Cities across the country are inadvertently underestimating their greenhouse gas emissions, new research suggests. And in many cases, they’re undercounting by a lot.

In a study of 48 U.S. urban areas, researchers found that the average city had underreported its carbon dioxide emissions by nearly 20%. But in some cases, the report was more than 100% off.

Torrance, Calif., and Blacksburg, Va., had some of the largest underestimates, at over 145% and 123% respectively. A few cities actually overreported their emissions—Benicia, Calif., was the most extreme case, overestimating by about 63%. Most reports, however, were too low.

These errors could hamper cities’ efforts to fight climate change, the study’s authors warn.

Cities across the nation have set ambitious targets for reducing their greenhouse gas emissions over the next few decades. But it’s hard for cities to keep track of how well they’re meeting their own goals if they aren’t sure exactly how much carbon they’re emitting today.

Cities must have an accurate “starting point and assessment points along the way as they reduce emissions,” said lead study author Kevin Gurney, a climate scientist at Northern Arizona University. “And right now, with these kinds of levels of uncertainty and inaccuracy that we’re seeing, it really makes it impossible for them to do that.”

The main issue is that there’s no single standardized method for estimating urban emissions. That means cities are largely left to design their own systems—and it’s a notoriously complicated enterprise.

Generally speaking, cities must rely on a tedious bottom-up accounting method. They collect data on all the activities occurring in all the different emissions sectors—transportation, electricity and so on—and then calculate how much carbon is associated with each of those activities.

To calculate traffic emissions, for instance, cities must estimate the number of miles traveled by all the vehicles in that area. Then they must calculate out the carbon footprint based on assumptions about how much CO2 is associated with the miles traveled for specific vehicle types and road conditions.

And that’s just one sector.

“You can imagine in something as large as a major urban area, there are literally hundreds of emission sectors,” said Riley Duren, a carbon monitoring expert at the University of Arizona and NASA’s Jet Propulsion Laboratory, who commented on the new research for E&E News.

He added that cities are often underfunded or understaffed when it comes to these projects. That makes it even harder for them to identify gaps or errors in their own reporting and make improvements.

“You can imagine this is an extremely complex undertaking to identify those potential errors and account for them, and most cities just don’t have the resources to do it,” he said.

There are various research groups around the nation trying to develop more sophisticated systems, often focused on individual urban centers. Gurney’s team is among the first to attempt a large-scale evaluation of city emissions inventories across the country.

Gurney heads a research project at NAU known as the Vulcan Project—it’s a system for calculating fossil fuel emissions across the United States. The system mines data on fossil fuel carbon dioxide from around two dozen federal datasets, including bottom-up emissions statistics collected by EPA, the Department of Energy, the Federal Highway Administration and other agencies.

The Vulcan Project synthesizes these estimates and compares them with measurements of atmospheric carbon dioxide to make sure the calculations are accurate.

In the new study, Gurney and his colleagues compared Vulcan’s estimates with 48 U.S. cities’ self-reported greenhouse gas inventories. It found major discrepancies in many cases.

Some cities appear to actually overestimate their own emissions—in the most extreme case, by more than 60%. Some were more or less on the nose.

Most, though, underestimated their emissions compared with Vulcan.

These underestimates spanned a large range—from a few percentage points in places like Detroit or Richmond, Va., to more than 145% in Torrance.

The extreme variation from one city to the next was the most concerning finding, Gurney noted.

“When you look at individual cities, you see these very large departures,” he said. “A given city really can have an estimate that is far from physical reality, and that’s not what you want.”

Reporting errors can happen just about anywhere along the pipeline—but the researchers did notice a few patterns in their comparisons.

Many cities don’t fully account for petroleum use when calculating emissions from buildings, Gurney said. There are often gaps in estimates of industrial emissions. Different cities have different accounting methods when it comes to traffic emissions.

“We saw a few things like that, and I chalk them up generally to differing methods and gaps or holes, missing parts of the inventory," Gurney said.

The new study highlights the need for more sophisticated, standardized methods for emissions accounting in urban areas.

“I’m not at all surprised by the finding of this study, that self-reported emissions from many cities are largely inconsistent with independent estimates,” Duren said. “Nor am I surprised that the estimates are all over the map, including both under- and over-reporting. I think it reflects the diversity and the general incomplete understanding of the many processes that drive greenhouse gas emissions at local scales.”

Vulcan presents one potential opportunity; it also highlights the amount of work required to develop new and improved systems for urban emissions accounting. Gurney and his colleagues have been working on the system for the last 15 years, and only in the last couple of years have they felt that it’s ready for this kind of large, multi-city study.

“We feel the research is on the edge of what we call operationalization—it can become an operational system,” Gurney said. “We don’t know how that would be administered or distributed, but we certainly think we have a system now that has accuracy, has consistency across the entire landscape, that would save, frankly, cities from having to do a pretty tough task.”

Making these improvements is more important now than ever, Duren said—most greenhouse gas mitigation efforts occur at the subnational scale.

Cities are a front line in the fight to curb carbon emissions and halt climate change. They just need the resources to make the necessary improvements.

“Of course, it’s easy to say,” Duren added. “If it were easy, it would have been done.”

Reprinted from E&E News with permission from POLITICO, LLC. Copyright 2021. E&E News provides essential news for energy and environment professionals.