A higher average temperature means that areas that were able to grow crops will now be dryer, pushing growing areas to cooler places. Yes, higher average temps may mean that it stays 70 degrees for a longer period of time, but it also means that you will hit 100+ more often, drying the ground and drying plants.
"Roman Polanski makes his own blood. He's smart -- that's why his movies work." -- A brilliant director at "Frank's Place"