Climate TRACE underestimates vehicle CO2, study finds

New research led by Kevin Gurney of Northern Arizona University and published in Environmental Research Letters compared the urban on-road CO2 estimates in Climate TRACE with the Vulcan Project on-road inventory and found that Climate TRACE reports vehicle CO2 emissions in U.S. cities that are, on average, 70% lower than Vulcan's estimates, based on a matched comparison covering 260 U.S. urban areas (Environmental Research Letters; NAU press release). The study notes Vulcan's reported on-road uncertainty is about 14%, which the authors cite when assessing the magnitude of the discrepancy (NAU; Environmental Research Letters).

Reporting in Gizmodo and Phys.org summarises the same paper and includes commentary from co-authors; Gizmodo also reports that Climate TRACE co-founder Gavin McCormick told Gizmodo in an emailed statement that the organisation has compared its data to official city datasets globally and did not find results consistent with the study's claims (Gizmodo; Phys.org; NAU). The NAU press release and the paper additionally reference a prior comparison that flagged discrepancies at power plants, which the authors combine with the new urban findings (NAU).

The paper compares two fundamentally different approaches to allocating urban road-sector CO2. The Vulcan Project uses government-sourced traffic, energy and infrastructure data and established emission-factor methods to produce high-resolution on-road CO2 estimates; the study treats Vulcan as a benchmark because it is calibrated to official datasets (Environmental Research Letters; Gizmodo). Climate TRACE uses machine-learning methods that incorporate satellite observations, atmospheric measurements, and other remote-sensing-derived inputs to build a global inventory of emissions; the organisations publicised this satellite- and ML-forward approach when the database launched (Gizmodo). The study presents city-by-city comparisons and reports large systematic underestimates in the Climate TRACE on-road layer relative to Vulcan's on-road totals (Environmental Research Letters).

Industry-pattern observations: Independent, high-resolution inventories such as Vulcan and remote-sensing/ML-based global systems like Climate TRACE are complementary but not interchangeable for urban on-road allocation. When researchers compare them, methodological mismatches, including differences in activity proxies, emission-factor assumptions, spatial allocation algorithms, and temporal coverage, commonly produce systematic offsets. Such offsets can be large enough to matter for city-scale policy and verification questions, particularly when one inventory is used as a baseline for mitigation tracking.

For practitioners: The reported 70% average disparity is large relative to Vulcan's 14% uncertainty and therefore raises immediate questions about comparability, not only raw accuracy. The discrepancy is reported by authors who lead Vulcan, and the paper combines this new result with prior findings on power-plant allocations to argue broader concerns about the Climate TRACE dataset's urban fossil-fuel CO2 totals (NAU; Environmental Research Letters). Climate TRACE's reported rebuttal to Gizmodo, that internal comparisons to official city datasets have not shown the same pattern, means the disagreement currently stands as a contested, data-centric problem rather than a settled correction (Gizmodo).

For observers: independent replications using other high-resolution city inventories; release or disclosure of the specific Climate TRACE on-road methods and sensitivity tests used in the version compared in the paper; city-level audits where local traffic and fuel consumption records can be matched to both inventories; and whether subsequent versions or errata from either team change the magnitude or sign of the discrepancy. Editorial analysis: For practitioners building systems that depend on emissions inputs, this episode highlights the importance of cross-dataset validation, reporting of uncertainty bounds, and documentation of activity proxies and scaling factors when integrating third-party inventories.

The NAU-led paper reports a substantial mismatch between Vulcan and Climate TRACE on urban vehicle CO2 in the U.S., and the claim is publicly contested by Climate TRACE via reporting; the disagreement is primarily methodological and empirical and will require replication, method disclosure, and city-level audits to resolve (Environmental Research Letters; NAU; Gizmodo; Phys.org).