Greenhouse Gases (GHGs) are molecules which contribute to the greenhouse effect in our atmosphere. When the sun’s light hits the earth, it would normally reflect back into space; however GHGs are able to absorb some of this light and ‘trap’ extra solar energy in Earth’s atmosphere (1,2). Increased emissions of GHGs, such as carbon dioxide from the burning of fossil fuels, increases the trapping of heat in the atmosphere, driving the process of global warming (2). The most potent GHGs are carbon dioxide, methane, nitrous oxide and fluorinated gases (3). Each of these gases are emitted from multiple sources, and have a different impact on global temperatures (3). In Canada, the largest producers of GHGs are the transportation and the oil and gas industries, accounting for, respectively, 24.3% and 27.3% of total emissions in 2018 (Figure 1) (4).
Accounting for more than 75% of GHG emissions, carbon dioxide (CO2) is the largest contributor to anthropogenic climate change (3,5). CO2 is released into the atmosphere in several ways, including the burning of fossil fuels, setting of concrete, and deforestation, among others (3). Naturally, CO2 plays an important role in Earth’s carbon cycle (2); however, problems arise when forms of concentrated carbon, such as fossil fuels, are burned and released in large quantities into the air, causing unprecedented levels of CO2 in the atmosphere (2,4). Moreover, destruction of the natural environment (e.g. forest fires, logging) increases atmospheric CO2. Trees contain lots of carbon in many forms, and when they burn the carbon is released as CO2 (6). Forests further act as 'carbon sinks', taking in CO2 from the atmosphere to store as energy in the form of sugars. Deforestation and fires, in addition to releasing more CO2 into the air, also decrease the planet’s ability to store CO2 (7). As a result of human activity, the level of CO2 in the atmosphere has skyrocketed to over 400 parts per million (ppm), 100 ppm greater than the levels before the Industrial Revolution, and is continuing to rise (8).
Because CO2 is the most abundant GHG in our atmosphere, scientists use it as relative measurement for the warming effect of all other major GHGs. The global warming potential (GWP) is a measure of the ability of a gas to trap heat in the atmosphere, as compared to CO2 (which has a GWP of 1) (9). This measurement allows us to compare the effects of different GHGs and demonstrates that even GHGs released in smaller quantities have quite significant effects.
With a GWP of 298, nitrous oxide is a strong GHG – almost 300 times more effective at trapping heat in the atmosphere than CO2 (10). In addition to its effects as a GHG, nitrous oxide in the atmosphere may also contribute to destruction of the ozone layer, a layer of molecules high in the atmosphere that protects us from harmful UV radiation (11).
Nitrous oxide is produced through processes such as fossil fuel burning and production of materials such as nylon and explosives, but the main source is agriculture (12,8). Nitrous oxide in agriculture comes from the nitrogen used in commercial and consumer fertilizers (13). When these fertilizers are added to soil, around half of the nitrogen is taken up by the crops, helping them grow, while the rest is used by microbes in the soil and converted into nitrous oxide (14). With an ever-rising global population, there is an increased demand for fertilizers in order to meet food supply requirements. Since many fertilizers produce nitrous oxide, increased agriculture using current methods will increase GHG emissions and encourage ozone depletion (12,13,14).
Accounting for 15% of global GHG emissions, methane has a GWP of 36, meaning it is 36 times better at trapping heat than CO2 (9,13). In Canada, methane is mainly produced through the oil and gas sector, with 98% of our methane production originating from the ventilation of storage tanks (15). In addition, studies have shown that methane production at many natural gas production sites may be extremely under-reported, meaning its overall impact on the climate crisis may be underestimated as well (15). Methane can also be linked to the agriculture industry, specifically livestock. For example, one estimate suggests that, in one year, a single cow can create the same amount of methane as the emissions from a medium-sized car driving 20,000 kilometers (16).
Global warming will also indirectly cause large amounts of methane to be released when permafrost soils begin to thaw (16). As the ground heats up, huge amounts of CO2 and methane will be generated from carbon stores in the ground, by microbes that had been previously frozen. The freed GHGs will be released into the atmosphere, further worsening the effects of warming (17).
While ozone-depleting chlorofluorocarbons have been almost completely phased out in Canada under the Montreal Protocol, their replacements - fluorinated carbons – are still a strong GHG in our atmosphere (18). Fluorinated gases are synthetic compounds which are used in many household and industrial applications, such as air conditioning, production of aluminum and magnesium, and manufacturing of products such as electronics, appliances, and carpet (5).
These compounds have varying GWPs, depending on the specific chemical - as high as 23,500 in the case of sulfur hexafluoride, which is one of the most potent GHGs known (5,18). Due to the severe impacts of fluorinated gases, all 197 member nations of the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer agreed to the Kigali Amendment in 2016, adding chlorofuorocarbons to the controlled substances list (18). Each nation also agreed to gradually reduce production of a less potent form of these gasses - hydrofluorocarbons - between 2019 and 2024 (19).
Figure 1: Greenhouse gas emissions by economic sector, Canada, 1990 to 2017.