Satellite measurements of CO2 and CH4 can be subject to regional systematic errors that can consequently compromise their ability to infer robust flux estimates of these two gases. We develop a method to use retrieved ratios of CH4 and CO2 that are less affected by systematic error. We show that additional in situ data are needed to anchor these observed ratios so they can simultaneously infer fluxes of CO2 and CH4. We argue the ratio data will provide a more faithful description of true fluxes.