In a new study, scientists have used satellite observations from NASA’s OCO-3 (Orbiting Carbon Observatory-3) to provide a ‘top-down’ view of CO₂ emissions in 54 major global cities.
Traditionally, cities estimate their emissions using ‘bottom-up’ methods, which rely on data such as energy use or fuel consumption. However, these estimates are often difficult to verify independently.
The study combined CO₂ data from space with measurements of NO₂ pollution and weather models to pinpoint which areas were impacted by city emissions.
Overall, the satellite estimates closely matched traditional bottom-up estimates, differing by only 7% globally. But when looking at individual regions, the differences became much larger. It was found that traditional methods tend to underestimate emissions in African cities and overestimate them in parts of Asia, such as China and South Asia.
Another finding from the study is that wealthier cities tend to emit less CO₂ for every dollar of economic activity. Cities in North America emitted about 0.1 kilograms of CO₂ per dollar, compared to 0.5 kilograms per dollar in African cities. The researchers speculate this is because more affluent cities often have cleaner, less carbon-intensive industries.
The research also found that bigger cities are generally more efficient when it comes to per-person emissions. Smaller cities (under 5 million people) emitted around 7.7 tons of CO₂ per person, while very large cities (over 20 million) emitted just 1.8 tons per person. Presumably because larger cities are more densely populated and rely more on shared infrastructure and public transport, which lowers energy use per person.
According to the report, this research establishes that: ‘it is crucial to integrate various emission data sets, including high-resolution inversion model estimates, local inventories, along with the bottom-up data sets and satellite-based estimates presented in this study, to more effectively support global cities in their emission mitigation efforts and achieve their net-zero goals.’
The full research can be read here.
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