Defining Greenhouse Gas
A greenhouse gas - abbreviated to GHG - is a gas in the Earth’s atmosphere that absorbs and emits infrared radiation (heat). Because of their radiative properties, greenhouse gases produce the greenhouse effect, which leads to trapped heat warming up the planet.
The main greenhouse gases are water vapour (H₂O), carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). We hear a lot of bad press when it comes to greenhouse gases because of the climate emergency but in correct quantities they are not all bad. In fact without greenhouse gases and the greenhouse effect, the Earth’s surface temperature would be around −18 °C, approximately 33°C colder than the actual average temperature that supports existing life.
Composition of Earth’s Atmosphere
Earth’s atmosphere consists mainly of nitrogen N₂ (78.08%), oxygen O₂ (20.95%) and Argon Ar (0.93%), however these are not greenhouse gases. Molecules with two of the same atoms (N₂ and O₂) and monoatomic gases (Ar) are almost completely unaffected by infrared radiation therefore do not contribute to the greenhouse effect.
Greenhouse gases exist in very small quantities so are known as “trace” gases and make up less than 1% of the atmosphere (excluding water vapor which accounts for 0.2% - 4%). Despite their minute presence they have a huge effect over many years.
Long-lived vs. short-lived greenhouse gases
Depending on the lifespan of the greenhouse gas a distinction is made between the long-lived and short-lived greenhouse gases. Long-lived greenhouse gases (LLGHGs) such as carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) live in the atmosphere over time scales of decades to centuries or longer. They are chemically stable and they have a long-term influence on the climate. The Intergovernmental Panel on Climate Change (IPCC) categorises the following as LLGHGs:
- Carbon dioxide (
CO₂) - Methane (
CH₄) - Nitrous oxide (
N₂O) - Chlorofluorocarbons (
CFCs) - Hydrofluorocarbons (
HCFCs,HFCs) - Trichloroethane (
CH₃CCl₃) - Carbon tetrachloride (
CCl₄) - Sulfur hexafluorid (
SF₆) - Tetrafluoromethane (
CF₄) - Hexafluoroethane (
C₂F₆)
Carbon dioxide - the gas primarily responsible for climate change - can act as a natural thermostat over millions of years: as temperatures rise, natural chemical weathering of rocks accelerates and absorbs more CO₂ thus weakening the greenhouse effect and decreasing the temperature. As temperatures decrease the chemical weathering process slows, taking less CO₂ which in turn intensifies the greenhouse effect and raises the temperature again. The long residence of CO₂ and other long-lived greenhouse gases indicates the importance of keeping the correct concentration in the atmosphere to avoid sever climate anomalies as it takes hundreds to thousands of years for the concentration to adjust naturally.
On the contrary, short-lived greenhouse gases (SLGHGs) like water vapor (H₂O), carbon monoxide (CO) and ozone (O₃) are chemically reactive and live in the atmosphere for time scales of days to years, making their concentrations highly variable. Water vapor covers by far the most mass of atmospheric greenhouse gases and its concentration is mostly a function of the temperature. If the temperature increases, the amount of water vapor in the atmosphere will increase within just a few weeks and vice versa - were the temperature to cool down, the amount of water vapor will decrease. Gaseous H₂O in the air is considered a feedback in the climate system: Warmer air has more water vapor, which through the greenhouse effect makes the system even warmer. Conversely colder air has less water vapor meaning a less powerful greenhouse effect which cools the system down.
Greenhouse gases produced by humans
Today humans produce three greenhouse gases mainly responsible for the climate crisis: CH₄ a product of livestock farming and landfills; N₂O from agriculture, fossil fuel combustion and industrial processes; and the main contributor, CO₂. CO₂ is generated in particular by the burning of fossil fuels that send large quantities of gases into the atmosphere. Unfortunately in the last 300 years our society has burned an increasing amount of coal, oil and gas for electricity, industry and transport, emitting more gases than the earth is naturally able to deal with.
The amounts and speed of these gases produced and released into the atmosphere by humans is too big and fast for the natural system to remove on human time scales causing a continuous temperature increase. Warmer temperatures cause more water to evaporate and, as stated above, this feedback loop will amplify the effects of the long-lived greenhouse gases and accelerate our climate crisis.
To truly slow and reverse the greenhouse effect we must remove these greenhouse gases from the atmosphere.