I know Inside Climate News won a Pullitzer Prize, and good for them, but is this the kind of hype it takes to come out on top?
I'm sure that not a single scientist was actually "anxious" about CO2 levels crossing above 400 ppm.
(And by the way, that should be "ppmv" and not "ppm.")
Of anyone, scientists know that atmospheric CO2 content is a smoothly varying function, and so crossing a value based only on how many toes we have is utterly meaningless.
It's a number for activists only, not for scientists.