Interestingly enough, the century XX saw a tremendous shift of agriculture from northern, wet lands to southern, sunny land. You could drain your aquifer but you could not add extra sun up north.
I wonder if the trends reverses as aquifers are done with, and agriculture in places like US's original 13 colonies or middle Russian lands or Prussia or northern France becomes trendy again.
If global warming is real and weather will change, it's possible that some shifts. Cold places will become warmer.
It's also important to think about predictability. Issue with many lands is unpredictable weather. You put your grains in the soil, month later you've got -2C and grains are dead. I feel that global warming will not help here, but rather will make things worse: weather will be less predictable, even if average temperature goes up, local extremums will occur often.
I don't think this is entirely true. I know in Canada we grow wheat, soy, rapeseed and rye in the Prairies, which has a harder time with traditional yellow human corn.
And wheat is a very important staple. We feed a lot of the world!
So while there is some fallow land, this exists everywhere, and I don't think repurposing will help.
I wonder if the trends reverses as aquifers are done with, and agriculture in places like US's original 13 colonies or middle Russian lands or Prussia or northern France becomes trendy again.