Source: AGU Advances
Because the hustle and bustle of cities is driven largely by fossil fuels, urban areas have a critical role to play in addressing global greenhouse gas emissions. Currently, cities contribute around 75% of global carbon dioxide (CO2) emissions, and urban populations are projected only to grow in the coming decades. Members of the C40 Cities Climate Leadership Group, a network of nearly 100 cities that together make up 20% of the global gross domestic product, have pledged to work together to reduce urban greenhouse gas emissions. Most of the cities have pledged to reach net zero emissions by 2050.
To meet these pledges, cities must accurately track their emissions levels. Policymakers in global cities have been relying on a “bottom-up” approach, estimating emissions levels on the basis of activity data (e.g., gasoline sales) and corresponding emissions factors (such as the number of kilograms of carbon emitted from burning a gallon of gasoline). However, previous studies found some regional variations in emissions estimates depending on which datasets are used, especially in certain geographic locations.
Ahn et al. tried a “top-down” approach, using space-based observations to estimate emissions for 54 C40 cities.
They used data from NASA’s Orbiting Carbon Observatory 3 (OCO-3) mission on board the International Space Station (ISS) to collect high-resolution data over global cities. OCO-3 uses a pair of mirrors called the Pointing Mirror Assembly to scan atmospheric CO2 levels as the ISS flies over a target city.
The researchers found that for the 54 cities, the satellite-based estimates match bottom-up estimates within 7%. On the basis of their measurements, the researchers also found that bottom-up techniques tended to overestimate emissions for cities in central East, South, and West Asia but to underestimate emissions for cities in Africa, East and Southeast Asia, Oceania, Europe, and North America.
The team also examined the link between emissions, economies, and populations. They found that wealthier cities tended to have less carbon intensive economies. For example, North American cities emit 0.1 kilogram of CO2 within their boundaries per U.S. dollar (USD) of economic output, whereas African cities emit 0.5 kilogram of CO2 per USD. They also found that residents living in bigger cities emit less CO2—cities with under 5 million people emit 7.7 tons of CO2 per person annually, whereas cities with more than 20 million people emit 1.8 tons per person, for instance.
The authors note that their findings show that satellite data may help cities better track emissions, improve global monitoring transparency, and support global cities’ efforts to mitigate emissions. (AGU Advances, https://doi.org/10.1029/2025AV001747, 2025)
—Sarah Derouin (@sarahderouin.com), Science Writer
