That's an interesting point about temperatures, but I don't think that's the main concern we should have about global warming. People living in Arizona don't seem to be significantly worse off than people living in North Dakota. And a few degrees will not change much in terms of heating and A/C. I think the real worry of global warming is what it will do to ecosystems.
Within the limits of where a human can survive outdoors cooling will look like a very linear process. Higher temperatures do make ground sinking that much more attractive.
1) Heating a room up is easy, cooling it down is hard. High temperatures exponentially decrease the productivity and/or increase the need for energy.
2) It is very hard to purify water and clean water is very scarce. Water is the most important dependency of life.
So my answer would be:
1) Lack of clean water supplies
2) Global warming
3) Disastrous combination of both