"Most people tend to think of climate change as something that will impact the future," Christopher Field, a co-author on the study and ecology expert at the Carnegie Institution in Stanford, California, said in an e-mail response to questions.

"This study shows that warming over the past two decades has already had effects on global food supply," he added.

Not every scientist agrees that agriculture is suffering from warmer temperatures.

A draft UN report obtained by Reuters on Thursday said warming is expected to turn the planet a bit greener by spurring plant growth, but crops and forests may wilt beyond mid-century if temperatures keep rising. That report, by the Intergovernmental Panel on Climate Change, will be released on April 6.

Field said the Carnegie study was the first to estimate how much global food production has already been affected by climate change. It was funded by the Carnegie Institution and the Lawrence Livermore National Laboratory, which is managed by the University of California for the US government.

Average global yields for several of the crops suffered from warmer temperatures, with yields dropping by about 3 to 5 percent for every 1 degree Fahrenheit increase, the study said.

Average global temperatures increased by about 0.7 degree F during the study period, with even larger changes in several regions.

If the past is an indication, agriculture will also suffer going forward, Field said. "We expect future warming to continue to be a drag on yields, essentially like driving with the parking brake engaged."

The cereal crops hit by global warming account for at least 55 percent of non-meat categories consumed by humans, according to the study. They also contribute more than 70 percent of the world's animal feed.