6.1 Conventional energy sources – fossil fuels and nuclear energy

Fossil fuels

Although we think of them very differently, fossil fuels, like wind and solar energy began with the sun. Fossil fuels come from the organic matter of plants, algae, and cyanobacteria that were buried, heated, and compressed under high pressure over millions of years. The process transformed the biomass of those organisms into three types of fossil fuels: oil, coal, and natural gas. Different types of coal, oil, and gas are a result of differences in heat, pressure, and time, which affect the energy content of the fuel.

The energy content or energy density of fossil fuels is a measure of the amount of energy provided by each unit of fuel. Natural gas has the highest energy density, followed by oil and then coal. In a world in which climate change is an urgent concern, we can also discuss the carbon intensity of these fuels, which is related to how much CO₂ is produced when we produce a set amount of energy (different from burning a fixed amount of the fuel). Coal produces the highest volume of CO₂ for the energy produced (96 kilograms of CO₂ per million British thermal units – kg/BTU), ^[1], followed by oil and gasoline (mid-70 kg/BTU) and then natural gas (53 kg/BTU).

Mineral resources occur in varying concentrations and purities. Any occurrence is termed a resource, whereas a resource that is technically, economically, legally, and socially recoverable is termed a reserve. Many billions of tons of coal, oil, and gas will never by mined from the Earth due to limitations in one or more of these criteria.

Coal

Coal is an abundant resource that is relatively inexpensive to produce, particularly if the impacts of mining are not well regulated. Harder varieties – anthracite and bituminous coal – have a higher energy density. Soft coal, subbituminous (harder) and lignite (softer), contains lower amounts of energy, and therefore more needs to be burned to get the equivalent amount of energy. All coal contains some sulfur – it was part of the proteins of the living things that became fossil fuel. When burned, this becomes SO₂, an air pollutant that becomes part of acid rain. Soft coal has higher levels of sulfur than hard coal. Coal is primarily used to generate electricity.

Hard coal reserves are greatest in the US, China, Russia, and India, in that order; soft coal reserves are greatest in Russia, Australia, Germany, and the US.^[2] Coal use has flattened in recent years; at current rates of use, coal reserves can last for centuries. However, distribution of coal reserves among countries is very uneven (Fig 1).

Figure 1. Confirmed (proved) reserves of coal in the world.

Although coal consumption in the US has significantly declined in the last couple of decades, globally its consumption continues to grow, roughly doubling in the last few decades from 4 to 8 gigatonnes per year, dominated by growth in Asia. (Fig 2). China leads the world in the pace of its build-out of renewable energy, but it also burns the largest share of coal.

Figure 2. Global coal consumption 2022-2026. From Coal 2024: analysis and forecast to 2027. International Energy Association (IEA) CC BY.

Coal is plentiful and inexpensive when looking only at the market cost relative to the cost of other sources of electricity, but its extraction, transportation, and use produces  environmental impacts that the market cost does not truly represent. Coal is mined from surface, open-pit mines, from underground mines, and by mountain-top removal (practiced only in the US).

Coal mining, if not carefully regulated, acidifies and otherwise pollutes surface water, groundwater, and soils. Mountain-top removal buries streams under large volumes of earth, and pollutes streams that flow from the burial sites. Burning of coal contributes to global warming and also releases sulfur dioxide, nitrogen oxide, and mercury, which are linked to acid rain, smog, and health issues. Coal combustion byproducts include large volumes of solid ash that can be rich in toxic materials such as metals and organic compounds that can pollute air and water. Mining of coal in countries with fewer safety regulations continues to claim lives, often due to explosions that trap miners underground, and black-lung disease, caused by inhaling coal dust over long periods, is a threat, as well.

Oil 

Petroleum or crude oil is a liquid energy resource, mostly derived from marine algae. Crude oil is a complex mixture of organic chemicals and is refined into a series of products, but because of its high energy density and ease of handling, it is most used as a transportation fuel. As with coal, many billion tons of oil resources will never be recovered.

Oil reserves are more evenly distributed geographically than coal resources, but many countries lack both resources (Fig 3). In countries with rich oil reserves, such as Saudi Arabia, oil flows to the surface, making extraction very inexpensive. However, most oil is pumped from wells 1-2 miles below the Earth’s surface (0.6 – 1.2 km). Oils range widely in quantity and ease of production, ranging from thin, light, “sweet” oils that are easy to produce and refine to thick, heavy, sour (sulfur rich) crudes that are expensive to produce and refine and that cause more air pollution. Refining uses energy, and which contributes to the carbon intensity of the resulting fuel.

Oil is also found in oil sands, tar sands, and shales; these are all considered unconventional sources, and are proportionally much more expensive to produce. Energy analysts are increasingly skeptical of the accuracy of reported oil and gas reserves because of the economic and political implications of the data and the incentives for countries to misreport their holdings.^[3]

Figure 3. Oil reserves around the world. OurWorldinData.org. CC BY.

During oil production, leakage during drilling and pumping can contaminate soil and water; spills cause similar damage at a larger scale. Often, wells are drilled through aquifers that contain salt water, leading to salt water being carried to the surface, along with oil. Usually, saltwater is reinjected back into the aquifers. If freshwater aquifers overlie the salt-water aquifers, they may be contaminated by both oil and salt water. Disposal of produced saline water into surface water can pollute surface and groundwater, as well as soil.

Oil and tar sands are mined or extracted by heating the oil in situ, rather than drilled. Extraction produces more greenhouse gases than conventional oil drilling^[4], and produces large volumes of contaminated water. Burning oil results in air pollution from SO₂, NOx, VOCs, and particulate matter, and global warming from CO₂ and methane.

Fracking – hydraulic fracturing – is regularly used to increase production of oil and gas in deposits in which oil and gas are trapped in rock. Perforated pipes are placed into the rocks that hold the oil and gas reservoir, and a mixture of water, sand, and a suite of proprietary chemicals (including the forever chemicals PFAS) is injected into the reservoir under high pressure to crack the rock and release the oil and gas (Fig 4).

Fracking consumes large volumes of water, often 10 million gallons (3.8 million liters) or more^[5], and also produces large volumes of polluted water, because much of the water-chemical mix used to pressurize the oil and gas deposits returns to the surface along with the oil and gas. If wastewater escapes or contaminates overlying freshwater aquifers, then surface and groundwater contamination can result.^[6] Recycling of wastewater is common in areas with limited freshwater availability, and this can reduce water use in an oil field – especially important in arid areas. Methane, one of the major GHGs, is released during fracking. A variety of public health hazards have been reported, as well. ^[7]

Figure 4. Diagram of hydraulic fracking machinery and process. Emiliawilkinson from Wikimedia Commons, CC BY-SA.

Natural gas is a suite of gases mainly composed of methane (CH₄) and is a very potent greenhouse gas. Thermogenic gas was formed in the past, from the compression of deeply buried, ancient organic matter with deep heat, underground. Thermogenic gas is found with petroleum in reservoir rocks and with coal deposits, and these fossil fuels may be extracted together. Biogenic gas is found at shallow depths and arises from present-day bacterial decay of organic matter where oxygen is unavailable, as in landfill gas or swamp gas. Most plastics are made from natural gases (e.g., ethylene, C₂H₄).

As with coal and oil, the global distribution of natural gas reserves varies geographically. Up until the first decade of the 21st century, all natural gas was produced in the same manner as oil, by the drilling of vertical wells that produced a flow of gas to the surface, powered by its own pressure. With the advent of two new technologies in the US, the drilling of wells into reservoirs horizontally and the fracturing of reservoirs to enhance their productivity, vast new reserves of natural gas were established. This radically changed the energy profile of the US. The major decline in the consumption of coal for the generation of electricity was made possible by large increase in the production of natural gas which is now the predominate source energy for generation. Natural gas has a lower carbon density than coal, so US production of GHG decreased as a result of this switch.

Approximately 35% of methane released to the atmosphere during 2010-2019 was produced naturally, particularly from wetlands. ^[8] Our attention here is to anthropogenic natural gas.

Approximately 30% of anthropogenic natural gas is produced by fossil fuel production and use. Reported leakage during production is relatively low – less than 1% of US production in 2023, but this accounted for approximately 30% of overall US methane emissions, and not all leakage is reported. As we learned in Chapter 2, methane is a potent GHG. It often leaks from all phases of production, transportation, and consumption. This leaked or “fugitive” methane is now being recognized as a significant contributor to climate change, but whose abatement is moderately easy to accomplish.

When natural gas is produced but cannot be captured and transported economically – as when it comes up from oil wells equipped for liquid oil capture rather than gas capture – it is “flared” or burned at well sites, which converts most of it to CO₂. This is considered safer and better than releasing methane into the atmosphere because CO₂ is a less potent greenhouse gas than methane.

When natural gas comes up from a well, other gases may also be present, including hydrogen sulfide, H₂S, which is toxic. Gas with H₂S can be flared, producing SO₂, or treated to remove the H₂S, or released directly into the atmosphere where it is a health hazard first as H₂S and eventually as SO₂. Like oil, natural gas is produced by drilling, which may be accompanied by fracking. As a result, the environmental impacts of production are similar to those for oil. When burned, natural gas produces less CO₂ for a given amount of energy (heat) produced than oil or coal, making it the preferred fossil fuel. However, the methane that can be released during production of natural gas can cause it to have GHG effects similar to coal unless the methane is addressed. Natural gas produces less SO₂ and less NOx than coal.

Nuclear energy

Nuclear power is energy released from the radioactive decay of elements, such as uranium, which releases large amounts of energy as heat. Unlike fossil fuel power plants, because of the inherent dangers associated with the radioactive material that is used as fuel, nuclear power plants are highly complex and have many systems that other power plants don’t require to ensure the safe containment of the fuel. In contrast with the use of other energy fuels, nuclear power plants produce no carbon dioxide and nuclear fuels are often considered alternative fuels (fuels other than fossil fuels). A 2025 report from International Energy Association indicates that the proportion of world electricity provided by nuclear power peaked at the end of the 1990s, at approximately 18%; currently, nuclear energy provides about 9% of global electricity generation^[9]. Some reactors are also used for heat generation – a form of co-generation that uses the heat of the nuclear fission reaction that creates the power for electricity.

Many nuclear reactors are near the end of their original licensing periods. At the end of 2023, the average nuclear reactor in the developed world was 36 years, whereas reactors in emerging and developing countries averaged less than 18 years. However, the demand for electricity, ballooning with the huge energy needs of artificial intelligence, and growing even as climate change impacts are increasingly visible, is renewing interest in and plans for nuclear energy.

Mining and refining uranium ore and making reactor fuel demand a lot of energy. Large-scale nuclear power plants are very expensive and require large amounts of metal, concrete, and energy to build. However, small modular reactors are being designed, with lower construction costs and times, and faster times to profitability; the reduced costs make financing easier, and enable more players to be involved. The first of the so-called SMRs are due online in 2030.

Nuclear reactors generate considerable heat during operation and are often water-cooled. However, most of the water is returned to the source. In countries with relevant regulation, plants are limited in the amount of warming they can cause in receiving bodies of water, and may employ cooling towers or other methods of reducing the temperature of the water leaving the facility. Under global warming, as the temperature of the incoming water warms, cooling processes for the plant become less effective.^[10]

Although nuclear power requires mining of uranium, and is associated with the usual environmental impacts of mining, the main environmental challenge for nuclear power is waste. By volume, the waste produced from mining uranium, called uranium mill tailings, is the largest volume of waste and contains the radioactive element radium (Ra), which decays to produce radon (Rn), a radioactive gas with a half-life of over 1,000 years. The half-life of a radioactive element is the time it takes for 50% of the material to decay radioactively, and is one measure of how long wastes containing the element will be dangerous.

Waste from nuclear fuel rods – small uranium fuel pellets in long metal tubes – is considered high-level radioactive waste. Decay of radioactive uranium in nuclear fuel rods produces several different decay products, some with half-lives as short as 30 years, others with half-lives of thousands of years. Practically speaking, then, radioactive wastes from nuclear power can be considered another “forever” pollutant. Only Finland and Sweden have started construction on long-term storage facilities for nuclear waste; Finland’s facility is due to open in 2026. For now, everywhere in the world, spent nuclear fuel rods are first held in “temporary” storage pools, where water continuously cools their ongoing, but energetically less useful, radioactive decay, and later are held in outdoor steel or concrete containers cooled by air.

The potential for significant harm in the event of a failure at a nuclear plant is not a hypothetical issue. The 1986 failure at the Chernobyl nuclear reactor in Ukraine led to the evacuation and resettlement of over 200,000 people from three nations. Thousands of cases of thyroid cancer were linked to the radioactive fall-out, and economic loss and hardship occurred across a wide area^[11]. Thousands of square kilometers around the site are uninhabitable for thousands of years.

The 2011 accident at the Fukushima Daiichi Nuclear Power Station in Japan was caused by an earthquake and tsunami that overtopped the protective seawall and greatly complicated relief efforts. Radioactive material was spread both through the atmosphere and in flood waters. Initially, over 100,000 people left the area, with 79,000 still considered evacuees in 2017. ^[12] Clean-up continues to the present day.

A unique hazard associated with nuclear energy is the potential for this energy resource to be weaponized. Some 31 countries operate nuclear power plants today. Securing nuclear power stations and the waste they produce requires a high degree of technology, which does not necessarily accompany the technology for building nuclear power stations. These additional risks associated with nuclear energy bring an additional level of ethical consideration to its use. However, despite the demonstrated potential for harm, interest in nuclear energy is returning after a prolonged decline. Decarbonization of the world’s energy supply through renewable energy has not advanced quickly enough to slow, much less halt, climate change; nuclear energy is seen as an alternative means of meeting growing demands for clean energy. The smaller, modular reactors may meet less resistance from voters despite the risks accompanying their use.

Closing and decommissioning traditional-fuel and nuclear production facilities

Coal, oil and gas mining operations must eventually close when the target resources are reduced below economically recoverable levels. In many countries, mining permits include payment of a deposit to ensure reclamation or safe capping and clean-up; reclamation and clean-up of operations that started before such regulations were imposed and sites that have been abandoned generally become the responsibility of the state, and may never be undertaken, due to lack of funds.

Coal

The goal of reclamation of surface coal mines is to return the land to beneficial uses and minimize impacts from the site. Reclamation typically involves recontouring the site so that it matches the surrounding landscape and, as nearly as possible, the original topography of the site. Waste materials that contain recoverable energy may be removed or burned for energy. Methods of reducing acid mine drainage in the long term should be put in place to avoid contaminating groundwater and surface waters that receive percolation or runoff from the mine. A variety of treatment approaches is available but this phase of reclamation can be particularly costly.^[13] Unless the site is to be developed immediately, some form of revegetation is appropriate to reduce erosion. Reclaimed sites have very different soils from the original soils on the site because of the highly disturbed nature of the subsoil and, often, the presence of a clay cap to reduce water percolation into the mined material.  As a result, revegetation requires use of plant species that can tolerate the new soil conditions. If the price of coal rises substantially, deposits that were left behind for financial reasons may become attractive again, and reclaimed sites may be reopened and subject to additional mining, particularly if they were reclaimed to wildlife habitat, agriculture, or recreational uses.

Underground coal mines should also reclamation, which employs many of the same processes as surface-mine reclamation – recontouring, addressing acid-mine and other drainage issues, and revegetating. An important difference is the need to backfill or stabilize the underground area of the mine to prevent formation of sinkholes and subsidence of the ground. When large volumes of material have been removed, stabilization – often undertaken by injecting specialized “grouts” into the empty (or void) spaces – can be costly.

Because coal mining began long before relevant regulations were in place anywhere in the world, abandoned coal mines and their acid-mine wastes are a reality in most places that have (or had) easily mined coal deposits. They continue to acidify local waters and the lands they occupy cannot be made useful without some form of reclamation.

Oil and gas

Oil and gas drilling sites also need reclamation to ensure safety and to minimize environmental impacts. Wells should be capped and infrastructure removed. Spills of toxic materials should be remediated. Salt water that may come to the surface in large volumes along with the recovered oil and gas should be disposed of, often by injecting it back into deep rock formation. Where fracking was employed, reclamation should include a search for methane leaks and assessment of impacts to local hydrology, and treatment of any that are found. Recontouring and revegetation may be undertaken, as needed depending on intended future uses of the land. Regulations that require some form of deposit or bond to cover these costs can help to ensure that reclamation occurs to a good standard.   

Nuclear

The goal of decommissioning a nuclear-energy facility is to safely return the site to alternative uses such as agriculture, industry, or housing. Disposal of the radioactive fuel is an ongoing part of operation of the facility, and is discussed above. However, parts of the facility itself are also radioactive, at lower levels. The fastest route to decommissioning is to remove and dispose of the radioactive components and then dispose of the rest of the facility – this approach can be completed in less than a decade but is still costly. Nuclear facilities can also allowed to sit, unused, with appropriate monitoring, for a long enough period of time to reduce radiation levels to the point that special disposal is not required, which takes decades. Lastly, facilities can be permanently entombed in concrete, as was done with the Chernobyl facility.^[14]  Decommissioning will eventually be a concern for the new, modular nuclear reactors planned for 2030 and later. Although these will be smaller than existing reactors, they will contain similar components.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.