True, but just measuring carbon dioxide for the whole planet does show a distinct seasonal variance and that's for the reasons previously stated - there's a lot

True, but just measuring carbon dioxide for the whole planet does show a distinct seasonal variance and that's for the reasons previously stated - there's a lot of land mass in the northern hemisphere and terrestrial plant life has a significant effect. Possibly on exoplanets there might be similar asymmetries, or the asymmetry of an elliptical orbit. Seasonal variation in atmospheric gasses aren't necessarily a certain sign of life since there could be other non-biological causes, but they're a big clue.