U.S. Cities Are Under-Counting Their CO₂ Pollution By Almost 20%

(Bloomberg) -- At least 48 U.S. cities are under-counting their carbon dioxide pollution by nearly 20%, according to a new study that compares local disclosures against a national database that can now estimate the same information. The new analysis could create confusion about how much cities emit—and therefore how much pollution they must cut—at a time of increased attention to climate change from the White House, state capitals, and city officials. 

About three-quarters of fossil-fuel CO₂ pollution comes from cities. As populations swell, reducing those emissions becomes even more critical, said Northern Arizona University professor Kevin Gurney and his co-authors in the journal Nature Communications.

The difference between the cities' and the study authors' estimates is consequential at a national scale—amounting to 70 million tons of CO₂, or about the output of the entire state of Massachusetts. If the difference between the 48 cities included in the study and the research estimates were extrapolated across the country, the under-counted emissions would amount to 474 million tons, or 24% more than California emitted in 2015. 

“We hope that this will stimulate a significant reevaluation of how to go about the entire endeavor,” Gurney said. “We are recommending that a science-driven estimate be generated for all cities and provided to them.”

Gurney likened his vision for cities' CO₂ data to weather forecasting. “We don't expect each locality to measure their own weather, run their own weather models, and crunch the numbers,” he said. “We have a system that accomplishes that difficult task and delivers an apolitical, reliable estimate  to everyone. Then they can use that information as they see fit.”

Standards by which cities estimate their own emissions vary widely, which has long been an obstacle to building a consistent picture of urban emissions in the U.S. Measuring emissions can be expensive and time-consuming, which is a problem in places where resources are already stretched. Cities commonly miss industrial or commercial uses of gasoline, or emissions from individual facilities. They may also calculate emissions related to shipping and driving in different ways.

The paper's authors checked the cities' disclosures against the Vulcan Project, a national emissions database developed by Gurney's lab at Northern Arizona University that estimates emissions nationwide down to the square kilometer. The Vulcan work brings uniformity and scientific rigor to national emissions calculations in a way that isn't always captured by the multiple approaches deployed by cities nationwide. The researchers rely on multiple overlapping datasets, which reinforce their confidence in the numbers. They also compare the emissions estimates to atmospheric CO₂ levels, providing another way to test the data accuracy, Gurney said.

The comparison suggests that the U.S. has much work to do in determining how much CO₂ it emits, even as it tries to push levels down. The city of Indianapolis, for instance, wants to cut its emissions from buildings to 20% below 2016 values by 2025, the authors write. But given that its current CO₂ count is 27% below the Vulcan Project's estimate, Gurney and his co-authors write, “it will be difficult to know when and if this target is truly achieved or track progress towards it.”

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