claim: as the pathbreaking work by Rosamond Naylor and David Battisti has shown, the tropics are already too hot to efficiently grow grain, and those places where grain is produced today are already at optimal growing temperature — which means even a small warming will push them down the slope of declining productivity.
evidence: The polar stratospheric clouds had a warming effect on the poles, increasing temperatures by up to 20 °C in the winter months.