Greenhouse gas

Greenhouse gas is a term used to describe atmospheric gases that absorb and trap heat emitted by a planet, thereby increasing the planet's surface temperature through what is known as the greenhouse effect. Greenhouse gases include carbon dioxide, methane, nitrous oxide, ozone, and fluorinated gases. Water vapor also contributes to the greenhouse effect, although it is often considered separately from other greenhouse gases due to its unique properties.

Most greenhouse gases are a natural part of Earth's atmosphere, and they play a key role in making the planet's surface warm enough to be inhabitable. However, human activity—especially the burning of fossil fuels—has greatly increased greenhouse gas emissions. This increase is considered the main driver of global warming. As such, scientists have identified major reductions in greenhouse gas emissions as crucial to avoiding existential threats from anthropogenic climate change.

Overview

While greenhouse gases can exist on any planet or other body with an atmosphere, they are most commonly discussed in the context of Earth and its global climate. Earth's atmosphere is made up of various gases held in place by gravity. Some of these gases (including the three most common: nitrogen, oxygen, and argon) have a simple molecular structure that does not reflect thermal radiation. However, other gases do absorb and reflect thermal radiation due to their more complex molecular structure. These include water vapor, carbon dioxide, methane, nitrous oxide, ozone, various chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs), and others.

The atmospheric gases that absorb thermal radiation get the name "greenhouse gases" from their role in the so-called greenhouse effect. When energy from the Sun reaches Earth's surface, it is absorbed and partially radiated back outward as heat. While some of this heat travels back out into space, some is reflected by greenhouse gases back down toward the planet's surface. The result of this heat-trapping effect is a warmer average planetary surface temperature. Scientists have estimated that if there were no greenhouse gases, Earth's surface temperature would be below freezing, making the planet inhospitable to most forms of life.

The natural levels of greenhouse gases in Earth's atmosphere have varied throughout the planet's geologic history. Over many millions of years in the distant past, concentrations of carbon dioxide, in particular, are believed to have shifted significantly due to many complex factors affecting the carbon cycle, and in turn impacted the global climate. The concentration of a greenhouse gas is the amount of that particular gas in the air, measured in parts per million, billion, or trillion. One part per million is roughly equivalent to one drop of water that has been diluted into about thirteen gallons of liquid. Individual molecules of each greenhouse gas remain in the atmosphere for different amounts of time, which can range from a few years to thousands of years. The gases remain in the atmosphere long enough to become mixed, meaning that the amount that is measured in the atmosphere is roughly the same all over the world, regardless of where the original emissions came from.

While most greenhouse gases are a natural part of Earth's atmosphere, human activity—especially since the Industrial Revolution—is responsible for drastic increases in emissions of these gases. The most significant source of human-generated greenhouse gas emissions is the burning of fossil fuels. Deforestation and certain industrial and agricultural processes also emit greenhouse gases. These emissions have accelerated the greenhouse effect, driving global climate change.

Nearly 80 percent of human-created greenhouse gas emissions are due to carbon dioxide. Carbon dioxide enters the atmosphere through the burning of fossil fuels, such as coal, natural gas, and oil, as well as burning solid waste, trees, and other biological material. It can also be produced due to certain chemical reactions such as those that occur during the manufacturing of cement. Carbon dioxide can be removed from the atmosphere through a natural process called sequestration, in which plants absorb it as part of their biological carbon cycle.

Methane accounts for approximately 11 percent of greenhouse gas emissions. It is primarily emitted during the production and transport of fossil fuels like coal, natural gas and oil. Methane is also produced by livestock, agricultural practices, land use, and the decay of organic matter at landfills.

About 7 percent of greenhouse gas emissions are due to nitrous oxide. This compound is emitted during agricultural, land use, and industrial activities. In addition, combustion of fossil fuels and solid waste as well as wastewater treatment cause nitrous oxide emissions.

Fluorinated gases like hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride account for another 3 percent of greenhouse gas emissions. These powerful, synthetic greenhouse gases are emitted from household, commercial, and industrial sources. While these greenhouse gases are typically emitted in smaller quantities than the others, they are extremely potent in comparison. They are sometimes referred to as high global warming potential gases because they trap substantially more heat in the atmosphere per their mass compared to carbon dioxide. Scientists use carbon dioxide as a baseline for indexing the global warming potential of various gases because carbon dioxide is considered to have the most significant impact on climate change.

Researchers also use various other measurements and methods to understand concentrations of greenhouse gases and their effects. One notable project is the Annual Greenhouse Gas Index (AGGI) developed by the US National Oceanic and Atmospheric Administration (NOAA). The AGGI tracks the ratio of heat being added to the atmosphere by greenhouse gas emissions in a given year compared to the baseline year of 1990 (chosen to align with the baseline year of the landmark Kyoto Protocol). From 1750 to 1990, a period of 240 years, the AGGI steadily increased from 0 to 1. This rise continued to accelerate almost exponentially over the following decades. For instance, in 2021, the AGGI hit 1.49, which indicates a 49 percent increase in the warming influence of greenhouse gases compared to 1990.

Bibliography

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Lindsey, Rebecca. “Climate Change: Annual Greenhouse Gas Index.” Climate.gov, 17 June 2022, www.climate.gov/news-features/understanding-climate/climate-change-annual-greenhouse-gas-index. Accessed 8 Aug. 2022.

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