CO2e - Methods of Calculation

Human activities don’t just emit carbon dioxide (CO2) but also a variety of other greenhouse gases that heat up the atmosphere.

Emission accounting largely happens in carbon dioxide equivalents (CO2e) , so a way of converting the other gases into CO2e is needed. This is done by assigning each gas (referred to as “constituent gases” here) a Global Warming Potential  (GWP), which in essence means “how many kg of CO2 would one kg of this gas correspond to.” You can read more about emission factors and GWP in our introduction to emission factors.

As models and data have improved, our best guess of the Global Warming Potential of the different gases have changed. The calculation methods that used for these GWP are the IPCC ’s Assessment Reports. The Assessment Reports are generally shortened to “AR”, so the “4th Assessment Report” becomes “AR4”

The last three Assessment Reports released were:

• AR4 (2007)
• AR5 (2014)
• AR6 (2021)

At Climatiq we aggregate data from a wide variety of sources that use different calculation methods. We will always inform you what calculation method our calculated CO2e is based on. Take the following example of a response from the /estimate endpoint:

{
    "co2e": 0.31875,
    "co2e_unit": "kg",
    "co2e_calculation_method": "ar4",
    "co2e_calculation_origin": "source",
    "constituent_gases": {
        "co2e_total": 0.31875,
        "co2e_other": null,
        "co2": null,
        "ch4": null,
        "ch4_fossil": null,
        "ch4_biogenic": null,
        "n2o": null
    }
    //... more
}

You will see a co2e_calculation_method field, and calculation_origin field. You can see that this particular estimate is calculated with the methodology from the 4th Assessment Report (ar4), and that the CO2e has been calculated by the source.

Climatiq-calculated CO2e

Not all CO2e values are calculated by the source - some are calculated by Climatiq. We do this if we have enough data to provide it and the source has not provided it pre-calculated (for example, emission factors from the US Environmental Protection Agency)

An example, if the source:

• Specifies all constituent gases used for their calculations
• Uses those gases based on the ar4 calculation method to calculate a co2e

Then Climatiq has enough data to also calculate CO2e with the help of the ar5 method. You can make an estimation with the following parameters to get the more recent AR5 numbers:

// /estimate endpoint
{
    "emission_factor": {
        "activity_id": "some-activity-id",
        "calculation_method": "ar5"
    },
    "parameters": {
        "distance": 10,
        "money_unit": "km"
    }
}

And you will get the following response back

{
    "co2e": 2.9658171172938,
    "co2e_unit": "kg",
    "co2e_calculation_method": "ar5",
    "co2e_calculation_origin": "climatiq",
    "constituent_gases": {
        "co2e_total": null,
        "co2e_other": null,
        "co2": 2.9328720167999998,
        "ch4": 0.1180605261,
        "ch4_fossil": null,
        "ch4_biogenic": null,
        "n2o": 0.11184681419999999
    }
    // ... other fields
}

The key differences here are that the co2e_calculation_method has switched to ar5, and the co2e_calculation_origin has gone from source to climatiq

Fossil and biogenic methane

Methane (CH4) is reported across three constituent-gas fields — ch4, ch4_fossil and ch4_biogenic — because IPCC AR6  assigns methane a different GWP100 depending on where its carbon comes from.

At the end of its life in the atmosphere methane oxidizes into CO2. The split exists because whether that CO2 should be counted as an additional contribution to warming depends both on the origin of the carbon and on whether it has already been counted elsewhere:

• ch4_fossil — methane from fossil fugitive and process emissions (for example, leaks from oil and gas infrastructure). This methane is never combusted, so its carbon has not been counted as CO2 anywhere. When it oxidizes it adds genuinely new fossil CO2 to the atmosphere, so AR6 includes the effect of that CO2 on top of methane’s own radiative forcing, giving the higher GWP100 of 29.8 kgCO2e per kgCH4.
• ch4_biogenic — methane from biogenic sources. Its carbon was recently drawn from the atmosphere (for example, by the plants an animal ate), so the CO2 produced when it oxidizes is part of the natural carbon cycle and adds no net forcing. AR6 gives it the lower GWP100 of 27.0 kgCO2e per kgCH4. Biogenic emissions also frequently need to be reported separately in GHG inventories.
• ch4 — methane that has not been identified by the source as biogenic or fossil. This is most commonly methane from the combustion of fossil fuels. This carbon is fossil and does oxidize to fossil CO2, but the standard IPCC Tier 1 CO2 emission factors are based on the fuel’s total carbon content and assume 100% of it converts to CO2. That assumption already counts the carbon that actually escaped as CH4 as though it were CO2, so the CO2 from the methane’s later oxidation is already in the books. Applying the 29.8 fossil factor would count it twice, so AR6 directs that the biogenic GWP100 (27.0) be used for combustion methane instead. It therefore stays in ch4. This field may contain some biogenic or non-fossil CH4 if the data source has not separately identified these.

So ch4 (combustion) and ch4_biogenic use the same 27.0 value, but for opposite reasons: for biogenic methane the oxidized CO2 is climate-neutral, while for fossil-fuel combustion the CO2 is fossil but already accounted for in the source’s CO2 figure.

References: The fossil versus non-fossil methane values come from IPCC AR6, Working Group I, Chapter 7 , Table 7.15. The rule that methane from fossil-fuel combustion uses the biogenic value — to avoid double-counting the CO2 from its oxidation — is set out in IPCC AR6, Working Group III, Annex II , Table 9, footnote 6.

Defaults

Climatiq tries to stay as close to the sources as possible.

This means that we will always default to providing the CO2e emission factor provided by the source, preferring the most recent calculation method if the source provides more than one.

If you want to make your own choice about which calculation methodology to use, Climatiq allows you to define calculation_method in all endpoints that accept a Selector.

Possible Returned Calculation Method Values

The co2e_calculation_method field is a standard part of Climatiq’s API responses. Below is a table that describes potential values for this field across Climatiq endpoints.

Due to the need for backwards compatibility, there are multiple values that mean the same. You should check the documentation of the endpoint to see which version it uses or ensure your applications can recognize and handle both versions of each value.

Endpoints will always list the values it might return. Climatiq might add new values as a non-breaking change, e.g. when a new Assessment Report is released. Your application should not treat this list as exhaustive.