Climate expert Bob Scholes, the professor at the University of Switzerland explains that wheat is a so called C3 crop. This is a cereal which is quite responsive to rising carbon dioxide concentration. As a result of climate change and rising carbon dioxide, wheat production goes up about 10%, but at the same time we face changes in the amount of rainfall and temperature which also affect the cultivation.
Temperature will increase quite significantly in the future and rainfall will rise as well. So, there is kind of a trade-off between those two factors. If there is more rain, the productivity usually rises. If the temperature goes up however, it can have different effects.
In areas with an already hot climate, the weather can become too hot for wheat cultivation. In other, colder regions, the agriculture can benefit from the increasing temperatures and produce more wheat. As for the cold, northern parts of Europe, production will go up by a few percent. In the southern parts, where temperature is already quite high for grain cultivation, there will be a decrease in crop production.
Overall, the number of places in the world that see the increase and the decrease are almost balanced at the present time. In the (close) future, this will change however. Many countries' agriculture will benefit from the climate change while others suffer drastically.