Lean burning increases mileage, but produces nitrous oxides.
Rich burning produces carbon monoxide, which eventually converts to CO2 in the atmosphere.
Unit for unit, NOx emissions are 265 times as damaging as CO/CO2.
https://scied.ucar.edu/learning-zone/how-climate-works/some-greenhouse-gases-are-stronger-others
To successfully convert NOx, catalytic converters need a stoichiometric or slightly rich fuel/air mixture to the engine.