Glossary: Environmental Issues
Definitions of terms relating to global climate change, pollution,
and other environmental topics.
Acid rain: Sulfur dioxide (SO2) and nitrogen oxides (NOx), along with other chemical compounds, are released during the combustion of fossil fuels. When these gases react in the atmosphere with water, oxygen, and other chemicals, they form acidic compounds. Sunlight increases the rate of most of these reactions. The resulting substances are wet (acid rain, snow, or fog) or dry (acidic gases or particulates) and may drift far from the original source before falling to the earth. The negative effects of these acidic deposits include damage to forests, soil, and aquatic ecosystems, damage to infrastructure and human health, and reduced visibility.
Carbon dioxide (CO2): Carbon dioxide is one of the most common greenhouse gases in the atmosphere and is regulated through the natural carbon cycle, where carbon dioxide is emitted into the air and reabsorbed by vegetation and water. This cycle is upset by the emission of additional carbon dioxide from human activities. Because natural cycles cannot absorb these additional emissions, a large portion of carbon dioxide remains in the atmosphere and increases climate change. The primary human source of carbon dioxide is the burning of fossil fuels for electricity, heat, and transportation.
Carbon monoxide (CO): This gas is created when the carbon in fossil fuels is not entirely burned during combustion and can have serious impacts on human health. The majority of carbon monoxide emissions come from the use of fossil fuels in transportation. Lesser quantities come from electricity production and natural events like forest fires. Improperly-adjusted gas stoves can also release high levels of indoor carbon monoxide. When released into the air, carbon monoxide can exacerbate heart disease and damage the human nervous system. Carbon monoxide also has an indirect effect on global climate change, and is a criteria pollutant.
Criteria pollutants: These are man-made pollutants such as carbon monoxide, nitrogen oxides, nonmethane volatile organic compounds (NMVOCs), and sulfur dioxide, that have indirect effects on global warming. They are primarily emitted as byproducts of fossil fuel and biomass combustion. Lead and particulates are also criteria pollutants. Although these pollutants only remain in the atmosphere for a short time, the chemical reactions that remove carbon monoxide, NMVOCs, and nitrogen oxides from the atmosphere promote the formation of ozone, which is harmful to people and animals at ground level.
Emissions: Emissions are gases and particles released into the air as byproducts of a natural or man-made process. One of these processes is the burning of fuels to create electricity and other forms of energy. The emissions from burning fossil fuels contribute significantly to global warming and poor air quality. A small set of emissions are responsible for the majority of human impacts on global climate change and health. These gases and particulates come from a variety of sources and can be categorized as greenhouse gas emissions (which affect climate change) and air quality emissions (which affect health as well as the environment). One of the primary benefits of clean energy is that it typically produces no emissions, significantly reducing climate change and health impacts.
Global climate change (GCC): Global climate change is a significant alteration from one climatic condition to another, beyond the usual alterations in various climates throughout the globe, as the result of human activities. The greatest of these is fossil fuel combustion, which traps greenhouse gases in the atmosphere that cause gradual changes in Earth’s temperatures over hundreds of years. The term “global warming” may also be used but refers more specifically to temperature, whereas global climate change encompasses the broader changes associated with elevated greenhouse gas levels, such as dryer deserts, increased numbers of hurricanes, and warmer oceans. Learn more about global climate change.
Global warming: See global climate change.
Greenhouse gases: While gases like carbon dioxide, methane, nitrous oxide, ozone, and water vapor naturally occur in earth’s atmosphere, human activities can artificially increase concentrations, notably through fossil fuel combustion to produce heat and electricity. These gases are dubbed greenhouse gases because they remain in the atmosphere and intensify the sun’s heat as it radiates to the earth, similar to a greenhouse’s glass walls heating and moisturizing the air inside of it. Greenhouse gases are the primary source of global climate change (GCC). Learn more about greenhouse gases and GCC.
Halocarbons: Halocarbons are entirely manmade chemicals, meaning that they are not found on their own in nature. When released into the atmosphere as greenhouse gases, they can significantly disrupt global climate patterns. There is a lower concentration of atmospheric halocarbons than other greenhouse gases, but the warming effects of halocarbons ranges from 3000 to 13,000 times that of carbon dioxide, and they remain in the atmosphere for hundreds of years. The most commonly known halocarbons are CFCs (chlorofluorocarbons), HCFCs (hydrochlorofluorocarbons), and HFCs (hydroflurocarbons). Their most common use is in refrigeration and air conditioning technologies but they are also used heavily in the electric system infrastructure.
Methane: Methane is a greenhouse gas. The primary natural source of methane is the decay of plants, but, like carbon dioxide, the amount of methane in the atmosphere has increased significantly due to human activities including fossil fuel production, decay in landfills, and the digestive processes of farm animals.
Nitrogen oxides (NOx): Nitrogen oxides are byproducts of nitrous oxide from fossil fuel combustion. They are called criteria pollutants (along with carbon monoxide, sulfur dioxide, nonmethane volatile organic compounds, lead, and particulates). They contribute to acid rain, smog, and respiratory problems, and have an indirect impact on global climate change.
Nitrous oxides (N2O): Nitrous oxides are greenhouse gases. The natural sources and cycles of nitrous oxides are not as well understood as those of carbon dioxide and methane, but their primary natural source appears to be bacterial breakdown of chemicals in soil. Human activities that increase nitrous oxide levels in the atmosphere (and the corresponding risk of climate change) include fossil fuel burning, use of nitrogen-based fertilizers in farming, and emissions from industrial processes.
Ozone (O3): Ozone is a unique emission because it is not directly produced by human sources. Instead, it is created as a result of chemical reactions between human-produced emissions and other gases in the atmosphere. Ozone is also unique because it is considered beneficial in some places and detrimental in others. When ozone is in the earth's upper atmosphere it is considered good because it protects the earth from the sun's radiation. But when ozone is created in the lower atmosphere, it creates smog which can cause respiratory problems and damage to plant and animal life. In the lower atmosphere, ozone is typically created when volatile organic compounds (VOCs) or nitrogen oxides react with other atmospheric gases.
Particulates: Particulates are criteria pollutants that include dust, dirt, soot, smoke and other miniscule solids released into the air and can affect heart and respiratory health. Particulates can be composed of many different chemicals. Their human sources vary but come largely from construction activities like road building. Particulates can also form when emissions from fossil fuels react with sunlight and water vapor to create solid particles in the air.
Smog: Smog is air pollution mainly consisting of ozone and nitrogen oxides, which creates a visible brownish haze (particularly in cities in the summer). Smog can cause breathing problems and greatly reduces visibility in the air. Power plants and vehicles are major causes of smog. Learn more about smog from the U.S. Environmental Protection Agency.
Sulfur dioxide (SO2): Sulfur dioxide is a criteria pollutant that contributes to respiratory problems and the creation of acid rain. Sulfur dioxide is created by burning fossil fuels with trace amounts of sulfur, like coal and oil. Smaller amounts can be created during industrial metal processing. The major source of sulfur dioxide is the use of fossil fuels in electricity production. Lesser sources include other energy production and transportation.
Sulfur hexafluoride (SF6): Sulfur hexafluoride is a manmade greenhouse gas that is currently released only in very small quantities due to electric power equipment gas leaks and losses. These small quantities can still make a large impact on the global climate; SF6 has a global warming potential almost 24,000 times greater than carbon dioxide over a 100-year time horizon.
Volatile Organic Compounds (VOCs): VOCs are found in a wide variety of consumer products from paints, solvents, and adhesives to carpeting, deodorants, and cleaning fluids. When released into the air, they react with other chemicals to create ozone in the lower atmosphere, impacting human health. They can also have direct air quality impacts inside buildings when off-gassed by indoor paints, carpeting, and adhesives. Nonmethane volatile organic compounds (NMVOCs) are a type of VOC.