Recent changes in thermal coal use in the United States, illustrated in Figure 2 , impacted by switching from coal to natural gas, are substantial and will likely continue to trend downward due to overall market influences of natural gas prices relative to coal, along with current and pending federal and state policy. However, it is important to note that Pennsylvania's rise into the top four producing states is a relatively recent occurrence. The Henry Hub in Erath, LA, has been the basis for U.S. natural gas prices, in part because of the numbers of natural gas pipelines that intersect that area. Gas sold on the global market is commonly indexed to crude oil market prices. Concern has been expressed from some voices in the industrial and utility sectors that LNG exported overseas will raise natural gas prices in the United States. Along with the push for diversification, expected U.S. LNG export to the European Union is also affected by the advent of shale development successes in North America; the realization of shale potential in regions of Europe; the planned dismantling of nuclear power generation, particularly in Germany; the broad increase in renewables; the Eurozone economic crisis, which is causing demand erosion; the permitting of LNG import facilities in several European Union countries; and the potential for LNG exports from the United States to reach Europe in the foreseeable future. And in a similar fashion in the United States, DOE, the State Department, and the Department of the Interior have been coordinating an effort to develop greater global energy security, with an emphasis on the development of natural gas resources from shale in countries where dependence on countries such as Russia and Iran is high. First, export terminals may be built in ecologically sensitive coastal areas. Expanding Use of Natural Gas

With a growing awareness of both the size of the natural gas resource likely to be sourced from shale and the rate at which it is being produced, interest in domestic use is increasing in all sectors. Utility companies are increasing construction of new distribution lines for all types of customers due to the predicted favorable pricing of natural gas in the mid term based on increased supply. Transport Fuels Alternatives for Natural Gas

Another key trend in utilization is the expected displacement of traditional liquid fuels for transportation by natural gas. This is currently being accomplished through the compression of natural gas to compressed natural gas (CNG) or chilling and liquefaction to LNG, which is used in vehicles and trucks with standard engines modified to utilize natural gas. The change in U.S. natural gas supplies has raised the prospect of energy independence and possibly the United States becoming an exporter.
The United States is in the process of discovering the true extent of its unconventional natural gas resources primarily derived from shale, with estimates rising every year. Natural gas has become an increasingly available, low-priced commodity, and U.S. natural gas is in demand at home and abroad. The price of natural gas relative to coal has led to a trend of switching from coal to gas for electric generation.
Estimates of undiscovered technically recoverable natural gas have multiplied in recent years, rewriting the map on natural gas production. Pennsylvania is newly one of the top four gas-producing states (since 2011), with Texas holding the top spot. This rise has necessitated pipeline network expansions; however, progress has been slow, leading to constraints and price differentials. Appalachian natural gas has historically been priced higher than that from the Gulf Coast's Henry Hub due to lower availability, but increased production in Pennsylvania's Marcellus Shale resource has reversed this differential.
The earning potential for liquefied natural gas (LNG) exportation overseas, particularly in high-demand areas such as Asia, has led to an increase in applications to construct U.S. export terminals. The prospect of increased LNG exports raises three main issues. First, exporting may raise domestic prices just as domestic natural gas usage is beginning to expand. Second, an influx of LNG into the global market may create new geopolitical concerns. Natural gas production in the United States has prompted other countries to attempt to produce from shale deposits. Third, onshore export terminals may be built in ecologically sensitive areas with the alternative being offshore terminals. Additionally, an increased reliance on LNG may shift focus away from exploring renewable energy sources.
The increase in U.S. natural gas production also affects the domestic market. The percent of new housing constructed to be heated by natural gas has become almost equal to the percent heated by electricity. This increase has been due largely to economic and environmental considerations. In the transportation sector, compressed natural gas (CNG) and LNG are beginning to displace traditional liquid fuels; some companies are producing vehicles that run on various forms of natural gas. However, the main obstacles are the cost of the new technology required and that of reformatting fueling stations.