You are assuming that a warming climate is more helpful, but you could have a warm dry desert
Wrong. A warmer climate releases more moisture into the atmosphere from the oceans, which winds up on land. You always have a net positive effect on moisture...
This has also been noted in explanations of why snowfall amounts are up in some areas.
Deserts are the result of specific weather patterns not allowing moisture to flow to a region, but it always goes somewhere...
We also have proof of this simple fact, the medieval warm period was a fair amount warmer than we are now, and it was in fact a great time for agriculture.
Lastly, you are again ignoring jungles which are as hot as deserts... you seem to think that a great amount of heat automatically means desert which is very far from the truth.
But the most rise we are predicted to see anyway is about 2-3C.