I'd guess there'd be a lot of sudden interest in energy-efficient LED grow lights and there'd be an over-supply of beef in the short term because there's no food for the cattle, so we might as well eat them.
Long term, it's all about being able to have enough indoor hydroponic/aeroponic space to grow enough food for ~7.5 billion people, which sounds really difficult and impractical.
I find it interesting that one possible benefit of Elon Musk's Mars plan is that the same space/energy/resource efficient food production technology that will be needed on Mars is the same technology we would need on Earth in order to ride out certain catastrophes. By creating an interplanetary "insurance policy" to ensure human survival, we may improve our ability to survive on Earth.
I can't find the research I read a few years ago but from what I understand growing indoors is crazy practical. Plants need less sun light, essentially zero pesticides, can grow much faster and if you build your structures vertically you'll be able to farm the same amount of food on orders of magnitude less space.
I don't know if we're at the point where it can be practical today, especially with so much subsidy in farming, but I'm very optimistic about it moving to automated systems within buildings that wouldn't even need the sun.
Ideally, we need a couple of years food in storage together with detailed plans for how to grow food indoors (if needed).
It's just not realistic. Sure we can significantly enlarge our buffer but two or more years? I can't see that happening in any practical sense.
Even having a single years buffer would be very worthwhile. As I mentioned in the OP if we have a catastrophic event in the northern summer the poor are in serious trouble.
Once the stockpile was established and the process of cycling it set up, then the cost should be relatively minor.
More importantly what is the alternative? Let everyone starve?
Moreover, wrt to the 72 days raised by ggpp, one would just shrink the surplus to 72 days for the smaller population.
Food insecurity is purely a function of the inefficiency of our society's means of food distribution.