What are Greenhouse Gases?

Greenhouse gases (GHG) are gases that can trap heat from the sun within our atmosphere and warm the earth's surface. Too little of these gases would result in a world covered in ice; just the right amount is what makes the earth habitable. Human activity has caused GHG concentrations to increase, which has resulted in global warming. It is important to remember that some GHGs can trap heat more efficiently than others and thus can have a greater impact on global warming.

The major types of GHGs include the following:

1. Carbon Dioxide (CO2)

2. Methane (CH4)

3. Nitrous Oxide (N2O)

4. Fluorinated Gases (FGs)

Percentages of GHGs based on GHG type. (Source: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2022.)

The contribution of each type to total GHG emissions is shown for 2022 in the figure to the right (source). Though CO2 has the largest contribution, it is actually the least potent. CH4 and N2O are about 25 and 300 times more efficient, respectively, at trapping heat than CO2. FGs are the most efficient at 12,200 - 22,800 times the efficiency of CO2.

Another factor to consider when looking at the overall warming caused by a GHG is how long it remains in the atmosphere. All GHGs will eventually break down, but the longer this takes, the higher their contribution to global warming. The lifetime of CO2 is between 300 and 1,000 years, while CH4 and N2O break down after about 12 and 114 years, respectively. FGs have much longer lifetimes that can range anywhere from 270 to 50,000 years.

After looking at the numbers above you may be thinking that reducing FGs needs to be a top priority due to their high potency and long lifetimes. The concern is that even though the concentrations of CO2, CH4, and N2O, have decreased over the last several decades, the concentration of FGs has increased significantly. This is because the vast majority of FGs come from substances used to replace refrigerants beginning in the 1990s; previous refrigerants were the primary cause for the hole in the ozone layer. This is a classic example of fixing one problem while (unintentionally) causing another potentially worse problem.

Sources of GHGs

1. CO2: transportation (e.g. highway/passenger vehicles, air travel, marine transportation, rail); electricity sources from fossil fuels (e.g., coal and natural gas extraction and use)

2. CH4: digestive processes of domestic livestock; storage of animal manure; production, processing, storage, transmission, and distribution of natural gas; production, refinement, transportation, and storage of crude oil; landfills; wastewater treatment

3. N2O: production & application of synthetic and organic fertilizers & other cropping practices; management of manure; production & application of synthetic nitrogen fertilizers to urban soils; fuel combustion in motor vehicles; production of synthetic fibers (e.g. nylon); domestic wastewater treatment

4. FGs: refrigerants in vehicle and home air conditioning systems; aerosol propellants, foam blowing agents, solvents, and fire retardants; semiconductor production; byproduct of aluminum production; insulating gas in electrical transmission equipment (e.g. circuit breakers)

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Helpful Hints

Some ideas for reducing your footprint in terms of the four types of GHGs (some of which will address more than one GHG) are listed below:

• CO2: get a home energy audit and implement suggested improvements; improve building insulation; convert to fuel-efficient vehicles; install more energy-efficient electric appliances; unplug & turn off; reduce required travel distances; produce/support renewable energy (e.g. solar and wind)

• CH4: convert appliances to electric; identify and fix any leaky pipes; reduce consumption and waste; eat less meat

• N2O: convert to fuel-efficient vehicles; reduce required travel distances; eat less meat; produce/support renewable energy (e.g. solar and wind)

• FGs: convert to refrigerant-efficient devices/vehicles and/or those that use refrigerant substitutes