Greenhouse gases (GHGs) are those gases that trap heat in the atmosphere; this allows an average temperature on Earth of approximately 15°C. Without this natural "greenhouse effect," the average ambient temperature would be about 33°C lower than it is now, making most life as we know it today impossible.
But since the Industrial Revolution, human activities have added significant amounts of greenhouse gases to the atmosphere, enhancing the natural greenhouse effect. This is causing an increase in the global average temperature, resulting in severe effects on the climate.
Some of these gases, such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), are emitted to the atmosphere from both natural and anthropogenic processes, while other greenhouse gases, such as chlorofluorocarbons (usually known as CFCs), stem exclusively from industrial activities.
The principal greenhouse gases released from human activities, particularly from fossil fuel combustion, are carbon dioxide, methane, and nitrous oxide.
GHG emissions, regardless of the gas concerned, are commonly reported in terms of equivalent emissions of carbon dioxide.
This measure is used to compare the ability of each GHG to trap heat (Global Warming Potential, or GWP) in the atmosphere relative to that of CO2, which is taken as a reference gas. The carbon dioxide equivalent for a gas is derived by multiplying the amount of gas emitted by its GWP.
Brief descriptions of the principal GHGs are given below:
Carbon dioxide (CO2) is a nontoxic and innocuous gas. The steady increase in CO2 concentration in the atmosphere that is of concern for its effects on climate change is mostly due to human activities.
It has been estimated that global atmospheric concentrations of CO2 in 2005 were 35% higher than the values observed before the Industrial Revolution.
The main source of this gas is the burning of fossil fuels, (of which electric power sources contribute between 17 and 40% of total CO2 emissions); other sources are forest and grass fires, and combustion processes in producing material for cement [48, 56–58].
Methane (CH4) remains in the atmosphere for 9 to 15 years and is 21 times more effective in trapping heat in the atmosphere than carbon dioxide. Like carbon dioxide, methane is emitted to the atmosphere from diverse natural and anthropogenic sources.
Natural sources include wetlands, termites, oceans, wildfires, etc., while anthropogenic sources are mainly combustion of fossil fuels, enteric fermentation, landfills, natural gas systems, fossil fuel production, rice cultivation, biomass burning, and waste handling. It is estimated that natural sources contribute approximately 37% of the total methane emitted into the air every year; therefore, anthropogenic sources are the principal sources of its release to the atmosphere.
Nitrous oxide (N2O) is a colorless gas with a slightly sweet odor and it is about 310 times more effective in trapping heat in the atmosphere than carbon dioxide.
As with carbon dioxide and methane, nitrous oxide is also emitted from natural and human-related sources, but contrary to the situation with the two other gases, natural sources of this gas contribute approximately 64% of the total inputs to the atmosphere.