Climate Change Already Impacting Wheat, Rice, Corn, Soybean Yields Worldwide

Increased temperatures from climate change will reduce yields of the four crops humans depend on most—wheat, rice, corn and soybeans—and the losses have already begun, according to a new meta-study by an international team of researchers.
Humans depend for two thirds of their calories on these four staple crops, but yields of wheat are expected to decrease by 6%, rice by 3.2%, maize by 7.4%, and soybean by 3.1%.
"By combining four different methods, our comprehensive assessment of the impacts of increasing temperatures on major global crops shows substantial risks for agricultural production, already stagnating in some parts of the world," the scientists say in the study, which appears in the latest issue of the Proceedings of the National Academy of Sciences.
"Yield increase has slowed down or even stagnated during the last years in some parts of the world, and further increases in temperature will continue to suppress yields, despite farmers’ adaptation efforts." The study, led by Chuang Zhao of Peking University, cites three other studies documenting declines in crop yields in Europe, Africa, India, China, Central and South America and other regions.
The study of studies was conducted by scientists in China, Germany, Belgium, Italy, France, Spain, The Philippines, and the United States, including the University of Florida, Stanford University, the University of Chicago, and Columbia University in New York. They hoped to settle a question that seemed to have produced conflicting results in the many studies they reviewed: what are the effects on crop yields of temperature increases from anthropogenic climate change?
The study rebuts an argument made by those who argue against mitigating climate change because they say higher CO2 concentrations will increase crop yields. That argument, the scientists say, fails to account for higher temperatures:
"While elevated atmospheric CO2 concentration can stimulate growth when nutrients are not limited, it will also increase canopy temperature from more closed stomata," the scientists say. The stomata are the pores plants use to exchange gases and moisture with the atmosphere. When plants close stomata because of higher temperatures they may conserve water but lose the ability to absorb CO2.
Higher temperatures can also increase atmospheric absorption of water in the plants and in the soil, provoke heat waves and stimulate pests and weeds.
The study anticipates that crop yields will improve in some areas because higher temperatures will lengthen the growing season, but it finds net losses worldwide.
The scientists acknowledge uncertainty about the interactions between temperature, rainfall and increased CO2 concentrations in different regions. They note that different crops respond differently in different regions and under different conditions, so they call for increased local analysis and local strategies:
"Differences in temperature responses of crops around the world suggest that some mitigation could be possible to substantially affect the magnitude (or even direction) of climate change impacts on agriculture. These impacts will also vary substantially for crops and regions, and may interact with changes in precipitation and atmospheric CO2, so a reinvigoration of national research and extension programs is urgently needed to offset future impacts of climate change, including temperature increase on agriculture by using crop- and region-specific adaptation strategies."