We often hear about greenhouse gas (GHG) emissions, but do you know how they are calculated? Do you know the impact of the calculations on emissions estimates?
The use of emission factors
Emissions are calculated using emission factors. An emission factor is a ratio that refers to the average emission rate of a given source, activity, or process. For example, there is an emission factor that determines how many GHGs are emitted when a liter of gasoline is burned. There are also emission factors that determine the GHG emissions associated with the landfilling of waste. These emission factors are calculated by experts, revised periodically, standardized, and recognized internationally to allow comparisons between countries and periods.
An emission factor reflects the emissions related to the different greenhouse gases for each activity. The main gases emitted by human activity are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and fluorinated gases. The main source of nitrous oxide is agriculture, while methane comes from coal and natural gas production, decomposition of waste materials, rice paddies, cattle farming, and thawing of permafrost, among others.
Determining the global warming power of GHGs
The different greenhouse gases each have their own global warming power (GWP). They have a different capacity to absorb heat (radiative efficiency) and a different lifetime in the atmosphere.
Each of these gases is assigned a value based on these parameters. The ton of carbon dioxide (CO2) is the reference unit, i.e., 1. The other gases are evaluated in relation to this unit. Nitrous oxide, for example, has a GWP of 265 because one ton of nitrous oxide is 265 times more potent than one ton of CO2 in terms of global warming.
There are several ways to calculate the GWP of GHGs. One can calculate how much energy GHGs absorb over 20 years or over 100 years. Currently, the impact of GHGs is calculated over
100 years, i.e., effects beyond this time are not considered. This time frame was chosen based on CO2.
The impact of a gas with a lifetime longer than 100 years will therefore be underestimated. As for short-lived gases, their weight in the calculations is also reduced by this choice. Thus, the GWP of methane over 100 years is 28 (i.e., a GWP 28 times that of CO2), while over 20 years it is estimated to be between 84 and 87. If we calculate over 100 years, the effects of methane are somehow spread out over a century, whereas in fact, they are very intense for about 15 years. Beyond that, the gas is degraded into CO2 by chemical reactions in the atmosphere.
Therefore, at COP26, nearly 90 countries, including Canada, committed to reducing methane emissions by 30% by 2030 compared to their 2020 levels when they signed the Glasgow Pact (2021). The reason for this decision is related to the predicted explosion of global methane emissions (agriculture, waste, permafrost thawing, etc.) by 2050, taking us further away from the “net zero” target by 2050.
Conclusion
Should we change the time frame used to calculate the GWP of methane? Choosing a GWP calculated on a twenty-year basis could give a clearer picture of the real impacts of methane emissions in the short (2030) and medium (2050) term. This would perhaps lead us to question our consumption of meat, which is the basis of demand for livestock production and the primary source of methane emissions today.