Exxon Mobil, the world's largest publicly traded oil company, is making a $29 billion bet that pressure to curb climate change will mean natural gas — cleaner than coal and suddenly much easier to reach — will become a crucial source of U.S. power.

Exxon agreed to buy XTO Energy in an all-stock deal at a 25 percent premium, showing how eagerly a company that is among the most conservative in a conservative industry is jumping into the market for natural gas.

As negotiators haggled in Copenhagen over a global plan to curb carbon emissions, the deal suggested Exxon sees change coming for an energy source best known now for heating homes.

The deal announced Monday was also the largest for the U.S. energy sector in at least four years and Exxon's biggest acquisition since it bought Mobil Corp. for $75 billion in 1999.

The technology to unlock natural gas from tight rock formations has advanced so rapidly that energy experts have raised their estimates of how much fuel is available by 35 percent in just two years.

The emergence of massive supplies of natural gas in the U.S. coincides with the nation's focus on cutting emissions. Much of this gas is under Western Pennsylvania in the Marcellus Shale formation.

The newfound supply and looming climate legislation have been cited by utilities this year as they have shuttered old coal-fired power plants and scrapped plans to build new ones.

Climate legislation would put utilities in the crosshairs, and many are aggressively seeking new fuels like natural gas to minimize the economic hit.

"From the outside view, it does look like this move makes much more sense in a world where there's carbon policy because that ensures a growing market for natural gas," said Amy Jaffe, a fellow at the James A. Baker III Institute for Public Policy at Rice University.

Just this month, Progress Energy became the latest utility to announce it would close coal-fired power plants in favor of natural gas. Exxon Mobil expects global demand for gas to grow 50 percent by 2030.