Just weeks after Hong Kong sweltered through record-breaking heat, a team of scientists has delivered an unexpected ray of hope: cities that team up on cutting pollution can lift their overall sustainability by more than 10%. The finding, from a massive study led by The Hong Kong University of Science and Technology (HKUST), suggests that regional teamwork may beat going it alone in the fight against climate change.
The research, published in the journal Nature Communications and led by Professor Lu Mengqian of HKUST's Department of Civil and Environmental Engineering, crunched 11 years of data across 4,116 cities in 103 countries — including Hong Kong, Paris and Los Angeles. Together those cities produced more than 90% of the world's economic output between 2010 and 2020. The team built an innovative statistical model to map how each city's environmental progress ripples through its neighbors.
The heart of the study is a simple but powerful idea: no city is an island. Air pollution and greenhouse gases drift across borders, so a city's success at cleaning up often depends on what its neighbors do. The numbers make this interdependence vivid. A whopping 93.51% of cities are measurably influenced by the environmental performance of other cities. In concrete terms, a 10% improvement in pollution indicators like fine particles (PM2.5 and PM10), gases (SO2 and NOX) and greenhouse emissions (CO2 and CH4) in neighboring cities can boost a focal city's own score by 0.2% to 0.45%.
But these ripples can flow two ways. When effects are positive, scientists call it a "synergistic effect" — neighbors lifting each other up. Hong Kong, for example, benefited from its Greater Bay Area neighbors' industrial upgrading in cutting CO2 emissions, while Copenhagen's progress dovetailed with its connected cities through shared climate policies, integrated transport and low-carbon technology. Negative links, called "trade-off effects," strike when pollution just moves somewhere else. Beijing illustrates this: its air-quality performance worsened as pollution-intensive industries relocated to nearby cities.
Encouragingly, the upsides outnumber the downsides. Over the full study period, roughly twice as many cities gained from synergistic effects as suffered trade-offs. The work also flags that less-developed cities, especially fast-industrializing ones in Africa and Asia, are most vulnerable to outside influence because they lean heavily on cross-border resources and trade. The good news keeps coming: about 75.66% of intercity influences have weakened over time, suggesting the world is slowly steadying.
For a globe racing to meet the United Nations Sustainable Development Goals, the message is clear: collaboration, not isolation, is the way forward. When cities work together on emissions, everybody breathes easier.