As you and Izzy point out 2100 is a few years away.

A lot can happen between then and now.
Peak oil, wars, disease outbreaks, rising oceans, storms... The list of things you can let terrorize you is endless.

I am a bit alarmed by the "Technology will fix it!" attitude of the people trying to debunk it, but I don't think Global warming will be the major cause of reduced food production.

For one thing Peak Oil factors into agriculture much more heavily.

Most of us only think of oil as fuel for home heating or vehicles.
We forget about how much oil goes to produce electricity, how much gets turned into plastics for containers and clothing, how much is turned into chemical fertilizers, and how much is used to plow fields and run factory farm livestock operations, to fish the oceans.
After all that how about the oil used to save and to transport all the food into the cities.
I would bet that the rising cost of refrigeration alone would cost more in lost food than the temp increase would in the near future.All of the oil used to replace agricultural product would need to be replaced by raising even more crops and animals.
Add to this the need for fuel and building materials, and consider that we have already depleted most of the world's forest reserves... again this could be a very long list of effects.
Without a plentiful supply of oil at this time we are already well above the carrying capacity of the planet for humans.

And Peak oil is only one of the factors.

Remember the great Banana scare?
There was a new disease which looked like it might destroy the Cavendish type banana.
All food type bananas are Cavendish and banana is a staple food for many tropical nations, not just desert.
The disease is being controlled by quarantines and inspections because the geneticists have not found a way to make an immune banana yet. It should remind history buffs of the Irish Potato Famine.

I will stop there with the scenarios. It is too easy to become paranoid about it.

I recognize there may be big problems over the distant horizon, and if we continue sailing the course we are on we might sail right over the edge of the Earth!
(how about that for a stretched metaphor)

Unfortunately I have a few things which are more immediate on my priorities list than saving the planet right now.

"I find it a bit ironic that air conditioning is a cause of global warming."