For data going back to 1990 see [1] and [2].

In figure 2, the paper [3] tries to fit the data to an exponential curve starting at 2004. The longer term data at [2] does not appear to support this fit.

[1] https://en.wikipedia.org/wiki/List_of_riots#1990s_-_2000

[2] http://www.fao.org/worldfoodsituation/wfs-home/foodpricesind...

[3] http://arxiv.org/pdf/1108.2455v1.pdf

UN Food and Agriculture Organization website:

http://www.fao.org/worldfoodsituation/wfs-home/foodpricesind...