Nowadays, a farmer who can't grow crops anymore doesn't necessarily have to find a farm elsewhere, but could also move to a nearby city and work in a factory. That option is enticing enough for many that they'll choose it even when their farm is doing fine. Agricultural employment is generally on a downward trend. https://data.worldbank.org/indicator/SL.AGR.EMPL.ZS?most_rec...
You may wonder where that city is going to get their food if it can't be grown nearby, but food can be traded across long distances and often already is. So a more likely scenario is that Canada massively expands agriculture with a relatively small number of agricultural workers, many of whom might certainly be immigrants, and then trades the surplus.
The one case where long-distance migrations will be unavailable are island countries that can't handle rising sea levels, but fortunately even put together they don't have very large populations, and in some cases resettlement agreements are already in place.
You may wonder where that city is going to get their food if it can't be grown nearby, but food can be traded across long distances and often already is. So a more likely scenario is that Canada massively expands agriculture with a relatively small number of agricultural workers, many of whom might certainly be immigrants, and then trades the surplus.
The one case where long-distance migrations will be unavailable are island countries that can't handle rising sea levels, but fortunately even put together they don't have very large populations, and in some cases resettlement agreements are already in place.