Space-based quantification of per capita CO2 emissions from cities

Urban areas are currently responsible for∼70% of the global energy-related carbon dioxide (CO2)
emissions, and rapid ongoing global urbanization is increasing the number and size of cities. Thus,
understanding city-scaleCO2 emissions and how they vary between cities with different urban
densities is a critical task. While the relationship betweenCO2 emissions and population density has
been explored widely in prior studies, their conclusions were sensitive to inconsistent definitions of
urban boundaries and the reliance uponCO2 emission inventories that implicitly assumed population
relationships. Here we provide the first independent estimates of direct per capitaCO2 emissions (Epc)
from spaceborne atmosphericCO2 measurements from the Orbiting Carbon Observatory-2 (OCO-2)
for a total 20 cities across multiple continents. The analysis accounts for the influence of meteorology
on the satellite observations with an atmospheric model. The resultant upwind source region sampled
by the satellite serves as an objective urban extent for aggregating emissions and population densities.
Thus, we are able to detect emission ‘hotspots’ on a per capita basis from a few cities, subject to
sampling restrictions from OCO-2. Our results suggest that Epc declines as population densities
increase, albeit the decrease in Epc is partially limited by the positive correlation between Epc and per
capita gross domestic product. As additionalCO2-observing satellites are launched in the coming
years, our space-based approach to understandingCO2 emissions from cities has significant potential
in tracking and evaluating the future trajectory of urban growth and informing the effects of carbon
reduction plans.