What is the CO2 equivalent?

Published on August 10, 2021

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Each GHG has a different impact.

It would be much too simple if all GHGs had the same effect. In fact, each gas has specific characteristics.

Residence time

GHGs stay in the atmosphere for different periods of time. This is called “residence time”. As a result, they exacerbate the greenhouse effect differently.

Examples:

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Radiative forcing

Radiative forcing is the notion used to show the diverse contributions of different gases to global warming. It is the contribution made by a chemical species to the Earth’s energy efficiency. In other words, its contribution to the additional greenhouse effect. It is measured in W/m2

The adjective “radiative” is used because it measures differences in radiation. More precisely, it measures the difference between solar radiation entering the atmosphere and infrared emissions leaving the atmosphere.

For example, positive radiative forcing means there is an increase in the contribution to the greenhouse effect, causing global warming. On the other hand, negative radiative forcing is when the Earth receives less energy than it releases. A gas with negative radiative forcing reduces the effectiveness of the greenhouse effect and therefore cools the planet.

Each GHG has different radiative forcing and therefore, contributes differently to global warming.

Comparing different GHGs and their impact.

You can’t compare apples and oranges. As we have just explained, GHGs have two main characteristics which impact on global warming: residence time and radiative forcing. The indicator created to make simple comparisons of GHGs is the GWP or "Global Warming Potential".

Calculating GWP: take a GHG’s radiative forcing over 100 years to neutralize the question of residence time, then compare it to the CO2 value. By definition, the GWP of CO2 is 1.

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This shows that the average global warming capacity of methane is 30 times that of CO2. If 1kg of methane is emitted into the atmosphere, the greenhouse effect over a century is the same for 30kg of CO2 for the same period.

It is thus possible to compare the impacts of different GHGs using the “CO2 equivalent”. The CO2 equivalent of a GHG is the same amount of CO2 capable of warming the planet over a given period of time. This concept is used by the media because it is easier for the general public to understand.

For example: A dairy cow emits about 100kg of methane per year, which has the same heating capacity as 3 tons of CO2 per year. This means that a dairy cow emits 3 tons of CO2 equivalent per year. That’s the same emission as a new car over 25,000 km.

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GWP is still a common and practical indicator for comparing the contribution of GHGs to global warming, regardless of the gas, sector or country being analyzed.