The use of natural gas at U.S. power plants surpassed coal for the first time last year, and its share of the domestic electric market has only expanded since then, growing from 20 percent a decade ago to 34 percent this year, according to the Energy Department. Coal's share, meanwhile, has plunged by half, to 24 percent this year from 49 percent in 2006.

"You can go back as long as you want in history, coal had always been the dominant source of fuel in U.S. power, until last year," Spencer Dale, chief economist for British oil giant BP, recently told Houston business and civic leaders.

Globally, coal still dominates electricity generation, but it is slowly losing its grip, according to BP's Statistical Review, which each year details energy supply and demand around the world. Coal's share of the worldwide power market slipped below 30 percent last year, the lowest in a decade. During that same period, natural gas's stake rose from 23 to 24 percent, according to the BP report.

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