the idea that climate causes civilizations to fall is pretty standard, nowadays. there's nothing obscure about that, and the same science that helps us understand climate change helps us piece together how changes in the climate may have affected different settlements. so, the premise is sound.
but, you can tell that the narrator has at some point entertained some less than ideal sources, as he tends to reference things that you don't hear legitimate scientists talk much about, in ways that sound like climate denial.
but, i haven't caught him say anything wrong yet. so, i don't have anything to add except a little caution that the source sounds compromised, without providing any definitive data that it actually is.
so, for example, he talked about the "medieval warm period" and the "little ice age" during the cambodian episode, which made me cringe. these things happened, but they were not global events - they were examples of regional climate variations restricted strictly to northern europe. nothing of the sort would have been apparent in far away cambodia, so it's really not appropriate to drop the language in those terms. but, when he actually worked out the climate effects, he point to relevant tree ring and ice core samples, and there wasn't anything to really correct him on - except the language he used, and the fact that it made it clear his background isn't in science.
so, he might fuck this up still.
but, he hasn't yet.
some caveats are consequently required. but, i've pointed out before that the interdisciplinary nature of the videos is actually fairly developed, and he's broadly doing a pretty good job, in the context of what he's broadcasting he's doing - a series of short films about the rise and collapse of various historical cultures.