To meet the Obama administration’s targets for cutting greenhouse gas emissions, some researchers say, Americans may have to experience a sobering reality: gas at $7 a gallon.

To reduce carbon dioxide emissions in the transportation sector 14 percent from 2005 levels by 2020, the cost of driving must simply increase, according to a forthcoming report by researchers at Harvard’s Belfer Center for Science and International Affairs.

The 14 percent target was set in the Environmental Protection Agency’s budget for fiscal 2010.

In their study, the researchers devised several combinations of steps that United States policymakers might take in trying to address the heat-trapping emissions by the nation’s transportation sector, which consume 70 percent of the oil used in the United States.

Most of their models assumed an economy-wide carbon dioxide tax starting at $30 a ton in 2010 and escalating to $60 a ton in 2030. In some cases researchers also factored in tax credits for electric and hybrid vehicles, taxes on fuel or both.

In the modeling, it turned out that issuing tax credits could backfire, while taxes on fuel proved beneficial.

“Tax credits don’t address how much people use their cars,” said Ross Morrow, one of the report’s authors. “In reverse, they can make people drive more.”

Dr. Morrow, formerly a fellow at the Belfer Center, is a professor of mechanical engineering and economics at Iowa State University

Researchers said that vehicle miles traveled will increase by more than 30 percent between 2010 and 2030 unless policymakers increase fuel taxes.