Okay, I'm sorry, that does indeed say a great deal of what you alleged. I made an assumption based... if you'll believe this, and I swear it's true... not on anything I may have perceived about you, but on the fact that, if you squint, it might almost seem like people might bring up global dimming as a counterargument to global warming. Now, of course, two things can both be true: global temperatures can increase while the net insolation decreases due to atmospheric effects. In fact I can see how there could actually be a correlation between increased reflectivity of the atmosphere and greenhouse effect. I hyperfocused on the contrails example, and while that seems discredited or at least dubious, the balance of what you said appears to be true at least to this layman. I should have read you more carefully and seen that the contrails thing was not your only point.
Moreover I am sorry that I spoke in ridicule while you remained calm and civil.
I do wonder what the actual photographic effect would be... bear in mind that these are global averages. Dealing with specifics... I live in an area where cloud cover varies wildly and approaches 0% at various times. And as I said my latitude is relatively low so my sunlight of often more like sunny 22... but it is of course very possible that at one time it was brighter here on average. Seems impossible from how bright it gets here but it's very possible, since we are speaking in averages.
The difficulty is in isolating variables thst contain a lot of random noise.. I greatly doubt whether my thermometer is accurate to the kind of precision that is called for by standard color development procedures. There's every chance the temperature when I develop is off by more than two degrees, leading to variations in film density equivalent to under or overexposure by a fractional number of stops... Moreover the transmission of a lens is often about a third of a stop less than the actual mathematical value of the focal length/aperture equation. F/1.8 lenses often transmit the amount of light that would nominally be transmitted by an f/2 lens. This is taken for granted, as it affects many lenses about the same, and sometimes factored into exposure calculations, as in some cinema applications, but from what I can see usually it is not. Actually isolating the overall reduction in incoming sunlight as a meaningful photographic datum would be bloody difficult in my estimation.
Sunny 16 still works, in my regard. Because one should use it in bright sunlight only. As for OP I consider it a cinch that his meter (which he did use to confirm his manual estimates) was off due to alkaline batteries, which is a b*tch with his particular model of camera, unfortunately. Your meter, I can't speak to.
All that being said, I'm sorry for jumping to conclusions and bringing up dead issues out of the past.