The Obama administration has sought more stringent fuel standards for new cars, and Congress is debating regulations that would progressively limit carbon dioxide emissions throughout the economy. The administration has taken a cautious approach to conventional energy resources, freezing leases to develop oil shale reserves and carefully reviewing future offshore leases for oil and gas.

Instead, the administration seeks to increase the share of renewable energy, especially wind and solar power. But experts say that meeting these goals will prove challenging given the scale of the nation’s energy use and the costs involved in switching from fossil fuels.

Shale gas currently provides a small fraction of the nation’s total gas production. But many experts believe the rising supply of natural gas means it can substitute for other fossil fuels. With the output of conventional gas forecast to decline, the Energy Department expects that shale production will rise substantially to meet higher demand, as will imports.

Natural gas accounts for about a quarter of the nation’s total energy use, and 22 percent of electrical production. Coal accounts for about half of the nation’s power generation, while oil dominates transportation fuels. While gas generates less carbon dioxide than oil or coal, it still accounted for about 20 percent of domestic energy-related emissions in 2006.

The Energy Department estimates that demand for natural gas will rise by 13 percent by 2030. In the power sector, utilities have been switching to natural gas from coal, but further increases in the use of gas will most likely depend on whether Congress puts a price on carbon dioxide emissions, as it is considering. That would favor cleaner fuels like gas.

“It’s nice to have aspirations about renewable energy and efficiency, but we need to recognize these are long-term goals and that we need something to get us there in the meantime,” said Guy F. Caruso, a former administrator of the Energy Information Administration. “Natural gas has a role to play as a bridge because of the long lead time and scalability issues of renewable fuels.”

That the nation’s gas reserves were bigger than expected does not mean they will necessarily be developed, Mr. Caruso warned. “There are some things to be cautious about,” he said, “and obviously one of them is cost, and the other is regulatory risk.”