@drgroftehauge @davidho they're correctly referring to the development of the CO₂ mole fraction (as a percentage) which is a common way of describing concentrations in the atmosphere. It has changed from ~0.027% to ~0.042% over time, an increase of 0.015%. But they're of course pretty far off base by saying it's negligible as if greenhouse gases need large concentrations to be of any significance.