WHM using Amdaporian White Magic techniques.

[Sylvain]

I won't repeat why everyone said but the case of Amdapor and Mhach is also a bit special IMO in the sens that they did abuse magic thus creating their own cataclysm.
It was an all out war with heavy use (to not say abusive) of magic.
We know Ether is everywhere. Everything is made of it. Magic is the manipulation of the said Ether. Like natural ressource, you can effectively deplete it. Once you destabilise the "ethereal equilibrium" of a said region, a massive shift of ethereal type occurs thus the cataclysm. In the case of Mhach and Amdapor you had a huge amount of Ether shifting its alignement to water which created the flood.
For instance, (if I'm right but I think I've read it in the book or somewhere in the dialogue), when a conjurer use Stone/Aero, it temporarely shift the surrounding Ether to the said alignement to create the effect. (The Ether than goes back to it's former form which is why spamming stone doesn't create rumbles) However this is a very minor and localised "shift" of Ether.
You changed half a meter cube of something to rock for a few seconds.

After Bahamut oblitarated half of Eorzea, the region of coerthas had reacted by shifting the etheral balance toward ice. This is why there's a place where you see dragons and warrior litteraly frozen in place. The entire zone, this point especially, froze in an instant. (That's a very quick rebalance if you ask me). Here we aren't talking about a half a meter cube but half a continent worth of stuff. It's like burning 1L of fuel vs 1M tons of fuel. The first one won't affect the atmosphere, the second one will deplete oxygen in a huge radius around the center of explosion.

I guess we could somewhat compare abusing ether (in any form) to climate change on a much shorter time scale
If you use too much of a specific natural ressource (ether in this case), changes are bound to happen because you unbalance the "ethereal environnement".
For instance, if you'd suddently melt all the icecap, you'd have an overflow of water around the world.
if you'd release all the greenhouse gaz trapped in the iceground in north Canada, (and Russia too maybe? I dunno), you'd see temperature rise quickly (which would then lead to other changes)

The only real difference between that and the "natural" events I took as an example are
1 : No one really knows how ether work but SE tend to enjoy theme such as "natural balance, order of things" etc. So you could easily relate to our own world when it comes to ressource manipulation and their effect on the environnement. Just remember that in this case everything is made of ether which has elemental alinement and that those alignement CAN change. If by some unique way a character is like "I've found a way to shift Earth aligned ether to Water for a short period of time on grand scale" and "grand scales" includes object of mountain size, he could litteraly turn a mountain to water (which would then turn back to stone since he said it wouldn't last long)

2 : Unlike our world where things happen slowly, ether seems to enter a chain reaction MUCH faster. Like, once you've broken a specific threashold of unbalance the whole system rebalances itself within a few minutes resulting in catastrophic events for the living (Flood, Instant freeze of Coerthas are the two main example coming to my mind).



So, I think the only reason Black Magic and White Magic are banned is because there were abuses in the past and instead of learning how to use it properly, they rather ban it altogether.
Which is dumb imo, I like to think of those as Nuclear powerplant vs Coil powerplant.
The former is much more potent than the later but fail to use it adequatly and the result might not be so good. Mhach vs Amdapor sounds is a bit like an Eorzean Tchernobyl where they pushed the system too far (ether - the powerplant) and made it explode. So now we're back to Coil because coil can't explode over a continent.

And the WoL is a special snowflake that is learning to use Nuclear fission power again.


However one important point is that the flood was created by the Elements and honestly, we don't know much about them.
Could it be that they're conscious ether? Dunno.
Looking at the previous cataclysm, all of them have a specific alignement and only the flood was caused by the element.
The only other case we have of "man made catastrophe" (that we know of) is the Fourth caused by the crystal tower power surge when it transfered energy from Dalamud to the tower.

What we know is that every case of cataclysm was caused when a lot of Ether was moved around.
Crystal Tower power surge caused an Earthquack that destroyed an entire civilisation
the Magi war enraged the elements causing a flood (something would have happened anyway)
the 7th was cause by Dalamud powerplant (Bahamut) breaking free

We don't know what happened during the First, second, third and Fifth cataclysm beside their alignement. But one thing we can safely bet is that those event were caused. They didn't occur naturally. Eorzea and the way Ether can return to equilibrium really quick suggests (to me at least) that you can't have a cataclysm popping randomly for no reason.