Global warming potential is calculated from the sum of CO2-equivalents from N2O and CH4 emissions. Why is carbon dioxide emission NOT included in global warming potential?
GWP (kg CO2 equivalent ha ) = CH4 (kg CH4 ha-1 ) X 28 + N2O (kg N2O ha ) X 265.
CH4 emission is greater 28 times and N2O is 265 times greater than CO2 emission in greenhouse potential, nevertheless total CO2 emission is much greater than CH4 or N2O.
The reason carbon dioxide (CO2) emissions are not included in the global warming potential (GWP) calculation is because CO2 is used as the reference gas with a GWP of 1. GWP is a metric used to compare the warming potential of different greenhouse gases over a specific time period, typically 100 years. By convention, CO2 is assigned a GWP of 1 for this calculation, and other greenhouse gases are assessed in relation to CO2.
The reason CO2 is chosen as the reference gas with a GWP of 1 is because it is the most abundant and persistent greenhouse gas in the atmosphere. CO2 is responsible for the majority of the long-term climate change and its concentration has increased significantly due to human activities, particularly the burning of fossil fuels. As a result, the GWP of CO2 is set to 1, and other greenhouse gases are assessed relative to its warming potential over a specified time horizon.
Methane (CH4) and nitrous oxide (N2O) have higher GWP values because, while they are less abundant in the atmosphere, they are much more effective at trapping heat on a molecule-for-molecule basis. This is why CH4 has a GWP of 28-36 times that of CO2 over a 100-year period, and N2O has a GWP of about 265-298 times that of CO2 over the same time frame.
Even though CO2 emissions are much greater in total quantity, the GWP calculation focuses on the relative warming potential of different gases, considering their different abilities to trap heat and their lifetimes in the atmosphere. This approach helps provide a more comprehensive understanding of the different contributions of various greenhouse gases to global warming and allows for more accurate comparisons between different emission sources. It's important to note that this GWP calculation is just one tool used in assessing the climate impact of different emissions, and it's often used in conjunction with other metrics and considerations in climate policy and research.
If you want to understand the full radiative impact of greenhouse gas exchanges from a system, I think you *should* include CO2 fluxes. There might be scientific reasons why someone might not do that (e.g., they know that the net CO2 flux from their system is zero) or perhaps there are logistical reasons (e.g., they didn’t measure CO2 fluxes). I usually work in wetlands, where there is often net carbon sequestration (that is, net CO2 uptake) in the soils and that CO2 uptake offsets some of the radiative effect of CH4 and N2O emissions. So, in wetlands, not including CO2 would cause you to overestimate the radiative impact of the system.
I don’t think that any of what I just wrote conflicts with the good information provided by Anas Ghawanmeh.
One last point I want to make is about your equation. While it is common to use an equation like yours to calculate “global warming potentials,” what you are calculating would be better described as the CO2-equivalent greenhouse gas flux. The term global warming potential has a specific meaning as a metric that expresses the relative radiative impact of a greenhouse gas (compared to CO2) over a specific time horizon; the 28 and 265 values in your equation are the global warming potentials for CH4 and N2O, respectively, over a 100-year period. There are multiple reasons why we shouldn’t use GWP to refer to both the greenhouse gas metric and the CO2-equivalent greenhouse gas flux. Those reasons include the consideration of definitions and units, the desire for scientific clarity, promoting clear communication with people in other research fields, and the unintended meanings that may arise when terms are misused.
I recently wrote a mini-review in Ecosystems that covers the use (and mis-use) of global warming potentials in the ecological literature: Article [accepted manuscript] Global warming potential is not an eco...
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