How do you explain the increase in food prices despite the long term upward trend in cereal yield?

Justify using simple economic concepts like markets/ costs/ innovation developed in class.

The figure shows cereal/grain yields have consistently increased over the years. The average world price of food was declining till 1990s, but has steadily gone up in the last twenty years.

How do you explain the increase in food prices despite the long term upward trend in cereal yield?

Justify using simple economic concepts like markets/ costs/ innovation developed in class.