For the past three years, promoters of shale gas and environmentalists opposed to coal-fired power plants have hailed the sudden abundance of U.S. natural gas as a bridge to a renewable-energy future.

But natural gas has become so cheap that many energy experts and environmentalists now wonder whether it will turn into a long, bumpy detour.

U.S. natural gas prices, which hit more than $13 per thousand cubic feet in 2008, have tumbled to about $2.50 per thousand cubic feet. Rapidly rising production of shale gas and a warm winter have created a glut and pushed supplies in storage to 21 percent above the average of the past five years.