One of two new Romney ads includes footage of his visit last month to an Ohio coal mine, with a voiceover of a Romney speech where he says, "We have 250 years of coal, why wouldn't we use it?"

Various industry publications have cited that same estimate, saying, "The United States has more than a 250-year-supply of coal if it continues using coal at the same rate at which it uses coal today."

But in a major report five years ago, the National Academy of Sciences concluded that the best estimate it could confirm was that U.S. coal reserves would last less than half that long.

"The United States is endowed with a vast amount of coal," said the report, written by a panel of geologists, engineers and industry officials for the National Academy's National Research Council.

"Despite significant uncertainties in generating reliable estimates of the nation's coal resources and reserves, there are sufficient economically mineable reserves to meet anticipated needs through 2030," said the report, written at the request of the late Sen. Robert C. Byrd, D-W.Va.

"Further into the future, there is probably sufficient coal to meet the nation's needs for more than 100 years at current rates of consumption," the report said. "However, it is not possible to confirm the often-quoted suggestion that there is a sufficient supply of coal for the next 250 years."

Questions about coal reserve figures and future production forecasts are important, especially in the Appalachian coalfields. A variety of factors have the region's mine operators struggling and miners losing their jobs, and experts project major production declines during the next quarter century.

Along with stiff competition from low-priced natural gas and from other coal basins, experts have long warned of an impending downturn in Central Appalachian coal because much of the region's highest quality and easiest-to-produce coal has been mined out.

"In general, the remaining reserve potential is in thinner, deeper coal beds that are of poorer general quality than the coal that has already been mined," Robert C. Milici, a geologist with the U.S. Geological Survey, wrote a dozen years ago in the International Journal of Coal Geology.

Earlier this week, Romney's campaign unveiled its two new pro-coal ads as part of an ongoing strategy to convince voters that various Obama regulatory proposals would destroy a coal industry that would otherwise flourish, providing jobs for local communities and cheap industry for the nation.

"Coal is America's most abundant energy source," says the Romney campaign's energy plan. "But rather than focus on refining technologies that burn coal cleanly, President Obama has been waging war on the entire coal industry."

At the same time, House Republicans are planning a Friday vote on a package of bills blocking Obama initiatives aimed at curbing mining-related water pollution, reducing hazardous air emissions from coal-fired power plants, requiring greenhouse gas controls on new electrical generating units, and toughening rules for the handling and disposal of toxic ash produced when coal is burned.

Most of the measures included in the new legislation -- dubbed the "Stop the War on Coal Act" -- were already approved by the House, but have stalled in the Democratic-controlled Senate and now face a veto threat from the White House.

President Obama, though, has also touted the nation's abundant coal reserves, repeating the often-used line that, "The United States is the Saudi Arabia of coal."

The 250-year-supply estimate the Romney campaign repeated is based at least in part on 2006 U.S. Department of Energy projections that put the nation's recoverable coal reserves at about 267 billion tons and on current projection levels of about 1 billion tons a year.

But when it examined that estimate, the National Research Council warned that, "a combination of increased rates of production with more detailed reserve analyses that take into account location, quality, recoverability, and transportation issues may substantially reduce the estimated number of years supply."

Tad Patzek, chairman of the Department of Petroleum and Geosystems Engineering at the University of Texas at Austin, has written about how estimates of world coal reserves could be greatly inflated, at least if you want to use them to project future production capabilities.

In a paper published two years ago by the journal "Energy," Patzek warned that society is likely to more quickly use up not necessarily all of the available coal, but chip away at the reserves that are easiest to reach and of highest quality. It's possible industry developments could make thinner and deeper seams more economical to mine, but there's no way for sure to predict such changes, that paper said.

"When people refer to 250 years, there are a lot of assumptions about a lot of things," Patzek said this week. "My hunch is that we are just way, way too optimistic."