Greenhouse gases, or GHGs, are compound gases that can absorb infrared radiation, trapping heat 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. GHGs are electrically charged molecules. The molecular structure of GHGs allows them to absorb the heat released or trap it in the atmosphere and re-emit it back to the earth. This process is known as the greenhouse effect. The accumulation of GHGs since the industrial revolution has accelerated this greenhouse effect, contributing to 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. Carbon dioxide enters the atmosphere through the burning of fossil fuels, solid waste, biomass and wood products, and certain industrial processes, such as the production of cement. It is removed from the atmosphere or sequestered naturally by plants as part of the biological carbon cycle. CO2 is also absorbed by the ocean. Methane is released into the atmosphere by the production and transport 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 the combustion of fossil fuels and solid waste. Fluorinated gases are synthetic gases from various household, commercial, and industrial processes. They include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF6), and nitrogen trifluoride (NF3). They are sometimes known as stratospheric ozone-depleting substances or high Global Warming Potential (GWP) gases because they can be 1,000 times more powerful than CO2 and can be in the atmosphere for thousands of years.
GHGs can be classified into two different types: forcing GHGs and feedback GHGs. Forcing GHGs are the gases mentioned above: CO2, CH4, N2O, and fluorinated gases. They take many years to leave the atmosphere and don’t react to changes in temperature or air pressure, so they are not easily removed. Feedback GHGs are water vapor. They last in the atmosphere for only a few days and are highly active components of the climate system, meaning they respond rapidly to changes in conditions. They act as feedback to GHGs by circulating the greenhouse effect or by accentuating the warming effect of forcing GHGs. Each gas’s effect on climate change depends on three main factors:
1. How much is in the atmosphere?
This refers to the concentration, or abundance, of each gas in the atmosphere. CO2 is the most prevalent GHG, accounting for roughly 75% of greenhouse gases.
2. How long do they stay in the atmosphere?
CH4, N20, and CO2 have a relatively short lifespan in the atmosphere, ranging from 12-200 years. As we begin to reduce the amount of these gases emitted into the atmosphere, the concentration of these gases will decrease as those already emitted will break down over a few generations.
Alternatively, HFCs and PFCs can persist in the atmosphere for hundreds to thousands of years. These gases make up a small percentage of GHG emissions.
3. How powerful are the gases?
GHGs vary in their GWP. GWP is a value expressed in CO2 equivalent (CO2e). For example, CH4 has a GWP of 25, meaning that one molecule of CH4 is equivalent to 25 molecules of CO2. Some HFCs have GWP values of >10,000. That said, the figure above citing CO2 as roughly 75% of GHGs in the atmosphere considers the GWP of each gas, meaning CO2 is the most relevant GHG in our atmosphere, and the biggest contributor to anthropogenic (human-caused) climate change.
Human activities are the main source of GHG emissions. The burning of fossil fuel, 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 increase in GHG concentrations in the atmosphere. This can be attributed in large part to the burning of fossil fuels for electricity generation, heating of homes, industrial processes, and transportation.
GHG emissions are tracked by sector.
According to the Environmental Protection Agency (EPA), electricity production generated 25%, of U.S. GHG emissions in 2020. Approximately 60% % of U.S. electricity comes from burning fossil fuels, mostly coal and natural gas. Transportation accounted for 27% of 2020 U.S. GHG emissions, primarily from burning fossil fuels for 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 24% of 2020 U.S. 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 11% of 2020 U.S. GHG emissions. Emissions in this sector come from agricultural activities such as the cultivation of crops like rice and livestock. Livestock, primarily cows, produce methane as part of their digestion. The process is called enteric fermentation and represents almost 40% of the emissions from agriculture. Manure from livestock also contributes to methane and nitrous oxide emissions. Manure management accounts for about 13% of total GHG emissions in this sector in the U.S.
The commercial and residential sectors accounts for 13% of U.S. GHG emissions in 2020.
This includes all homes and commercial businesses (excluding industrial and agricultural activities). Emissions in this sector come primarily from fossil fuel combustion for heating and cooking. Other sources from this sector include waste and wastewater management, and leaks of refrigerants (HFCs) in homes and businesses. Emissions from natural gas consumption used for heating and cooking represent about 79% of emissions in this sector. Organic waste sent to landfills emits methane. Wastewater treatment emits methane and nitrous oxide. Refrigeration leaks emit fluorinated gases.
The Earth’s temperature depends on the balance between energy entering and leaving the atmosphere. Without GHGs the climate on earth would be much colder and less suitable for life as we know it. However, changes in the concentration of GHGs in the atmosphere can impact the greenhouse effect, which in turn causes climate change. Anthropogenic GHG emissions, primarily from the burning of fossil fuels, are rapidly increasing the concentration of GHGs in the atmosphere. Rising temperatures, changing snow and rainfall patterns, and more extreme climate events have been linked to the unnatural levels of carbon dioxide and other GHGs in the atmosphere.
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