I see where you're going, Rumbletiger, and I'm way ahead of you.
First, as noted, there aren't that many high level casters.
Second, who do you think is behind all this mess?
Yeah, we've decided that an Illumian cabal is behind the mess. The actual means of getting volcanoes to erupt all over the place hasn't been settled. We've actually seen a couple of different techniques, from imprisoning fire elementals inside them to magic rituals.
And their goal is, in the long term, to rule the world.
To FranktheDM: The world isn't pitch black, just perennially overcast. Crop yields are down, but not to the point of all plant life dying.
But you're on the right track in other ways. Enrichment will increase potential yield by a third, but that's only "potential" yield. With the lack of direct sunlight, field production is down by about 50%. Add a third of that to the total and you're getting 66% of normal yield.
Also, a lot of the ideas here are being presented as if magic wasn't already a factor in current food production.
Prior to modern land management techniques, including motor driven tractors and harvesters, weed control chemicals and insecticides, irrigation, fertilized and high yield hybrid seeds, starvation was a real factor for a lot of the world. It still is in many places.
(I read an article tracking the spread of the potato against the pattern of city growth and nation building. The correlation was amazing. The potato is the plant with the highest food value per acre of any cultivated crop, and it isn't native to Europe. Any pre-Columbian setting will be with it. The number 2 in terms of food value per acre? Corn! Another product of the Americas.)
Now the presence of magic will have alleviated that, but magical aid in agriculture should already be factored into the system, pre-catastrophe. An annual "Blessing of the Fields" would be an accepted part of life in any well developed area. Use of magic to produce high end foods will already be a relatively common practice among the wealthy.
So the question isn't, "How much food can magic produce", but "How much *more* food can magic produce."
Think about it from a DM's perspective: How many times have you had to remind adrenaline-junkie players that they aren't the first person to ever think about using Teleport to advance trade profits, or Major Creation to produce counterfeit gold?
So now let's remind ourselves that this kind of magic has been around for quite a while, in one form or another, in our game worlds. Its use is already built in to the setting.
One trick of the bad guys, by the way, has been to remove Druids who can help alleviate the problems with things like Plant Growth.