This is the sort of conversation to expect from deniers, and not from actual skeptics.
Regarding whether the accumulation is exponential, maybe step back and take a broader look?
"Whether or not 400ppm has any other detrimental affect other than temperature (which it obviously hasn't driven to Eocene levels)"
You're missing a word. "Yet".
Remember when I said "There is no claim that the system will respond instantly. A system as large as the earth will take some time to warm up." In this thread? No I figured you didn't. Forgetting counterarguments is a specialty you guys cultivate.
Well, I did my best. Go ahead and have the last word if you must.