There is very little evidence about what caused the downfall of Ghana Empire. Besides, new Mali Empire emerged only shortly thereafter.
Depends what you take as evidence
If you want pure historical written sources, then you are right. But there is increasing number of archaeologic evidence that climate played important role.
Not sure what you mean by
new Mali Empire emerged only shortly thereafter, as there was clearly an intermediate period between the two empires - at least some 2 generations. Of course when you use other sources than Wiki which dates the fall of Ghana to 1240 which is a date when Ghana was deffinitely absorbed.
It is clear that the whole area which used to be the core of Ghana was desertified during 12th century and the power base of the Sahel moved some 200-300 kilometers southwards - first to the "Soso" and later even further south to Mali.
We don't know all the reasons behind this shift, but current historiography has very little doubt that aridization of northern Sahel played important role in the downfall of Ghana.
And considering that deity bringing water was the main cult of pagan Ghana, we can assume that water was a precious rather than abundant resource in that empire. As proven by archaeology anyway.
But of course this may all be wrong and it might have been caused by something completely different.
I can't agree that the climate change was the main reason behind Maghreb medieval decline. If it was so, why NW Africa was more or less stable in terms of affluence since the rise of Carthage to the Aghlabid rule? Climate had changed, sometimes drastically, more than once through all these times.
Well, I didn't really say main reason. If you read my post, I desrribed the course of historiographic interpretations of the decline of Africa. For centuries it was explained purely as consequence of Hilalian migration/invasion, but later research - which included other than just written sources - has shown that there were also other reasons behind this decline and that the effect of said migration might have been overrated.
The data available to us is not enough to determin a clear one reason for that. It is most probably a combination of several factors which interacted with each other and increased each other's effect.
I've put North Africa to the contrary of Western Europe - while one was target of migration + like all other areas surrounding it witnessed natural aridization, the other - Europe - profited from the warming period and was not targeted by any external invasion at the same time.
As for climate changes - during the first millenium there were clearly climate changes. Some waves were smaller, some were larger. Isn't it strange that the relative wealth during the Aghlabids somehow corresponded with somehow colder period, the same goes with the revival during late 6th century after Justinian's conquest. I'm not drawing any conclusions, just pointing to some coincidence. There are of course many other factors which should be considered, but... well thinking that agricultural societies and their rise and decline was not influenced by climate is somehow silly, don't you think?
