In that scenario, humans with barely enough farmland to feed themselves are going to trade food against non-edible crops. Then the traders will be bombed by the humans now starving because their food was taken away, and you have the same situation except more local.

Not all land will grow food. We've invested our food growing capital where it's grown throughout the history of agriculture (about 10,000 years).

Good luck trading for enough food when your former high yield farmland only allows for light goat herding or poverty crops now because the glaciers that you relied on are no longer there and your aquifers are empty.