Major cereal crops have suffered annual losses of about 5 billion U.S. dollars since 1981 because of global warming, a new study has found.
Fields of wheat, corn and barley throughout the world have produced a combined 40 million metric tons less per year from 1981 to 2002 because of increasing temperatures caused by human activities, said the study conducted by the Lawrence Livermore National Laboratory (LLNL) in California.
The news was not completely dire as the damage has been offset by production gains from genetically modified crops and better farming techniques, it said.
This is the first study to estimate how much global food production has already been affected by climate change.
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 LLNL researcher and leading author of the study.
"Though the impacts are relatively small compared to the technological yield gains over the same period, the results demonstrate that negative impacts of climate trends on crop yields at the global scale are already occurring," Lobell said.
"Most people tend to think of climate change as something that will impact the future, but this study shows that warming over the past two decades already has had real effects on global food supply," said Christopher Field, co-author of the study and director of Carnegie Institution's Department of Global Ecology.
Lobell and Field studied climate effects on the six most widely grown crops in the world - wheat, rice, maize (corn), soybeans, barley and sorghum.
Production of these crops accounts for more than 40 percent of global cropland area, 55 percent of non-meat calories and more than 70 percent of animal feed.
Using global yield figures for 1961-2002 from the Food and Agriculture Organization, Lobell and Field compared yields with average temperatures in the major growing regions.
They found that, on average, global crop yields respond negatively to warmer temperatures with several of the crops. Lobell and Field then used these relationships to estimate the effects of observed warming trends.
"To do this, we assumed that farmers have not yet adapted to climate change, for example by selecting new crop varieties to deal with climate change," Lobell said. "If they have been adapting - something that is very difficult to measure - then the effects of warming may have been lower."
Most experts believe that adaptation would lag several years behind climate trends because of the difficulty of distinguishing climate trends from natural variability.
The importance of this study, the authors said, was that it demonstrates a clear and simple relationship at the global scale, with yields dropping by approximately 3-5 percent for a one-degree Fahrenheit increase.
"A key to moving forward is how well cropping systems can adapt to a warmer world," Lobell said. "Investments in this area could potentially save billions of dollars and millions of lives."