A study conducted at the Friedman School of Nutrition Science and Policy at Tufts University has found that the likelihood of extreme temperatures affecting crop yields has increased significantly in wheat-producing regions of the United States and China.
The study also predicts that heat waves that happened approximately once every hundred years in 1981 are now likely to happen once every six years in the Midwestern U.S. and once every 16 years in Northeastern China. The most recent report from the Intergovernmental Panel on Climate Change finds that the average global surface temperature in the last decade was 1.1 degrees Celsius higher than it was between 1850 and 1900. To evaluate how this has changed the risk of extreme weather, the researchers collected a large group of seasonal forecasts from the past 40 years. From this, they generated thousands of possible variations in temperature and rainfall, essentially showing all the things that could have happened in a given year. With this method, known as the Unprecedented Simulated Extreme Ensemble (UNSEEN) approach, the researchers were able to estimate the likely frequency of extreme temperatures that exceed critical growth thresholds for wheat.
Winter wheat crops grow in the fall and are harvested in the following summer. High temperatures in spring, when the plant is flowering, can affect wheat development. At temperatures over 27.8 degrees Celsius (about 82 degrees Fahrenheit), the plants start to suffer from heat stress. At temperatures over 32.8 degrees Celsius (about 91 degrees Fahrenheit), important enzymes in the wheat start to break down.

The study also indicates that the U.S. and China have been lucky in recent years as they have had fairly low temperatures, ending up with cooler weather than they could have had. However, climate change has changed these numbers, as the highest number is bigger than it used to be. These regions have not experienced the full extent of what is possible, and they might not be ready for it.

[now.tufts.edu]