I'm more concerned with rising mean global temperatures rendering areas surrounding the equator uninhabitable, when the average temperature gets up near 40c. You won't be able to grow crops anymore, and it will drive people away. If you move the human habitable zone away from the equator, you have less and less land, and even less flatland for ariable farming. So besides the slash and burn land quality loss, you are potentially losing degrees of latitude of ariable space.