A new study from Northern Arizona University (NAU) suggests that a major global greenhouse gas emissions database may be significantly undercounting carbon dioxide emissions from vehicles in cities. According to the researchers, the Climate TRACE database, developed by the Climate TRACE consortium co-founded by former Vice President Al Gore, underestimates urban vehicle CO2 emissions by an average of 70%.
The findings were published today in Environmental Research Letters by Kevin Gurney, a professor in NAU’s School of Informatics, Computing, and Cyber Systems (SICCS). The study focused on carbon dioxide (CO2) emissions from cars and trucks reported in the recently released Climate TRACE database.
Gurney said the results, together with an earlier study that identified similar issues in Climate TRACE estimates for power plants, raise concerns about the reliability of emissions data used to guide climate policy and decision-making.
“Given the importance of vehicle CO2 emissions in cities, we carefully examined the Climate TRACE data which relied on promising new artificial intelligence-based approaches,” Gurney said. “When combined with our previous study on Climate TRACE power plant CO2 emissions, our results suggest that the Climate TRACE data significantly underestimate over half of U.S. fossil fuel-based CO2 emissions in cities.”
Comparing Climate TRACE to the Vulcan Emissions Database
To evaluate the Climate TRACE estimates, Gurney and his colleagues compared them with data from Vulcan, an “onroad” emissions database developed by his laboratory. The Vulcan system is calibrated using official traffic records and energy consumption data, providing an independent benchmark for measuring vehicle emissions.
The researchers compared vehicle CO2 emissions data from 260 U.S. cities across the two databases.
“While the Vulcan onroad data is not perfect, with uncertainty of about 14%, this is far lower than the differences found when we compared 260 city vehicle CO2 emissions in the U.S. to the Climate TRACE database,” said Bilal Aslam, a SICCS postdoc and co-investigator on the study. “The Climate TRACE CO2 emissions were, on average, 70% lower than those same emissions in the Vulcan onroad CO2 emissions database.”
According to the researchers, the discrepancies were even larger in some locations.
“Individual cities such as Indianapolis and Nashville were lower by more than 90%,” added Pawlok Dass, a research associate in SICCS and contributor to the study.
The authors believe the underestimation may extend beyond the United States and could affect Climate TRACE data globally. They also expressed concerns about other aspects of the database that may warrant additional scrutiny.
Concerns About AI-Based Emissions Tracking
The researchers emphasized that artificial intelligence has enormous potential for monitoring environmental conditions and generating emissions estimates. However, they argue that strong scientific standards remain essential.
In their view, transparency, expert review, and rigorous scientific methods are necessary to ensure that emissions data are accurate and trustworthy. Reliable greenhouse gas measurements are a fundamental part of creating effective climate policies and evaluating progress toward emissions reductions.
The paper also outlines several recommendations intended to strengthen and improve Climate TRACE’s work, helping policymakers and budget planners make better-informed decisions about reducing greenhouse gas emissions.
“We will never estimate emissions with perfect accuracy, but we must ensure that the data shared with policymakers and the public is unbiased and meets best practices and the most rigorous scientific standards available,” Gurney said. “Without this, we mislead decision makers and potentially lose public trust in our ability to tackle climate change.”
Kevin Gurney’s Work on Greenhouse Gas Emissions
Gurney, whose expertise spans atmospheric science, ecology and public policy, has spent more than 20 years developing standardized approaches for measuring greenhouse gas emissions across the United States.
His Vulcan and Hestia projects, supported by multiple federal agencies, quantify and visualize greenhouse gas emissions nationwide, from individual power plants and roadways to neighborhoods. These systems help identify emissions “hotspots” and support more targeted strategies for reducing pollution. Gurney’s emissions estimates have also demonstrated strong agreement with direct atmospheric monitoring measurements.
Over the course of his career, Gurney has authored more than 180 scientific papers that have received more than 20,000 citations. His work includes contributions to a recent U.S. National Academy Report, “Greenhouse Gas Emissions for Decisionmaking.”
He has also participated in the United Nations Climate Change Framework Convention and the Kyoto Protocol process for more than 25 years and serves as a lead author for the Intergovernmental Panel on Climate Change (IPCC).