The most commonly used energy source globally are fossil fuels, which are a nonrenewable energy resource. As the world continues to industrialize and nations become more developed, the global demand for fossil fuels, and the reliance on them, increases.

Fossil fuels exist in three main forms: a solid (coal), a liquid (oil), and a gas (natural gas). The distribution of these is not uniform and varies from area to area based on the geologic history.

There are three types of coal that are used for fuel: lignite, bituminous, and anthracite. They are ranked by their energy density, with higher energy density providing more heat and more energy for the same amount of coal. This energy density is determined by the carbon content, with lignite having the least carbon and anthracite having the most, making anthracite the most energy dense. These properties are determined by the heat, pressure, and depth of burial. An increase in the heat, pressure, and depth will result in an increase in metamorphism, and an increase in carbon content and energy density.

Coal is the most abundant fossil fuel, but it is also the one with the worst environmental impacts. It is due to the environmental impact - particularly the large amount of air pollution - that has earned coal the title of the "dirtiest" fossil fuel.

First, coal must be mined, which causes a wide variety of environmental impacts including habitat destruction, land degradation, air pollution, and water pollution.

It is the combustion of coal that really causes problems. It releases more CO2 than any other fossil fuel when burned for the purpose of generating electricity - when compared to natural gas, it releases about twice as much. Along with releasing CO2 (with coal combustion being responsible for roughly 40% of global emissions), it releases air pollutants as well. Particulate matter, especially soot and ash, is released, which are respiratory irritants. In addition, the burning of coal often results in PM that is contaminated with toxic metals (such as lead, mercury, and arsenic), which can further adversely affect human health while also contaminating the environment. 

You've probably heard about people drilling for oil. Like all fossil fuels, it is necessary to extract the oil in order for humans to be able to use it. When reservoirs have been identified and characterized, oil wells will be drilled in order to bring the oil to the surface. After drilling into the resource, extraction devices are used to help extract the oil so that it can be collected. These wells can cause large environmental problems, as they may leak methane gas (a greenhouse gas) or toxic pollutants into nearby air, water, and soil. In addition, there is the risk of oil spills, which can be extremely hazardous and can kill a wide variety of organisms.

Oil can also be extracted from tar sands (also known as oil sands) - a mixture of clay, sand, water, and bitumen - a sticky and highly viscous semi-solid form of petroleum. Its most common use is in the construction of roads; it is what holds asphalt together. Due to its physical properties, it sticks to the sand and clay and is more difficult to extract than your standard liquid petroleum. It is common to use mining practices to extract it, after which it will then need to be separated from the sand and clay. However, if the bitumen is too deep, in-situ mining will be used where steam will be piped down to melt the bitumen so that it can flow through a well and be extracted at the surface. Bitumen mining is more energy, water, and land-intensive than standard oil wells, and can also lead to air, water, and soil pollution.

The gas itself is a greenhouse gas and contributes to climate change when released. In addition, it releases CO2 when burned for heat and energy. Leaks are common, as well as gas venting (the intentional release of unwanted natural gas during the extraction of other fossil fuels), which also contribute to the carbon footprint. There are actually areas where there are natural gas leaks are naturally ignited and can be found alight - such as the "Eternal Flame" at Chestnut Ridge in Western NY.

It is burned for heating, cooking (think of your gas stove), electricity generation, and is commonly used in the production of fertilizers and plastics. It is common to use a process known as cogeneration to use natural gas more efficiently. Electricity generation gives off heat as a byproduct, which in many cases results in the loss of energy. Cogeneration, also known as a combined heat and power system (CHP), uses this heat produced from the production of energy in a productive way to heat a building, rather than wasting it. It is a more sustainable process as it minimizes the loss of energy. This can be used with other sources of power as well but is common with natural gas.

Fracking, also known as hydrologic fracturing, is a technique commonly used to obtain more natural gas. It centers around the injection of high pressure "fracking fluid" into the ground, which creates cracks or "fractures" in the rock. This allows for natural gas that was trapped within the rock to be released, and increases supplies of natural gas. This has great economic benefits, as it greatly increases the supply and leads to lower prices for consumers. However, it has disastrous environmental effects.  Fracking can contaminate groundwater and surface water - it is not unheard of for people living near fracking sites to have the water in their homes become flammable due to contamination from the methane. In addition, fracking fluid contains salts, detergents, and acids, which can contaminate the air and/or water, which can be toxic to organisms. People who live near  fracking sites have been found to have a myriad of adverse health effects. Fracking has even been linked to the triggering of earthquakes.

Nuclear fission involves the firing of a neutron at a radioactive isotope, which splits the nucleus of the atom apart, releasing a large amount of energy in the form of heat.

The U-235 is stored in fuel rods, which are submerged in a reaction core. The process of fission releases heat, which turns the water into steam. This then follows the same general process as fossil fuels, with the steam spinning a turbine, and producing electricity through a generator.

There are a variety of components to a nuclear power plant aside from the fuel rods, turbine, and generator. Control rods can be lowered into the reactor core to absorb neutrons and slow down the reaction, preventing too much heat from being released and being used to prevent nuclear meltdowns. A water pump brings in cool water that serves two purposes: being turned into steam and also cooling the reactor to prevent overheating. In addition, the steam is brought to a cooling tower that allows the extremely hot steam to cool and condense back into a liquid before being reused or released.

One of the biggest reasons a lot of people are against an increase in the use of nuclear power is the dangers of nuclear meltdowns. These are not common, nor are they likely, but there are three famous disasters that occurred that have painted general sentiment.

Three Mile Island in the United States had a partial nuclear meltdown due to a testing error, which resulted in a release of radiation, however, no deaths were caused by this, nor were there any linked cancer cases.

Fukushima in Japan suffered a tsunami that caused a failure in the cooling pump, resulting in an explosion and widespread radiation release. There were no direct deaths, but many deaths were considered related to the disaster, such as cancer that had developed.

Chernobyl in Ukraine is the most famous nuclear disaster. A cooling valve was stuck during a test, which resulted in an explosion that caused several deaths, many cancer cases, and widespread release of radiation.

The release of radiation can result in genetic mutations, some of which can cause cancer, in affected organisms (including humans). This radiation can be spread through wind or water, affecting ecosystems far from the meltdown sites. In addition, the soil can be contaminated and become radioactive, harming future organisms.

A lot of work has been done since these disasters to help develop fail-safe after fail-safe, ensuring the safety and security of this method of energy generation. There are fewer deaths per unit energy generated by nuclear power than there are for any of the fossil fuels.

While nuclear energy does not result in air pollution or the production of greenhouse gases, it is not a perfect method that has no issues. It can cause thermal pollution, with the heated water harming organisms as it is added back to a water source. In addition, nuclear power produces hazardous solid waste - radioactive waste (or nuclear waste). This waste needs to be carefully stored and managed in order to prevent dangers associated with radiation. That said, nuclear reprocessing works to recycle the vast majority of spent nuclear fuel and minimizes the amount of waste that needs to be addressed.