Decomposing plants and other organisms, buried beneath layers of sediment and rock, have taken millennia to become the carbon-rich deposits we now call fossil fuels. These non-renewable fuels, which include coal, oil, and natural gas, supply about 80 percent of the world’s energy. They provide electricity, heat, and transportation, while also feeding the processes that make a huge range of products, from steel to plastics.

When fossil fuels are burned, they release carbon dioxide and other greenhouse gases, which in turn trap heat in our atmosphere, making them the primary contributors to global warming and climate change.

Major types of fossil fuels

There are several main groups of fossil fuels, including:

Coal: Black or brown chunks of sedimentary rock that range from crumbly to relatively hard, coal began to form during the Carboniferous period about 300 to 360 million years ago, when algae and debris from vegetation in swamp forests settled deeper and deeper under layers of mud. Mined via surface or underground methods, coal supplies a third of all energy worldwide, with the top coal consumers and producers in 2018 being China, India, and the United States. Coal is classified into four categories—anthracite, bituminous, sub-bituminous, and lignite–depending on its carbon content.

Carbon dioxide emissions from burning coal account for 44 percent of the world total, and it's the biggest single source of the global temperature increase above pre-industrial levels. The health and environmental consequences of coal use, along with competition from cheap natural gas, have contributed to its decline in the U.S. and elsewhere. But in other places, such as India, demand is expected to rise through 2023.

Oil: Crude oil, a liquid composed mainly of carbon and hydrogen, is often black, but exists in a variety of colors and viscosities depending on its chemical composition. Much of it formed during the Mesozoic period, between 252 and 66 million years ago, as plankton, algae, and other matter sank to the bottom of ancient seas and was eventually buried.

Extracted from onshore and offshore wells, crude oil is refined into a variety of petroleum products, including gasoline, diesel, and heating oil. The top oil-producing countries are the U.S., Saudi Arabia, and Russia, which together account for nearly 40 percent of the world's supply.

Petroleum use accounts for nearly half the carbon emissions in the U.S. and about a third of the global total. In addition to the air pollution released when oil is burned, drilling and transport have led to several major accidents, such as the Exxon Valdez spill in 1989, the Deepwater Horizon disaster in 2010, the devastating Lac Megantic oil train derailment in 2013, and thousands of pipeline incidents. Nonetheless, oil demand continues to rise, driven not only by our thirst for mobility, but for the many products—including plastics—made using petrochemicals, which are generally derived from oil and gas.

Natural gas: An odorless gas composed primarily of methane, natural gas often lies in deposits that, like those for coal and oil, formed millions of years ago from decaying plant matter and organisms. Both natural gas and oil production have surged in the U.S. over the past two decades because of advances in the drilling technique most people know as fracking.

By combining fracking—or hydraulic fracturing—with horizontal drilling and other innovations, the fossil-fuel industry has managed to extract resources that were previously too costly to reach. As a result, natural gas has surpassed coal to become the top fuel for U.S. electricity production, and the U.S. leads the world in natural gas production, followed by Russia and Iran.

Natural gas is cleaner than coal and oil in terms of emissions, but nonetheless accounts for a fifth of the world's total, not counting the so-called fugitive emissions that escape from the industry, which can be significant. Not all of the world’s natural gas sources are being actively mined. Undersea methane hydrates, for example, where gas is trapped in frozen water, are being eyed as a potential gas resource.

Reducing emissions from fossil fuels

Governments around the world are now engaged in efforts to ramp down greenhouse gas emissions from fossil fuels to prevent the worst effects of climate change. At the international level, countries have committed to emissions reduction targets as part of the 2015 Paris Agreement, while other entities—including cities, states, and businesses—have made their own commitments. These efforts generally focus on replacing fossil fuels with renewable energy sources, increasing energy efficiency, and electrifying sectors such as transportation and buildings.

However, many sources of carbon emissions, such as existing power plants that run on natural gas and coal, are already locked in. Considering the world's continuing dependence on fossil fuels, many argue that in addition to efforts aimed at replacing them, we also need to suck carbon from the air with technologies such as carbon capture, in which emissions are diverted to underground storage or recycled before they reach the atmosphere. A handful of commercial-scale projects around the world already capture carbon dioxide from the smokestacks of fossil fuel-fired plants, and while its high costs have prevented wider adoption, advocates hope advances in the technology will eventually make it more affordable.