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USRA Scientist Teams with NASA and University Collaborators to Find Links Between the Population Density of Cities and the Amount of Carbon dioxide they Produce

Columbia, Maryland--March 18, 2020.  A new NASA/university study of carbon dioxide emissions for 20 major cities around the world provides the first direct, satellite-based evidence that as a city's population density increases, the carbon dioxide it emits per person declines, with some notable exceptions. The study also demonstrates how satellite measurements of this powerful greenhouse gas can give fast-growing cities new tools to track carbon dioxide emissions and assess the impact of policy changes and infrastructure improvements on their energy efficiency.

The Orbiting Carbon Observatory-2 satellite is providing NASA’s first detailed, global measurements of carbon dioxide in the atmosphere. Credits: NASA/JPL-CaltechThe Orbiting Carbon Observatory-2 satellite is providing NASA’s first detailed, global measurements of carbon dioxide in the atmosphere. Credits: NASA/JPL-Caltech

Cities account for more than 70% of global carbon dioxide emissions associated with energy production, and rapid, ongoing urbanization is increasing their number and size. But some densely populated cities emit more carbon dioxide per capita than others.

To better understand why, atmospheric scientists Dien Wu and John Lin of the University of Utah in Salt Lake City teamed with Universities Space Research Association (USRA) scientist Tomohiro Oda working at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and Eric Kort of University of Michigan in Ann Arbor. They calculated per capita carbon dioxide emissions for 20 urban areas on several continents using carbon dioxide estimates from NASA's Orbiting Carbon Observatory-2 (OCO-2) satellite, managed by the NASA Jet Propulsion Laboratory in Pasadena, California.

The team carried out the calculations using a start-of-the art CO2 modeling system, X-STILT. USRA scientist Tomohiro has been involved in developing this model over the past years under the sponsorship of OCO-2 project. With his expertise in human carbon emissions and CO2 remote sensing, especially for urban domains, Tomohiro and the team used a combination of the estimated result from OCO-2 and an existing emission dataset in order to explain the relationship between CO2 emission and urban systems characteristics for 20 cities in this study.

Many scientists and policy makers have assumed the best way to estimate and understand differences in carbon dioxide emissions in major cities is to employ a "bottom-up" approach, compiling an inventory of fossil fuel emissions produced by industrial facilities, farms, road transport and power plants. The bottom-up method was the only feasible approach before remote-sensing datasets such as OCO-2 became available. The inventory-based approach can provide estimates of emissions by fuel type (coal, oil, natural gas) and sector (power generation, transportation, manufacturing) but can miss some emissions, especially in rapidly developing urban areas.

But for this study, researchers employed a "top-down" approach by using satellite-derived estimates of the amount of carbon dioxide present in the air above an urban area as the satellite flies overhead. The “top-down” approach provides an objective method of estimating CO2 emissions directly from atmospheric observations potentially for carbon emission monitoring even for areas where traditional inventor-based estimates might not be available.

Published on February 20 in the journal Environmental Research Letters, the study found that cities with higher population densities generally have lower per capita carbon dioxide emissions, in line with previous bottom-up studies based on emissions inventories.

"Our motivating question was essentially: When people live in denser cities, do they emit less or more carbon dioxide? The general answer from our analysis suggests emissions from denser cities are lower," said Eric Kort, principal investigator and associate professor of climate and space sciences and engineering at the University of Michigan. "It isn't a complete picture, since we only see local direct emissions, but our study does provide an alternative direct observational assessment that was entirely missing before."

The Density Factor, and Exceptions

Scientists have hypothesized that more densely-populated urban areas generally emit less carbon dioxide per person because they are more energy efficient: That is, less energy per person is needed in these areas because of factors like the use of public transportation and the efficient heating and cooling of multi-family dwellings.

The OCO-2 data show that not all densely-populated urban areas have lower per capita emissions, however, cities with major power generation facilities, such as Yinchuan, China, and Johannesburg, South Africa had higher emissions than what their population density would otherwise suggest.

"The satellite detects the carbon dioxide plume at the power plant, not at the city that actually uses the power," Lin said.

"Some cities don't produce as much carbon dioxide, given their population density, but they consume goods and services that would give rise to carbon dioxide emissions elsewhere," Wu added.

Another exception to the higher population density/lower emissions observation is affluence. A wealthy urban area, like Phoenix, Arizona produces more emissions per capita than a developing city like Hyderabad, India, which has a similar population density. The researchers speculate that Phoenix's higher per capita emissions are due to factors such as higher rates of driving and larger, better air-conditioned homes.

Looking Ahead

The researchers stress there's much more to be explored about urban carbon dioxide emissions. They believe new data from OCO-2's successor, OCO-3 – which is now on board the International Space Station since 2019- along with future space-based carbon dioxide-observing missions, will provide greater insight and potential solutions for mitigating cities' carbon emissions. Tomohiro Oda plans to obtain and analyze more data collected over more cities worldwide with new Snapshot Area Sampling (SAM) mode that allow obtaining observations for specific cities by OCO-3.

For more information on OCO-2 and OCO-3, visit:



About USRA

Founded in 1969, under the auspices of the National Academy of Sciences at the request of the U.S. Government, the Universities Space Research Association (USRA) is a nonprofit corporation chartered to advance space-related science, technology and engineering. USRA operates scientific institutes and facilities, and conducts other major research and educational programs, under Federal funding. USRA engages the university community and employs in-house scientific leadership, innovative research and development, and project management expertise. More information about USRA is available at www.usra.edu.