Climate science: Under-reporting of greenhouse gas emissions in US cities

Cities in the United States may under-report their own greenhouse gas emissions by 18.3 % on average, according to a new study published in Nature Communications this week. The research highlights that US cities may omit some fuels, and estimate transportation emissions differently, leading to questions of the robustness of the current self-reporting system.

Cities around the world are a major source of greenhouse gases from human activity, contributing up to 75% of all fossil fuel carbon dioxide (CO₂) emissions. Many cities estimate their greenhouse gas emissions using self-reporting methods that may follow one of a few publicly available global or local reporting framework protocols. These self-reported estimates are important for establishing baseline emissions and mitigation policy, but their accuracy has remained uncertain.

Kevin Gurney and colleagues investigated the self-reported inventories of 48 cities across the US. They found that cities used different accounting methods for marine, airborne and on-road emissions, as well as omission of petroleum fuel use in some cities. When comparing the self-reported emissions to independent estimates from the Vulcan CO₂ emissions data product (an estimate of annual CO₂ emissions from fossil fuels), the analysis suggests that the vast majority of cities under-reported emissions. For example, Indianapolis, Indiana, is found to be underestimating emissions by 26.9%. However, a few cities, such as Flagstaff, Arizona, and Madison, Wisconsin, on average were found to be over-reporting their emissions.

Inaccurate emissions assessments make it difficult to put effective mitigation strategies into place. However, the authors note that progress is being made on building a systematic emissions quantification system that may guide the implementation of the best mitigation strategies for a city.