How much do you reckon your city contributes to climate change? If you added up the emissions from all the homes, businesses, vehicles, industries—everything that makes a city a city—what would your local carbon footprint be?

If you haven’t a clue, you’re not alone. It turns out that city officials themselves are struggling to accurately self-report local emissions, according to new research out today in the journal Nature Communications. The study used Vulcan, a comprehensive emissions model developed by the researchers, to analyze 48 American cities. It found that, on average, officials are underreporting their greenhouse gas emissions by 18.3 percent. If you were to extrapolate that discrepancy to all the cities across the United States, the potentially unreported carbon would equal 129,000,000 metric tons—nearly 25 percent more than all of California’s emissions in 2015.

“We’re trying to avoid just being critical without having something redeeming to say at the end of all this,” says Northern Arizona University climate scientist Kevin Gurney, lead author on the paper. City officials themselves are not at fault, he insists. They’re working with limited resources, and they’ve still been able to build impressive, though imperfect, “inventories” of their cumulative emissions. “In some ways,” says Gurney, “they’re being burdened with doing a job they shouldn’t have to do.”

City officials build what’s called self-reported inventories, or SRIs, to get an idea of local emission levels. Municipal leaders then use these SRIs to develop climate action plans, aiming to trim emissions where they can.

But the problem is that local governments go about this quantification in unique ways, and some may have more resources to develop a comprehensive inventory than others. Typically, municipal officials will choose between three or four existing protocols, developed by nongovernmental organizations; these generally incorporate various forms of energy consumption data and tallies of vehicle miles traveled within the city. They can also count their city’s buildings to estimate how much the built environment contributes to emissions.

The result is a scattershot approach to quantifying emissions, with no national template. Officials in one city might find it difficult, for instance, to get numbers on fuel consumption in certain industries—so they leave that information out, thus underreporting overall emissions. Leaders in another city might go in the opposite direction and overreport their local footprint. Others might lose the team responsible for the SRI when an administration changes, so they will also lose their institutional knowledge of the intricacies of what went into that report; for instance, which emissions might have been left out of the tally.

“If you step back from it for a minute, it seems actually like kind of a silly thing to do,” says Gurney. “Not that they shouldn’t do it, but that you shouldn’t have every single individual city having to redundantly build this information. It’s expensive. It’s difficult. It’s hard to do it systematically.”

Take Flagstaff, Arizona. According to Gurney, officials there were incorporating retail sales from gas stations in the city’s SRI calculation, which is usually a perfectly reasonable way to estimate emissions from local vehicles. Emphasis on local. The issue is that Flagstaff sits on an interstate highway, and there’s no way to separate gas sales made to residents from sales made to people stopping by on their way between Albuquerque and Las Vegas. “There’s nothing technically wrong with doing that,” says Gurney of using retail gas sales as a data point. “But it’s probably not appropriate to reflect that in the emission account of the city of Flagstaff, unless they want to take responsibility for emissions that are occurring within 500 miles of the city.”