Chapter 1 : Understanding Global Warming Potential (GWP) and Carbon Dioxide Equivalent (CO2e) for GARP SCR Examinations

- June 21, 2023

Understanding Global Warming Potential (GWP) and Carbon Dioxide Equivalent (CO2e) for GARP SCR Examinations

You can expect a question based on the below topic in GARP SCR examinations. Please spare time to understand the concepts

Global warming potential (GWP) is a measure of how much infrared radiation (simply "heat") a greenhouse gas ,added to the atmosphere, would absorb over a given timeframe, as a multiple of the radiation that would be absorbed by the same mass of added carbon dioxide (CO2). GWP is 1 for CO2.

For other gases it depends on:

1. How strongly the gas absorbs infrared thermal radiation

2. How quickly the gas leaves the atmosphere (Half life period)

3. Time frame being considered for calculting GWP.

Carbon dioxide equivalent (CO2e or CO2eq or CO2-e) is calculated from GWP. For any gas, it is the mass of CO2 that would warm the earth as much as the mass of that gas. Thus it provides a common scale for measuring the climate effects of different gases.

It is calculated as GWP times mass of the other gas.

Methane has GWP (over 100 years) of 28 meaning that, for example, a leak of a tonne of methane is equivalent to emitting 28 tonnes of carbon dioxide. Similarly a tonne of nitrous oxide, from manure for example, is equivalent to 273 tonnes of carbon dioxide

Please refer to the table shown in the image.

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A substance's GWP depends on the number of years over which the potential is calculated. A gas which is quickly removed from the atmosphere may initially have a large effect, but for longer time periods, as it has been removed, it becomes less important. Thus methane has a potential of 28 over 100 years (GWP100 = 25) but 86 over 20 years (GWP20 = 86); conversely sulfur hexafluoride has a GWP of 22,800 over 100 years but 16,300 over 20 years (IPCC Third Assessment Report). The GWP value depends on how the gas concentration decays over time in the atmosphere. This is often not precisely known and hence the values should not be considered exact. For this reason when quoting a GWP it is important to give a reference to the calculation.

The GWP for a mixture of gases can be obtained from the mass-fraction-weighted average of the GWPs of the individual gases.

Commonly, a time horizon of 100 years is used by regulators.

Please refer to Figure 1. Number on each curve is half life period of that gas

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For the curious minds, how does gases like CH4 end up heating up the atmosphere. Well, the process is very similar to how the food is warmed in a microwave oven. In the microwave, the water molecules are made to vibrate at a very high frequency. This causes the water molecules in the food to vibrate and make them emit radiations in form of heat.

In the figure shown, it shows CH4 molecules are bombarded with various radiations. Carbon and Hydrogen atoms keep vibrating.The vibrating molecules emit radiations in the form of heat.How much heat each molecules emit depends upon how it is structured. For example: 1 CH4 molecule is equivalent to 34 molecules of CO2. Hence, it is emitting higher amount of heat radiation.

Please refer to Figure 2.