# 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 two Assessment Reports released were:

- AR4 (2007)
- AR5 (2013)

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, "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.

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": { "id": "some-emission-factor-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, "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`

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 filter on `calculation_methodology`

in both the /emissions-factor and /estimate endpoints.