[mmooss]

In addition to the other responses ... it's long been predicted and isn't hard to grasp:

Global warming changes many local climates. What was wet is dry, cold is warm, etc.

For 10,000 years, humans have invested in the current locations of productive farmland. They build infrastructure, economies, societies, and grew populations to what the farmland could support.

Now that farmland can't produce as much, so people start becoming poor and starving. They aren't going to sit there and starve and become impovrished; they will move to where there is food and income.