More generally, I hope a bunch of scientists somewhere have been granted a pot of money to tell us if the atmosphere has already become a significantly more dangerous place than our aircraft and systems were designed for.
I've applied for funding for something similar to this for several years, it's only in the last ~18 months that funding bodies have decided it's worth looking at.
Curiously enough, the average height of cloud tops is, in fact, falling.
The other poster was talking about CBs, not clouds in general. The reduction in height was due to a reduction in Cirrus clouds.