Fossil fuels, including coal, oil and natural gas, are currently the world's primary energy source. Formed from organic material over the course of millions of years, fossil fuels have fueled U.S. and global economic development over the past century. Yet fossil fuels are finite resources and they can also irreparably harm the environment. According to the U.S. Energy Information Administration, the burning of fossil fuels was responsible for 76 percent of U.S. greenhouse gas emissions in 2016. These gases contribute to the greenhouse effect and could lead to potentially catastrophic changes in the Earth’s climate. Technologies such as Carbon Capture and Storage (CCS) may help reduce the greenhouse gas emissions generated by fossil fuels, and nuclear energy can be a zero-carbon alternative for electricity generation. But other, more sustainable and less risky solutions exist: energy efficiency and renewable energy.

Oil

Oil is the world’s primary fuel source for transportation. Most oil is pumped out of underground reservoirs, but it can also be found embedded in shale and tar sands. Once extracted, crude oil is processed in oil refineries to create fuel oil, gasoline, liquefied petroleum gas, and other nonfuel products such as pesticides, fertilizers, pharmaceuticals, and plastics.

The United States leads the world in petroleum consumption at 19.88 million barrels per day as of 2017. Net petroleum imports for the United States were 3.8 million barrels per day. Top exporters to the United States include Canada, Mexico, Saudi Arabia, Venezuela, and Nigeria. The world’s heavy reliance on oil for transportation makes it difficult to reduce consumption. But oil poses major environmental problems. Besides the environmental degradation caused by oil spills and extraction, combustion of oil releases fine particulates which can lead to serious respiratory problems. Oil is also a major source of greenhouse gas emissions: petroleum is responsible for 45 percent of greenhouse gas emissions in the United States as of 2017.

Heavier crude oils, especially those extracted from tar sands and—through fracking—from shale, require the use of energy intensive methods that result in more emissions and environmental degradation compared to conventional oil. As conventional oil from underground reservoirs runs out, more oil producers are turning to unconventional sources such as tar sands and oil shale.

Coal

Coal is primarily used to generate electricity and is responsible for 30 percent of the electric power supply in the United States in 2017 (down from 39 percent in 2017 and 50 percent in 2007). The United States produces around 11 percent of the world’s total with Wyoming, West Virginia, Pennsylvania, Illinois, and Kentucky leading in production. China is the global leader in coal production, responsible for 48 percent of world supply (as of 2015).

The combustion of coal releases air pollutants such as acid rain-inducing sulfur dioxide, nitrogen oxides (NOx), and mercury. The mining process can also be very damaging to the environment, often resulting in the destruction of vegetation and top-soil. Rivers and streams can also be destroyed or contaminated by mine wastes. The combustion of coal is responsible for 32 percent of the greenhouse gas emissions in the United States.

The premise of "clean coal" has recently been promoted as a way to use this abundant energy source without damaging the environment. Carbon capture and storage (CCS), where carbon is separated from coal and injected underground for long term storage, could theoretically be used to mitigate the coal industry's greenhouse gas emissions. However, CCS has yet to be proven as a safe or realistic way to reduce greenhouse gas emissions from commercial power plants and the environmental and health costs of mining remain.

Natural Gas

Natural gas generates an increasing share of U.S. electricity and now represents close to a third of the country's energy use. It is most commonly used to produce heat or electricity for buildings or industrial processes; less than three percent of U.S. natural gas is used as a transportation fuel, typically for bus fleets. The United States produces around 17.6 percent of the world’s natural gas and consumes about 21.6 percent of it (as of 2015). Natural gas is most commonly transported by pipeline, which makes Canada the key exporter to the United States, while Russia remains the main supplier for much of Europe. Increasingly, however, natural gas is being transported by ship in a liquefied form (LNG) to meet greater global demand for the fuel.

Natural gas burns cleaner than coal and oil, with almost zero sulfur dioxide emissions and far fewer nitrogen oxide and particulate emissions. Natural gas releases almost 30 percent less carbon dioxide than oil and 43 percent less than coal. However, natural gas is still responsible for 29 percent of greenhouse gas emissions in the United States.

Natural gas, which is primarily composed of methane (CH4), is also generated by the decomposition of municipal waste in landfills and manure from livestock production. Methane is a greenhouse gas that is more than 20 times as potent as carbon dioxide. Capturing and burning the gas to produce usable heat and power prevents the methane from being released from the landfill or feedlot into the atmosphere directly.

Fossil Fuel Alternatives: Energy Efficiency and Renewable Energy

Despite current U.S. dependence on fossil fuels, several options exist to begin the necessary transition away from a harmful fossil fuel economy. Improving the energy efficiency of buildings, vehicles, industrial processes, appliances and equipment is the most immediate and cost effective way to reduce energy use. Planning communities where people can safely and conveniently use public transit, walk, or bike, instead of using private vehicles, also reduces energy demand. Finally, clean, renewable energy—such as water, biomass, wind, geothermal, and solar energy—can replace fossil fuels.