Natural gas is a mixture of light, gaseous hydrocarbons, consisting primarily of methane, the lightest gas in the mixture. It's exact composition varies, but nitrogen and sulfur compounds are present in much lower concentrations than in the other fossil fuels, coal and petroleum. Thus, natural gas burns with much less pollution and greater efficiency than coal or petroleum and is becoming the fossil fuel of choice for the production of electrical power. As of 2009, natural gas-fired plants contributed 22.1% of the electrical power generated in the United States, second only to coal (45.9%) and ahead of nuclear power (20.9%).
The potential supply of natural gas is quite large. Recent developments in the extraction of natural gas from shale formations have greatly increased the recoverable amount of domestic natural gas, thereby potentially decreasing US dependence on foreign sources.
Although natural gas is a finite, non-replenishable resource, its cleanliness, compared to oil and coal, and its relative domestic abundance have prompted some to propose natural gas as a possible "bridge" from our current reliance on oil and coal to a future based on sustainable energy sources.
There is, however, a dark side to natural gas, and that is to be found, strangely, in one of the coldest, most isolated spots on the earth, the Arctic region, including the Arctic Ocean and the bays and land areas surrounding it. Therein lie massive amounts of methane, either in frozen form or sequestered in submerged, frozen organic matter, which, upon thawing, releases methane, potentially in prodigious quantities. Methane is a highly potent greenhouse gas, much more so than carbon dioxide, and the warming taking place in those Arctic regions threatens to release methane in quantities that could threaten the climate of the earth in ways that are both rapid and extensive.