Any idea about the logic ITA uses to estimate CO2 emissions. Is it based on aircraft specific fuel consumption and capacity?

Noticed that for ATL-DAY:

- DL MD-88 ->245 pounds of CO2
- FL B717 -> 283 pound of CO2

I understand that the MD88 is larger, hence a lower pax-specific C02 release. However, the 717 should be newer and therefore consume a bit less.

Hence my question. Is it a back of the enveloppe calculation or something more involved?

Minos