Energy in the United States: 1635-2000
	Environmental Indicators
	The use of energy brings both benefits and costs. Some, but not all, of these costs show up on consumers' utility bills. The charges levied on consumers by an energy producer (an electric utility with a coal-fired generating plant, for instance) are designed to cover the producer's costs of building the power plant, extracting coal from the ground, transporting it to the power plant, crushing it to the proper size for combustion, maintaining the generating turbines, paying workers and managers, and so on.
	One important category of costs that is often not reflected in consumers' bills is energy-related environmental effects. These unwanted effects can be thought of as the tail end of the energy cycle, which begins with extraction and processing of fuels (or gathering of wind or solar energy), proceeds with conversion to useful forms by means of petroleum refining, electricity generation, and other processes, and then concludes with distribution to, and consumption by, end-users. Once the energy has rendered the services for which it is consumed, all that is left are the byproducts of energy use, i.e., waste heat, mine tailings, sulfur dioxide and carbon dioxide gases, spent nuclear fuel, and many others.
	All energy use has unwanted effects of one kind or another; even a simple campfire produces eye-stinging smoke as well as warmth. Such effects can be local or widespread and have long provoked concern. King Edward I of England, for instance, so objected to the noxious smoke and fumes from London's many coal-burning fires that in 1306 he tried (unsuccessfully) to ban its use by anyone except blacksmiths.
	The enormous scale of modern energy use has sharply increased concerns about unwanted environmental effects. No form of energy production is entirely free of them, including renewable energy. Damming rivers and streams for hydropower facilities radically alters natural stream flows in ways that can threaten or endanger aquatic species. Wind-turbine generators can make noise and kill birds. Biomass generating plants that rely on plantation forestry for fuel can displace natural forest habitat and reduce biological diversity.
	Among the most significant environmental effects of energy production and consumption is the emission of greenhouse gases. These gases--carbon dioxide, methane, nitrous oxide, and others--block infrared radiation from the Earth to space and retain the captured heat in the atmosphere. This greenhouse effect keeps the Earth's climate hospitable to life. But the possibility of carbon-dioxide-forced warming of the climate--postulated since 1861--concerns scientists, and in recent years many have come to believe that anthropogenic (human-caused) additions to greenhouse gases are raising global average temperatures and may produce harmful changes in the global climate. Energy-related greenhouse gas emissions make up a significant fraction of all such emissions, and the United States, as one of the world's largest producers and consumers of fossil fuels, is responsible for a major portion of global energy-related emissions.
	Carbon dioxide (CO2) accounts for the largest share of combined anthropogenic greenhouse gas emissions. In 1999 U.S. anthropogenic CO2 emissions totaled about 5.6 billion metric tons (of gas; 1 ton of carbon equals 3.667 tons of carbon dioxide gas), 17 percent higher than in 1980 and 28 percent higher than in 1983, the low point of the 20-year period from 1980 through 1999 (Figure 32). Nearly 99 percent of this total was energy-related emissions, especially from petroleum consumed by the transportation sector, coal burned by electric utilities, and natural gas used by industry, homes, and businesses.
	Figure 32. Carbon Dioxide Emissions
	Energy-related emissions of methane, another important greenhouse gas, remained at about 11 million metric tons in 1999. While about 37 percent of U.S. methane emissions stemmed from energy use, most came from landfills and such agricultural sources as ruminant animals (cattle and sheep) and their wastes. Emissions of a third potent greenhouse gas, nitrous oxide, remained about the same in 1999, at 1.2 million metric tons.
	All sectors of the U.S. economy contribute to energy-related greenhouse gas emissions, especially CO2. Of 1999 energy-related CO2 emissions of 1.5 billion metric tons of carbon (5.6 billion tons of gas), the industrial and transportation sectors each accounted for about one-third, the residential sector for about one-fifth, and the commercial sector for the remainder. Industry's emissions derive from a broad mix of fossil-origin energy, including electricity, petroleum, natural gas, and coal. Not surprisingly, the transportation sector emits carbon dioxide mostly via the consumption of petroleum (especially motor gasoline, distillate fuels such as diesel, and jet fuel). Residential- and commercial-sector emissions are owed mostly to the use of electricity and natural gas.