Hypothetically . . . and 500 feet is 500 feet . . .
Let's suppose the increase in temperature of about 0.5C since 1950 applies from the surface deep into the atmosphere.
The climate measurements show that although there has been this small temperature rise, the RH has remained more or less the dame.
If there were no changes in engine design, and there no growth in air traffic, then you could possibly detect a reduction in the occurrence of contrails of any type. But all those thing haven't been equal, changes in engine design have led to the raising of contrail critical temperature (for formation) which acts in the opposite sense. So the 500 feet (which as you say
is 500 feet) would be lost in the noise.
You seem to have the idea that all contrails are persistent. You don't seen to distinguish. To get a persistent contrail, you first need to get a contrail to form. That is, the exhaust / ambient air mixture needs to reach water saturation at some point. What happens after that depends on the RH... which is reckoned not to have changed (due to global warming effects).
Anyway, I think MJM's thinking about this matter only extends as far as imaging that the chemicals in the "spray" can somehow magically manipulate weather systems to both induce floods (a consequence of weather) and droughts (getting into the time-scale of a climate consideration), and other unspecified unpredictable weather at the same time, though possibly in different parts of the world.
Maybe he is trying to fold cloud seeding into the discussion, as so many other chemtrail leaders and activists seem to be doing
That whole statement of MJM's is very carefully crafted. See how he name-drops "unpredictable weather". We all know (especially those who are doubtful of the science) the weather is unpredictable Right? Truth! It's like he is using a
dog whistle. The believers will hear that phrase and a little light comes on; others will hear it and nothing happens - like it's inaudible.