WASHINGTON, June 20 - The United States may not have nearly as much coal as is popularly believed, and mining the remaining resources may be more dangerous for workers and the environment than current operations, the National Academy of Sciences said in a report Wednesday.
With domestic production of oil, gas and uranium far below peaks, coal has been promoted by elected officials and energy experts as the only bright spot in the national fuel supply picture. But as Congress considers billions of dollars in aid for projects to make gasoline and diesel substitutes from coal, and to build coal-fired plants that would capture their own carbon emissions, the study said that estimates of coal reserves were unreliable.
"There is probably sufficient coal to meet the nation's needs for more than 100 years at current rates of consumption," the study said. "However, it is not possible to confirm the often-quoted assertion that there is a sufficient supply of coal for the next 250 years."
The 250-year estimate was made in the 1970s and was based on the assumption that 25 percent of the coal that had been located was recoverable with current technology and at current prices, said one member of the study group, Edward S. Rubin, a professor of environmental engineering and science at Carnegie Mellon University.
But he said that more recent studies by the United States Geological Survey showed that at least in some areas, only 5 percent of the coal was recoverable with today's technology and at current prices. The 100-year forecast was based on current consumption rates, about 1.1 billion tons a year. By 2030, the rate of coal consumption could be 70 percent higher or 50 percent lower than it is now, the study found.
The impact of carbon constraints, if the government imposes them, are not clear, members of the study program said. The new report, which was requested by Congress at the urging of senators from two coal-producing states, Arlen Specter of Pennsylvania and Robert C. Byrd of West Virginia, raises the possibility that taxes on carbon dioxide emissions will sharply lower the demand for coal.
It also points out that mining will increasingly occur above or below seams that have already been excavated, raising questions about safety and the disruption of underground water flows.
The federal government spends hundreds of millions of dollars a year to research ways to use coal cleanly and tens of millions on miner safety. But the committee said more research was needed to find better ways to mine coal, to estimate reserves and to store carbon dioxide captured from plants. Carbon dioxide from burning fossil fuels is a major factor contributing to climate change, scientists say.