U.S. cities are underestimating their carbon dioxide emissions by an average of 18%, according to a study released this week from Northern Arizona University.

The study also points out cities are to blame for 70% of greenhouse gas emissions, which means that potential policies to combat climate change will not do enough if cities aren't measuring their emissions accurately.

Lead scientist Kevin Gurney said that makes it harder to create effective policies to combat climate change.

He said while the averaged is off 18%, the differences were large and varied widely across the 48 cities measured.

Cleveland reported emissions 90% below the NAU estimate while Palo Alto reported emissions 42% greater than the NAU report. Findings are published in Nature Communications.