It's very hard to argue without numerical data, but the graph on the Wikipedia version of the graph seems to increase linearly to my eyes: https://en.wikipedia.org/wiki/Keeling_Curve (Edit: No, I can see what you mean; the graph is steeper from 2010–2020 than from 2000–2010.)
(To anyone a bit confused, the Keeling graph is the cumulative amount of CO2 in the atmosphere, so emissions are the first derivative of this graph.)