Fossil fuels are buried deposits of petroleum, coal, peat, natural gas, and other carbon-rich organic compounds derived from the dead bodies of plants and animals that lived many millions of years ago. Over long periods of time, pressure generated by overlying sediments and heat from within Earth have concentrated and modified these materials into valuable energy sources for human purposes. In 2018, fossil fuels provided about 85 percent of all technological energy used in the world. They provide the power to move vehicles, heat living spaces, provide light, cook food, transmit and process information, and carry out a wide variety of industrial processes.

Modern agriculture is also deeply dependent on fossil fuels, since most of the nitrogen in fertilizers is derived from natural gas. It is no exaggeration to say that modern industrial society is nearly completely dependent on a continual supply of fossil fuels. How humans will adapt as supplies become too limited, too remote, too expensive, or too environmentally destructive to continue to use is a paramount question for society.

The amount of fossil fuels deposited over history is large. Total coal reserves, for example, are estimated to be in the vicinity of 10 trillion metric tons. If all this resource could be dug up, shipped to market, and burned in an economically and environmentally acceptable manner—which, at this point, it cannot—it would fuel all current commercial energy uses for several thousand years. Petroleum (oil) deposits are thought to have originally amounted to some four trillion barrels (600 billion metric tons), about half of which has already been extracted and used to fuel industrial society. At current rates of use the proven oil reserves will be used up in about 40 years. World natural gas supplies are thought to be at least 10 quadrillion cubic feet or about as much energy as the original oil supply. At current rates of use, known gas reserves should last at least 60 years. If one substitutes gas for oil or coal, as some planners advocate, supplies will be used up much faster than at current rates. Some unconventional hydrocarbon sources such as oil shales and tar sands might represent an energy supply equal to or even surpassing the coal deposits on which people now depend.

In the United States, oil currently supplies about 36 percent of all commercial energy use, while coal contributes about 13 percent and natural gas provides about 31 percent. Contributions of natural gas to the US energy mix have risen in the 2010s, with the great advantages in hydraulic fracturing (fracking). Oil and its conversion products—such as gasoline, kerosene, diesel fuel, and jet fuel—are the primary fuel for internal combustion engines because of the ease with which they can be stored, transported, and burned. Coal is burned primarily in electric power plants and other large, stationary industrial boilers. Methane (natural gas) is used primarily for space heating, cooking, water heating, and industrial processes. It is cleaner burning than either oil or coal but is difficult to store or to ship to places not served by gas pipelines.

The use of fossil fuels as the world's major energy source has many adverse environmental effects. Coal mining often leaves a devastated landscape of deep holes, decapitated mountain tops, toxic spoil piles, and rocky rubble. Acid drainage and toxic seepage from abandoned mines poison thousands of miles of streams in the United States. Every year the 731 million tons of coal burned in the United States (mainly for electric power generation) release 2.8 million tons of sulfur dioxide, 10.7 million tons of nitrogen oxides (the main components of acid rain), four million tons of carbon monoxide and unburned hydrocarbons, close to a trillion tons of carbon dioxide, and a substantial fraction of the toxic metals such as mercury, cadmium, thallium, and zinc into the air. Coal often contains uranium and thorium. Most coal-fired power plants emit significant amounts of radioactivity—more, in fact, than a typical nuclear power plant under normal conditions. Oil wells generally are not as destructive as coal mines, but exploration, drilling, infrastructure construction, waste disposal, and transport of oil to markets can be very disruptive to wild landscapes and wildlife. Massive oil spills—such as the deliberate oil spill in Kuwait during the Gulf War in 1991, the grounding of the Exxon Valdezin Prince William Sound, Alaska, in 1989, and the explosion at the Deepwater Horizon oil rig in the Gulf of Mexico in 2010—illustrate the risks of shipping large amounts of oil over great distances. Nitrogen oxides, unburned hydrocarbons, and other combustion by-products produced by gasoline and diesel engines are the largest source of air pollution in many cities, both in the United States and around the world.

Harvesting, processing, and distributing fossil fuels also pose environmental and health concerns. Offshore oil drilling is hazardous to aquatic organisms in the vicinity of oil rigs and beyond, while coal mining has historically been very dangerous for miners exposed to coal dust, which can dramatically affect lung function and Page 1853  |  Top of Articlecause chronic diseases, such as pneumoconiosis (also known as black lung disease).

One of the greatest concerns about people's continued dependence on fossil fuels is the waste carbon dioxide produced by combustion. While carbon dioxide is a natural atmospheric component and is naturally absorbed and recycled by photosynthesis in green plants, people now burn so much coal, oil, and natural gas each year that the amount of carbon dioxide in the atmosphere is rapidly increasing. Carbon dioxide levels today are higher than at any point in at least the past 800,000 years.

Because carbon dioxide is a greenhouse gas (that is, it is transparent to visible light but absorbs long wavelength infrared radiation), it tends to trap heat in the lower atmosphere and increase average global temperatures. Climatic changes brought about by higher temperatures can result in heat waves, changes in rainfall patterns and growing seasons, rising sea levels, and a potential increase in the frequency and severity of storms. These potentially catastrophic effects of global climate change may limit people's ability to continue to use fossil fuels as their major energy source. Many believe that all of these considerations suggest that people urgently need to reduce their dependency on fossil fuels and turn to environmentally benign, renewable energy sources such as solar power, wind, biomass, and small-scale hydropower.

There has been a concerted global effort to reduce dependency on fossil fuels and migrate to more environmentally friendly, renewable sources of energy, as many believe it is of paramount importance to keep the average rise of global temperature below 2.7°F (1.5°C) compared to pre-industrial levels. To this effect, they contend that no new fossil fuel power plants must be built, and there must be significant efforts to transition to alternative sources of energy, along with additional measures, such as reforestation.

In an effort to curb climate change, 189 countries (as of 2020) signed on to the Paris Agreement, an international accord. Under this agreement, each country must determine, plan, and report on its individual efforts to mitigate greenhouse gas emissions based on its set emission target. Under the Trump administration, the United States announced that it would withdraw from the Paris Agreement by November 2020, although it had originally signed on to the agreement under the Obama administration. Although changes in policy that are contrary to the agreement are already being put in place, many state and local governments continued to pledge their support for the agreement.

In early 2020, during the pandemic caused by the novel coronavirus (COVID-19), various parts of the world were put under lockdown regulations to curb the spread of the disease. Factories and other businesses were shuttered while government-ordered shelter-in-place policies were implemented for citizens. With so many people staying home, plants closed, and road and air traffic greatly diminished, scientists in May reported a major global emissions decline that reached 17 percent in early April. Researchers expected the reduction to be shortlived as businesses began to reopen.