This a very complicated chart, and that is its point.
There are a great many adverse impacts of GHG pollution on food productivity.
Fossil fuel air pollution damages crops by acid rain and by ground level ozone air pollution. This ozone is formed by heat catalyzing a chemical reaction of fossil fuel air pollutants (VOCs) to form ozone in the ambient air.
Ozone in our air is toxic to our health and it is also toxic to green plants impairing their capacity to grow by photosynthesis.
The many adverse impacts will exert an additive damaging effect and synergistic final effect (more than the sum of the separate impacts).