I took this [self-survey of CO2 emissions](https://uba.co2-rechner.de/de_DE/) run by the [German Environment Agency](https://en.wikipedia.org/wiki/Umweltbundesamt).

It computes for example that an economy class seat from London to San Francisco and back – about 22 hours flight – accounts for 5.28 tonnes of [CO2 equivalents](https://en.wikipedia.org/wiki/Carbon_dioxide_equivalent) (CO2eq or CO2e).

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> [![Calculator screenshot](https://i.stack.imgur.com/So96o.png)](https://i.stack.imgur.com/So96o.png)
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For comparison, 

* the average CO2-emission in 2017 per capita has been 9.7 tonnes in Germany according to [this Wikipedia article](https://en.wikipedia.org/wiki/List_of_countries_by_carbon_dioxide_emissions) – but that excludes other greenhouse gases.
* The [International Civil Aviation Organization](https://www.icao.int/environmental-protection/CarbonOffset/Pages/default.aspx) gives only 0.762 tonnes CO2 emission for the same flight. The methodology describes only CO2 emitted when the fuel is burnt.

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> [![Calculator output](https://i.stack.imgur.com/S2QjA.png)](https://i.stack.imgur.com/S2QjA.png)
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Is the German Environment Agency's estimate for CO2eq from such a return flight reasonable?