The scientists from the Norwegian Institute for Air Research and Aalto University in Finland have created a global map showing fossil fuel dependence of economies of the world’s largest cities. They analyzed over 5,400 cities with populations exceeding 100,000 and compared their economic growth with atmospheric pollution changes made by nitrogen dioxide, a gas produced primarily by combustion of oil, gas, and coal. The study showed that a lot of large cities had already learned to grow their economies without a proportional increase in ‘fuel-related’ pollution. However, in a number of regions around the world, economic development still remains directly linked to an increased consumption of fossil fuels.
Making assessment of the environmental sustainability of urban economies has been extremely difficult so far as emissions data are often unavailable, published with significant delays, or collected using different methodologies. Therefore, the scientists proposed an alternative approach: atmospheric observation from space. They have analyzed data from the European Sentinel-5P satellite which measured nitrogen dioxide (NO₂) daily concentrations above the Earth’s surface from 2019 through 2024. Since this gas breaks down quickly in the atmosphere and is closely linked to transportation, power plants, and industry, it allows for fairly accurate tracking of fuel-burning activity.
The researchers compared the data with per capita GDP trends in each city. Then they divided all the cities into four groups. The first one included the so-called ‘green’ cities where economies were growing along with declining of NO₂ level. The second group comprised ‘brown’ cities, where both the levels of economy and pollution were rising simultaneously. The third category included ‘gray’ cities, where economic activity was declining, but environmental indicators were improving and pollution levels were falling. Finally, the ‘red’ group, which included the cities distinguished by a simultaneous deterioration of the economic situation and rise of the pollution levels.
The analysis shows that about 80% of the cities with significant changes fall into the ‘green’ category. A particularly large number of such cities are found in China, Europe, and North America. For example, Beijing, Shanghai, Guangzhou, Paris, Berlin, Amsterdam, Seattle, and Denver have demonstrated a simultaneous economic growth and reduction in fuel combustion pollution. In China, the researchers link this to closure or relocation of ‘dirty’ industries, electric transportation development and stricter environmental regulations. In Europe, key factors include restricted-traffic zones, public transportation modernization, and transition to cleaner energy sources.
Another 16% of the cities fell into the ‘brown’ category, which means that they continued to grow economically by increasing their fossil fuel reliance. This situation was most common in India, Iran, Central Asian countries, and, to some extent, Russia. Among the examples cited by the researchers are Tehran, Tashkent, and Riyadh, where dependence is rising amid growth of motor vehicle traffic, industrial expansion, and insufficiently strict environmental regulations.
As for the ‘gray’ group, pollution reduction has nothing to do with successful environmental policies, but rather with economic downturns, reduced production, or conflicts. In total, about 4% of cities fell into this category, including Jerusalem, Beirut, Kabul, Khartoum, Lusaka, and Port-au-Prince.
The ‘red’ group is the most troubled because in its cities, both pollution and economic problems are rising simultaneously, in particular, Urmia, Pishva, Karchak, and Borujerd in Iran; Benghazi and Misrata in Libya; as well as Tehuacán in Mexico and Sragen in Indonesia.
It should be noted that nitrogen dioxide is just an indirect indicator and does not replace direct measurements of carbon dioxide emissions or a comprehensive assessment of air quality. A six-year observation period also partially overlapped with the COVID-19 pandemic, which might have influenced short-term trends. Furthermore, the method does not account for the emissions ‘imported’ from the other regions along with goods and services, and assumes that tropospheric gas concentrations reflect conditions on the Earth’s surface. To test these assumptions, the researchers conducted three validation runs: they compared the data with fossil fuel consumption statistics, with EDGAR emissions inventories, and with the ground-based measurements at 465 stations across Europe. The results show that the satellite correctly captures direction of change, although it underestimates an absolute magnitude of the trend by an average of approximately 16%.
