Greenhouse gases or GHGs refer to gases that trap infrared radiation when present in the atmosphere. Increases in the amount of and other greenhouse gases from human activities, like burning fossil fuels, contribute to global climate change. Greenhouse gases specifically let visible light through but absorb and re-emit radiation within the infrared range. This can be seen in figure 1. Other planets have greenhouse gases as well.
Approximate contribution of greenhouse gases:
- 36-72% water vapour
- 9-26% carbon dioxide
- 7-16% other trace gases.
It's important to note that water vapour is the largest contributor to the greenhouse effect, but is the one that is increasing.
Global Warming Potential
The global warming potential(GWP) of a gas is the relative capability for a gas to trap heat within the atmosphere. This measurement is done in comparison to an equal mass of carbon dioxide. Many restrictions and regulations placed on substances are due to high GWP values, as included in the Kyoto Protocol
Water vapour is merely water (H2O) in its gaseous form. It is abundant in the atmosphere, and the most common of the greenhouse gases. Water vapour is the most potent of the greenhouse gases due to the hydroxyl bonds in its chemical structure. Water vapour exists in the atmosphere in the form of moisture and clouds, and is a part of the water cycle as a precipitate creating rain. The GWP value for water vapour has yet to be calculated.
The most interesting attribute of water as a greenhouse gas is that it serves as a positive feedback mechanism for global warming. The warmer the world gets, the more water vapour there is in air, causing more warming. Concluding that water vapour itself doesn't drive climate change, however, amplifies the effects since more CO2 means more water vapour will be in the atmosphere.
Carbon Dioxide (CO2) is a naturally occurring gas, important to the carbon cycle for life and a byproduct of many forms of energy production. Carbon dioxide is primarily regulated by the consumption due to photosynthesis by plants to produce energy, and the respiration of animals in order to consume energy. Another major contribution of carbon dioxide into the atmosphere is as a byproduct of combustion. Forest fires and energy production both produce a substantial amount of carbon dioxide in the consumption of organic material. Due to its nature as the reference value for global warming potential, carbon dioxide has a GWP value of 1.
Methane (CH4) is a naturally occurring gas, making up 90% of natural gas. Although it is not as highly concentrated in the atmosphere like carbon dioxide, methane has a GWP that is 29 times larger, meaning it is still a very potent greenhouse gas. Methane levels have risen significantly in recent years—a rise 2.5 times greater than the pre-industrial era. Methane is regulated by natural processes in the atmosphere involving water vapour and oxygen, although human influence is capable of upsetting this regulation.
About 3.5 billion years ago, prior to photosynthesis utilizing plants, the concentrations of methane were 1000 times higher than they are today. The introduction of oxygen consumption in the carbon cycle due to photosynthesis led to a significant reduction in methane, which at the time was produced by some of the first bacteria through the consumption of hydrogen and carbon dioxide. Methane is produced naturally deep in the ground when organic plant and animal matter is compressed at high temperatures over thousands of years and is therefore considered a fossil fuel.
Nitrous Oxide (N2O) commonly known as laughing gas due to it's use as an anesthetic in surgery, is a naturally occurring gas often used in rocketry and racing to improve power output as an oxidizer. While in much lower concentrations than both carbon dioxide and methane, nitrous oxide has a GWP 298 times greater than that of carbon dioxide. Although it is a significant contributor to the greenhouse effect, nitrous oxide is also the primary regulator of ozone in the stratosphere. Nitrous oxide reacts with oxygen to produce nitric oxide, which in turn reacts with ozone maintaining concentration balances. Nitrous oxide is also commonly used as an aerosol propellant for canisters filled with substances like whipped cream and as an inert gas when filling potato chips and other snacks to protect them in transit from compression. Nitrous oxide is produced naturally in the environment in soil by various organic chemical reactions.
Ozone (O3) is an allotrope (alternative form) of oxygen found in the upper atmosphere, and is vastly more unstable than diatomic oxygen found commonly at sea level. Ozone primarily exists in the upper atmosphere as it is mainly produced by the absorption of ultraviolet rays from the sun and breaks down in the lower atmosphere due to instability, this absorption of UV radiation is itself valuable as it protects the surface from the harmful rays. Another common source of ozone is from electrical discharge in the atmosphere due to lightning. Although the GWP value for ozone is between 918-1022—one of the highest values recorded—it is a short lived chemical species due to instability and therefore when considered over a 20 year horizon the GWP is valued at only 62-69. Ozone concentrations measurable at ground level peak substantially higher in present day than in pre-industrial times, even in areas completely removed from industrial activity. Long term exposure to ground level ozone has shown to be dangerous, harming lung function, and has been linked to pre-mature death due to various respiratory diseases and heart attack.
Chlorofluorocarbon (CFCs) is an organic compound containing carbon, fluorine, and chlorine which is produced as a volatile derivative of fossil fuels like methane. CFCs have been used as refrigerants, propellants, and solvents in production, although the production of CFCs has been phased out in recent years due to the Montreal Protocol. CFC production has been phased out and replaced by hydrocarbons and carbon dioxide due to its contribution to the depletion of the ozone, and effect as a greenhouse gas.
For Further Reading
For further information please see the related pages below:
- PhET Simulations, Molecules and Light [Online], Available: https://phet.colorado.edu/en/simulation/molecules-and-light
- Prof. David Archer has graciously allowed the use of this and other videos in a private communication with Jason Donev.