The study by the Lawrence Livermore National Laboratory in California found fields of wheat, corn and barley throughout the world have produced a combined 40 million metric tons less each year from 1981 to 2002. Annual global temperatures increased by about 0.7 degrees Fahrenheit between 1980 and 2002, with even larger changes observed in several regions.

“There is clearly a negative response of global yields to increased temperatures,” said David Lobell, a researcher and leading author of the study.

Christopher Field, co-author and director of Carnegie Institution’s Department of Global Ecology, said while most people think of climate change as something that will impact the future, “this study shows that warming over the past two decades already has had real effects on global food supply.”