About Yellowstone, if it happen right now it wouldn't be quite so bad, worldwide stocks if grain are at an all time high. The primarily agrarian US states are ruined in the very short term, but the industrialized one might come out intact, distance from Yellowstone causing variation.
Southern Africa (A drought means the entire area is short of grain) and anywhere without a good distribution network (3rd and 2nd world) are probably looking starvation, food riots and political instability.
Some other points of note:
- No flight for at least a year, probably two in the US
- Energy, solar and biofuels are looking at a big problem
- So is oil, many of the big producers are looking at political instability
And there would be political changes due to all the refugees and the partial collapse of the free market in supply of food