What are Greenhouse Gases (GHGs)?
Greenhouse gases (GHGs) are compound gases that trap heat or longwave radiation in the atmosphere. Their presence in the atmosphere makes the Earth’s surface warmer. Sunlight or shortwave radiation easily passes through these gases and the atmosphere. This radiation is absorbed by the surface of the earth and released as heat or longwave radiation. The molecular structure of GHGs allows them to absorb the heat released or trap them in the atmosphere and re-emit them back to the earth. This heat trapping phenomenon is known as the greenhouse effect. The accumulation of GHGs since the industrial revolution has accelerated this greenhouse effect, causing global warming and climate change.
The principal GHGs, also known as heat trapping gases, are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and the fluorinated gases. CO2, the largest contributor among these, composes 64.3% of GHGs. It enters the atmosphere through the burning of fossil fuels, solid waste, trees and wood products, and certain chemical reactions. It is removed from and added to the atmosphere naturally by plants and animals as part of the biogenic carbon cycle. Methane is principally released to the atmosphere by the production and consumption of coal, natural gas and oil, livestock and agricultural practices, and the decay of organic waste in municipal solid waste landfills. N2O is emitted during agricultural and industrial activities, and during combustion of fossil fuels and solid waste. Fluorinated gases are synthetic gases emitted from industrial processes and in the form of fugitive refrigerants. They include hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, nitrogen trifluoride, and other fully fluorinated gases. They are sometimes known as stratospheric ozone-depleting substances or high global warming potential (GWP) gases because they are hundreds to thousands of times more powerful than CO2 and can be in the atmosphere for 1,000 years.
GHGs can be classified into two different types: forcing GHGs and feedback GHGs. Forcing GHGs are the four above-mentioned: CO2, CH4, N2O, and the fluorinated gases. They take many years to leave the atmosphere and do not react to changes in temperature or air pressure, so they are not easily removed. The principal feedback GHG is water vapor, which can remain in the atmosphere for only a few days and is a highly active component of the climate system. Water vapor responds rapidly to changes in atmospheric conditions and acts as a feedback effect to climate-forcing GHGs by circulating the greenhouse effect or by accentuating their warming effect.
The GHGs can be evaluated in three ways: how much, how long and how powerful. “How much”, or the concentration and abundance of GHGs, is important because some GHGs are produced in greater amounts than others. In popular discussions about global warming and GHGs, CO2 gets more focus because it is the most abundant GHG in the atmosphere and has been utilized as the common unit when describing the GWP of other GHGs.
Some gases stay a long time in the atmosphere; the duration (“how long”) or residence in the atmosphere depends on how well-mixed into the atmosphere they become. When a gas is well-mixed, the amount measured in the atmosphere is the same around the world.
“How powerful" refers to a gas’ GWP or impact on average global temperatures. Some gases are more effective at absorbing energy or trapping heat than others. For instance, one pound of CH4 traps up to 36 times as much heat as one pound of CO2.
Human activities are a major source of GHGs. The burning of fossil fuels, deforestation, intensive livestock farming, the use of synthetic fertilizers, and industrial processes all contribute. Since the Industrial Revolution of the 18th century, there has been a sharp and dangerous increase in GHGs in the atmosphere. This can be attributed in large part to industrial processes, electricity, heating of homes, and driving.
According to the Environmental Protection Agency (EPA), GHG emissions in the United States can be traced from different sectors. Electricity production generated 25% of US GHG emissions in 2019. About 62% of US electricity comes from burning fossil fuels; mostly coal, oil, and natural gas. Meanwhile, combustible fossil fuels accounted for about 67% of global electricity generation in 2019.
Transportation accounted for 29% of 2019 US GHG emissions. This is primarily from burning fossil fuel for our cars, trucks, ships, trains, and planes. Over 90% of fuel burned is petroleum-based, including gasoline and diesel. The largest sources of transportation-related GHG emissions include passenger cars and light-duty trucks, including sport utility vehicles, pickup trucks, and minivans.
Industry accounted for 23% of 2019 US GHG emissions. There are two classes of industrial emissions: direct and indirect emissions. Direct emissions come from burning fossil fuels for power or heat, from chemical reactions, and leaks from industrial processes. Indirect emissions come from burning fossil fuels at a power plant to make electricity used for industrial facilities or to power machinery.
Agriculture accounted for 10% of 2019 US GHG emissions. Emissions in this sector come from agricultural activities such as the cultivation of crops and livestock. Livestock produce methane as part of their digestion. The process is called enteric fermentation and represents almost 30% of the emissions from agriculture. Manure from livestock also contributes to methane and nitrous oxide emissions. Manure management accounts for about 9.54% of total GHG emissions in this sector in the US.
The commercial and residential sector accounts for 13% of the US 2019 GHG emissions. These include all homes and commercial businesses (excluding industrial and agricultural activities). Emissions in this sector come from fossil fuel combustion for heating and cooking needs, management of waste and wastewater, and leaks from refrigerants in homes and businesses. Emissions from natural gas consumption used for cooking and heating represent about 80% of emissions in this sector. Biogenic GHGs, or organic GHGs, are also generated at landfills and wastewater treatment facilities from the natural decomposition of organic waste.
The Earth’s average sea surface and land surface temperatures largely depend on the balance between energy entering and leaving the atmosphere. Imbalances can cause changes in the greenhouse effect, which in turn causes climate change. Prior to the Industrial Revolution, the climate varied naturally. However, average temperature increases indicating changes in the climate since the Industrial Revolution cannot be explained by natural causes. Rising temperatures, changing snow and rainfall patterns, and more extreme weather events have been linked to the unnaturally high levels of CO2 and other GHGs in the atmosphere.
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