Urban farming in a breadbasket

I've been spending time reading about vertical farming in urban environments as a possible way to feed future populations and conserve water supplies.

The biggest megacity regions in South America - Rio, Sao Paulo, Buenos Aires and Bogota - all exist right near some large agricultural areas that are fully capable of feeding them. Unlike other urban areas of the world, the need for vertical farming doesn't seem particularly urgent. It would help save water and it could help boost exports to regions like China where the demand for food continues to outstrip supply. However, those are secondary benefits not currently driven by local demand.

All this is to say that I think some form of vertical farming is likely in the future around the world, but without significant leadership or some economic incentive, South America may be one of the last regions to adopt it.

On the other hand, the Caribbean where land for agriculture and fresh water are at a premium could have the conditions to be an early adopter.

Shrinking islands

50 years from now rising sea levels will likely start to eat away at a number of coastlines. This will be a problem throughout the hemisphere, but will be a particularly big hit to the islands in the Caribbean. Most are rather small and don't have much space to lose.

Do they accept the lost land? Do they build walls to try to keep out the rising ocean? Do the build platforms over the sea? Do they build floating portions to the island? Do they build skyscrapers upwards, regaining lost ground vertically that they lose horizontally? Do they build underneath the water?

Every country will likely approach the problem differently. For those islands that choose to use structures to maintain their lost land, I would be interested to know some of the international law questions that could be raised by them building on top of or under the waves. This could go to how we define maritime boundaries.