Dr. Gurney said the mistakes appeared to be simple misjudgments. “I don’t think there is any attempt to systematically or deliberately underestimate emissions,” he said. Although some cities have been right about their emissions, he noted, “Whether this is right for the right reasons or right for the wrong reasons is hard to know.”

Dr. Gurney’s work is funded by the National Institute for Standards and Technology, and one of the authors, Kimberly Mueller, is a scientist there. James R. Whetstone, an official in the institute’s greenhouse gas measurement program, described the new paper as “an important step forward” in properly measuring urban greenhouse gases. “What will best serve the nation is if we find a unified way to move emissions from the city level to the national level,” he said.

The National Institute for Standards and Technology focuses much of its efforts on monitoring the atmosphere. Therefore, Dr. Gurney’s method help “measure the same thing in different ways” and gain confidence in the results.

Previous studies by researchers from the University of Michigan, Harvard, and the federal government found that emissions of methane, another powerful greenhouse gas, were also under counted by many cities. Dr. Gurney said that “both gases should really be part of this systematic approach.”

The cities’ efforts to date, said Dr. Gurney, was a laudable endeavor, but “they didn’t have many tools to do this.” He also said, “Cities are struggling to pick up trash and fill potholes, let alone keep detailed reports on their emissions.”

Reducing emissions in a city requires a deep understanding of its biggest issues, including specific pedestrianized highways and industries, so that authorities can take targeted action that will provide the greatest benefit at the lowest cost. Deploying busy vehicle lanes or fast bus lanes on any freeway can be wasteful. It is better to know which road projects could work best.