I like to think it is because of ifrit's influence that thanalan is bone dry. But it is calamity related I'm sure. (though i wonder if the burn was influenced by some primal like ifrit more than it's aether being taken to asz la. Although part of eastern thnalan is subject to monsoon or showers that is a direct result of the falling of dalamund/bahamut calamity.

Speaking of the burn why would they even need to take the aether away from the burn to asz la when they had three primals locked away. Plenty of aether to siphon off them.