* Yields to fall 6 percent per degree Celsius of warming -study
* Wheat yields most vulnerable in already hot regions
By Alister Doyle, Environment Correspondent
OSLO, Dec 22 (Reuters) - Global warming will cut average wheat yields by six percent for every degree Celsius (1.8 Fahrenheit) of temperature rise in a bigger-than-expected brake on food production in a hotter world, a study said on Monday.
The report, by a U.S.-led team of scientists, said a six percent drop would be 42 million tonnes of 701 million tonnes of wheat production worldwide in 2012, highlighting a need to breed more heat-tolerant crops.
In recent decades, wheat yields had declined in hotter sites such as in India, Africa, Brazil and Australia, more than offsetting yield gains in some cooler places including parts of the United States, Europe and China, the study showed.
"Global wheat production is estimated to fall by 6 percent for each degree Celsius of further temperature increase," according to the scientists who used wheat crop computer models and field experiments.
They said there were wide possibilities to limit the damage from higher temperatures by developing new types of wheat to tolerate extra heat, especially in warm regions.
The study examined only temperatures, not for instance the fact that more carbon dioxide - the main greenhouse gas emitted by burning fossil fuels - is an airborne fertiliser. It also did not try to assess possible changes in rainfall patterns.
"Wheat yield declines in response to temperature impacts only are likely to be larger than previously thought and should be expected earlier, starting even with small increases in temperature," they wrote.
Joergen Eivind Olesen, a professor at the University of Aarhus in Denmark who was one of the authors, said the focus on temperature alone meant the study "is not the complete picture".
"Even so, in many parts of the world there would still be a decrease in yields" even with small temperature rises, he said.
In March, the U.N.'s Intergovernmental Panel on Climate Change projected that yields of wheat, rice and maize would fall overall with temperature rises of 2 degrees C (3.6 F) above late 20th century levels - a higher threshold than in Monday's study.
Average world temperatures have risen by about 0.9 C (1.5F) since the Industrial Revolution and are projected to rise by between 0.3 and 4.8 C (0.5-8.6F) this century, depending on whether governments cut emissions or let them keep rising. (Editing by David Evans)
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