A new study finds rising production costs, not cheap natural gas, was the lead factor that drove thousands of coal mines across Appalachia to close. The analysis, published last week by the nonpartisan, environmental think tank, Resources for the Future, scrutinized the impact that natural gas prices, stagnant electricity demand and rising costs had on the ability of coal mines to stay in business. The researchers created a model that allowed them to study different factors that affected the profitability of coal mines using public data from the Mine Safety and Health Administration, U.S. Energy Information Administration and information reported by public coal companies in their annual reports to the Securities and Exchange Commission.