The United States is the world's largest energy producer and consumer. The nation's patterns of energy use are determined in part by its economic growth, land area, climate regimes, low population density and significant indigenous resources. Much of the infrastructure of U.S. cities, highways, and industries that has a long economic life was developed in response to abundant and relatively inexpensive energy resources.

U.S. government estimates suggest that undiscovered recoverable energy resources in the United States included 49.4 billion barrels of crude oil and 399 tril

Figure 14

lion cubic feet of natural gas in 1987. Proved reserves in the same year were 28.7 billion barrels and 196.4 trillion cubic feet for oil and gas, respectively. Except for small additions in 1980 and 1987, proved reserves of oil have been declining ever since the addition of reserves under Alaska's North Slope in 1970. U.S. energy resources also include some 265 million pounds (120 million kg) of uranium oxide, recoverable at $30 per pound or less.

The most abundant U.S. energy resource, however, is coal (Figure 14). At the beginning of 1991, the demon

strated reserve base of coal contained 470 billion short tons, about half of which is thought to be recoverable. The vast majority--88.9 percent-of this total is bituminous coal. Lignite and anthracite coal provide 9.5 and 1.5 percent of total coal reserves, respectively.

Coal, natural gas, and crude oil have comprised the bulk of U.S. energy production since 1960, accounting for 96.1 percent of production in 1960 and 85.7 percent in 1991. The commercial introduction of nuclear electric power and expanded hydroelectric generation displaced some of the production formerly contributed by fossil fuels (Figure 15). Other sources of renewable energy provide only a small part of domestic energy supplied. Geothermal energy, which is produced from hot-dry rock, steam, or hot water, has historically provided less than 1 percent of the domestic production of electricity. Production estimates for other renewableenergy sources, such as solar, wind, and waste products, are also small.

Growth in fossil fuel production has come mainly from coal and natural gas. These two fuels now contribute almost as much energy to the U.S. economy as was produced in 1960 by all fuels combined. Crude oil production, by contrast, first rose and then fell, with an overall increase of only 4.6 percent over 31 years. Figure 14 illustrates the flow of energy resources into and out of the U.S economy.

Before 1970, the United States imported a small amount of energy, primarily in the form of petroleum. Lower acquisition costs for imported crude oil in the early 1970s, however, put U.S. oil producers at a comparative disadvantage relative to their foreign counterparts. By 1971, the U.S. trends in energy consumption and production began to diverge (Figure 15).

On the consumption side, rising petroleum imports have permitted a 64.3 percent increase in petroleum use since 1960. However, increases were dampened by international oil price shocks in 1973 and 1979-80. Since 1981, however, energy prices have fallen almost as rapidly as they rose in the 1970s. After drop-offs following the oil price shocks, U.S. energy consumption again rose, until leveling off between 1988 and the present. Petroleum's share of total energy consumption fell from 45.5 percent in 1960 to 40.1 percent in 1991.

Similarly, domestic energy production has varied with energy prices over the past twenty years. However, that variation has been less pronounced than the variation in consumption. Overall, U.S. energy production has risen by approximately 9 percent since 1973.

Since the early 1980s, when energy prices began to fall, U.S. energy consumption has increased faster than domestic production. If that trend continues, the United States will need to import more than half of its oil by 2010. Since oil is projected to comprise nearly

half of U.S. energy needs by that date, the U.S. economy could, in such a scenario, remain vulnerable to variation in world energy prices.

Growth in the U.S. economy and population, rather than a worsening in the relative efficiency of energy use, is the major force behind recent and projected increases in energy use. Figure 16 shows a 27.2 percent decrease in the overall energy intensity of the U.S. economy since its peak in 1970. Similarly, the amount of energy consumed by U.S. households fell by 16.6 percent from 1970 to 1989; and by 1985, U.S. industrial energy intensity had declined 25 percent from its 1972 level. However, these improvements in per-unit energy use efficiency have not been large enough to counterbalance increases in the size of the population in the numbers of automobiles and households, and in the level of economic output.

In 1991, the generation, transmission, and distribution of electricity removed 20.49 quads from the stream of total usable energy, leaving 61.02 quads available for consumption by end users. Industry and transportation consumed nearly three-quarters of this energy, while the residential and commercial sector uses just 26 percent. However, because more than 60 percent of electricity is delivered to residential and commercial users, a larger portion of the conversion, transmission, and distribution losses is attributed to that sector. As a result, total energy consumption of 81.51 quads is distributed fairly evenly among residential/commercial, industrial, and transportation uses.

Industrial Energy Use

While definitive information on energy use for all industries is not yet available, the relative shift in the American economy away from manufacturing (Figure 10) may be partly responsible for the marked decline in the energy intensity of the U.S. economy. In 1960, more than 22,000 Btus of energy were used to produce one dollar of gross domestic product (GDP, measured in 1987 dollars). By 1991, U.S. industries were producing one dollar's worth of output with only 16,800 Btus. This 25 percent gain in efficiency was the direct result of (1) a changing mix of goods and services in the gross domestic product, as well as (2) technological improvements in the use of all forms of energy, and (3) increasing efficiency in the transformation of coal to electric energy, offset by the substitution of electricity for the less efficient uses of oil and gas. (The ratio of other forms of energy to GDP has been virtually unchanged since 1960.)

Energy intensity is also decreasing in the manufacturing sector. Of fifteen industry groups for which information is available, only one increased its energy intensity during 1980-88. Some groups decreased their intensity by as much as 54 percent, and the average improvement for all manufacturing industries was 27 percent (Figure 17). Most of these improvements (a portion of which may have resulted from structural shifts in the U.S. economy) were achieved in the first half of the 1980s, and reductions in overall energy intensity seem to have slowed since the early part of the decade (Figure 16).

Residential Energy Use

The number, density, characteristics, and distribution of private dwellings can be good indicators of resource consumption and some types of environmental impacts. The purchase and turnover of major appliances, motor vehicles, and other consumer products typically occur at the level of the household. In this sense, dwellings may be a better indicator of energy consumption than population statistics.

The United States has 102 million housing units, approximately half of which are detached and occupied by a single family. As of 1990, the average U.S. household consisted of 2.63 people, down from 3.33 in 1960, The median size of a U.S. dwelling is 1,604 square feet. The typical U.S. dwelling has five rooms and is situated on 0.12 hectares (0.39 acres); but average lot sizes

range from almost 0.20 hectares (one-half acre) in the South to less than one-quarter acre in the West. Twenty seven percent of U.S. dwellings are multiple-unit, attached complexes, and 7 percent of these contain 20 or more units. Approximately 6 percent of U.S. dwellings are classified as trailers or mobile homes. Most occupied dwellings in the United States were constructed before 1979, with 13 percent constructed in 1980 or later. The median age of dwellings in the United States is thirty years. During 1978-87, average energy consumption per household dropped over 25 percent, from 138 million Btus to 101 million Btus.

Transportation Energy Use

The transportation sector accounts for approximately one-quarter of U.S. greenhouse gas emissions. Since the 1920s, U.S. transportation has evolved into a sixmodal system of waterborne, highway, mass transit, air, rail, and pipeline transport (Table 4). The current U.S. surface transportation system is dominated by automo

biles. In 1990, the highway share of personal travel was 88 percent, in contrast to the mass transit share at 2.5 percent. At the same time, demand for freight transportation continued to increase. Over 3.6 trillion tonmiles of freight are moved in the United States each year. The predominant mode of intercity freight is rail, followed by waterways, highways, pipelines, and air. Between 1920 and 1985, the number of railroad cars in use declined, whereas the number of motor vehicles and air carriers increased dramatically, and the number of water-transport vessels and oil pipelines grew steadily. Motor vehicles provide another good indicator of a nation's energy consumption. As of 1991, there were 190 million cars and trucks registered in the United States, up from 74 million in 1960. The average age of U.S. automobiles is about 7.5 years, and 19 percent of the U.S. automobile fleet is over 12 years old.

In 1990, personal passenger vehicles in the United States traveled a total of 1.5 trillion miles (2.4 trillion km), using 273 billion liters (72 billion gallons) of fuel, with an average fuel efficiency of almost 21 miles traveled per gallon (8.9 km/liter) of fuel. In 1969, U.S.