The noble endeavor of major cities to reduce carbon emissions by substituting diesel-powered buses with natural gas may have a small, or even negative, effect on the climate, according to a new study released by Stanford University Thursday.

Powering buses with natural gas instead of diesel fuel makes the globe warmer, because diesel engines are relatively clean, Adam Brandt, the lead author of the new analysis, said.

The paper reviewed more than 200 studies examining emissions from natural gas production and concluded that the U.S. Environmental Protection Agency inventories underestimated methane emission from natural gas production, especially in vehicles that used to run on diesel.

Natural gas consists mostly of methane, which is about 30 times more potent as a greenhouse effect-producing gas than carbon dioxide. Methane has been found responsible for climate change by trapping heat within the planet's atmosphere.

"Fueling trucks and buses with natural gas may help local air quality and reduce oil imports, but it is not likely to reduce greenhouse gas emissions,” Brandt said. “Even running passenger cars on natural gas instead of gasoline is probably on the borderline in terms of [negatively impacting] climate," Brandt said.

While substituting diesel fuel in buses may take some toll on the climate, the study did acknowledge benefits of replacing coal-fired power plants with natural gas.