBU), The University of Hong Kong (HKU), and The Hong Kong Polytechnic University, involving their international partners-Michigan State University in the USA, Pusan National University in South Korea, and Monash University in Australia. Prof. Lu serves as a co-corresponding author, with Prof. XU Zhenci from HKU. Prof. XIAO Huijuan, formerly a Research Assistant Professor at HKUST and now an Assistant Professor at HKBU, is the first author.
