All SRES emission scenarios call for CO2 levels to increase significantly over the course of the 21st century. Even with drastic actions, levels are predicted to reach 550 ppm mid-century before decreasing. Worst-case scenarios have levels continuing to rise beyond 650 ppm at the end of the century. Compared to other climate variables, increasing CO2 generally has a positive impact on crops, leading to increased crop yields (see figure below). CO2 is key to photosynthesis; the gas is what is known as a limiting nutrient for plant growth. Without a certain amount of CO2, plants will fail to grow. CO2 acts as a fertilizer for crops like rice, soybeans, and wheat and enhances growth rates. The impact of increased CO2 is only known via experiments, and they show increases in production from 5-20% at CO2 levels of 550 ppm. The key uncertainty is how realistic experiments are at predicting the real world. There is a consensus among scientists that the real changes in yield might be slightly less than those in the lab.