Because temperate land areas will warm faster than the global average, this corresponds to roughly 1.25-2 °C in global average temperature.
For C4 crops, even modest amounts of warming are detrimental in major growing regions given the small response to CO2. C4 crops are essential to world food security as they include maize, millet, sorghum, sugar cane and many pasture grasses.
The IPCC assessment says that any CO2 fertilization effect for the lower latitudes will have a negligible impact and so crop yields will decline with a very small temperature increase.
For the mid and the high latitudes the IPCC chooses its language carefully saying that the CO2 fertilization effect may result in a small increase in crop yields for a while initially as temperature increases, after which crop yields will decline.
.