Greenhouse gases (GHG) can be described as substances with considerable potential to trap heat in the atmosphere – just like a greenhouse. GHGs are naturally present in the atmosphere, but their concentration has increased markedly since the industrial revolution. Carbon dioxide is the best-known greenhouse gas and its concentration has increased from less than 300 ppm to more than 420 ppm in less than three centuries.
The combustion of fossil fuels is the main source of GHG emissions, but they are also produced by many other human activities. The Kyoto Protocol identifies six main greenhouse gases:
- Carbon dioxide (CO2)
- Methane (CH4)
- Nitrous oxide (N2O)
- Sulfur hexafluoride (SF6)
- Hydrofluorocarbons or HFCs
- Perfluorocarbons or PFCs
Here we will discuss these six greenhouse gases and their main sources. These gases have high priority due to their heat retention potential and their correlation with human activity. However, there are also naturally occurring GHGs that are not on the Kyoto list, including water vapor and ozone.
According to Architecture 2030 , building operations are responsible for 27% of global GHG emissions. When considering the activity and materials of the construction industry, the total share of emissions for the built environment rises to 47%. Building owners can significantly reduce their environmental footprint with energy efficiency measures and renewable sources .
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Carbon Dioxide (CO2)
Carbon dioxide or CO2 is the most well-known greenhouse gas and makes up 79% of U.S. emissions, according to the Environmental Protection Agency . The combustion of fossil fuels is the main source of CO2, but the carbon footprint is not the same for all fuels:
- Coal = 95.74 kg CO2 per million BTU
- Distilled Fuel Oil = 74.14 kg CO2 per million BTU
- Kerosene = 73.19 kg CO2 per million BTU
- Finished gasoline = 67.38 kg CO2 per million BTU
- Propane = 62.88 kg CO2 per million BTU
- Natural gas = 52.91 kg CO2 per million BTU
In the USA, the transport sector is responsible for 33% of CO2 emissions, followed by the electricity industry with a share of 31%. Industrial activity represents 16%, the commercial and residential sectors account for 12% (excluding electricity) and other sources account for the remaining 8%.
Methane (CH4)
Methane can be considered the 2nd most important greenhouse gas, representing 11% of GHG emissions in the USA and surpassed only by carbon dioxide. Methane is 25 times more powerful than CO2 in terms of global warming potential, and its main sources are:
- Extraction and transportation of fossil fuels
- Agricultural activity, especially livestock
- Organic waste
According to data from the US EPA, 32% of methane emissions are released by natural gas and petroleum systems. 27% comes from the metabolism of cattle and other ruminants, while 17% is released into landfills. Manure management and coal mining account for 9% and 6% respectively, and the remaining 9% comes from other sources.
Methane is a naturally occurring greenhouse gas, but emissions have risen sharply due to industrial activity, fossil fuel extraction, agriculture and waste management. Currently, it is estimated that 50-65% of methane comes from human activity.
Nitrous Oxide (N2O)
Nitrous oxide is 298 times more potent than CO2 and is responsible for 7% of GHG emissions in the US. Agricultural land management is by far the main source of N2O, representing 74% of total emissions from human activity. The remaining 26% comes from multiple sources:
- Wastewater treatment = 6%
- Stationary combustion = 5%
- Chemical production and other product uses = 5%
- Manure management = 5%
- Transport = 4%
- All other sources = 1%
Like carbon dioxide and methane, nitrous oxide is also a naturally occurring greenhouse gas. For example, bacteria living in soil constantly release gas as part of their metabolism. However, N2O emissions have increased significantly due to human activity.
Fluorinated gases: SF6, HFCs and PFCs
F-gases account for just 3% of U.S. greenhouse gas emissions, but they are extremely potent.
- SF6 is 22,800 times more powerful than CO2
- HFCs are up to 14,800 times more powerful than CO2
- PFCs are up to 12,200 times more powerful than CO2
Carbon dioxide, methane and nitrous oxide have natural and artificial sources. On the other hand, fluorinated gases are produced almost exclusively by human activity.
Sulfur hexafluoride (SF6) is mainly used in electrical networks as it can reliably extinguish arcs in high voltage substations and is an excellent insulator. Hydrofluorocarbons (HFC) are widely used in semiconductor manufacturing and refrigeration applications, while perfluorocarbons (PFC) are mainly generated by aluminum production.
