This is the VOA Special English Agriculture Report.

A new study says climate change has reduced the world's wheat and maize production. The study says rice and soybean yields have also decreased in some places -- but increased in others. In the words of the researchers: "For soybeans and rice, winners and losers largely balanced out."

The researchers studied climate trends and global crop production from between 1980 and 2008. They found that climate changes "are already exerting a considerable drag on yield growth" and may have affected food prices.

The study used computer models linking crop yields to weather. Yield is the amount produced for each hectare or acre. The researchers compared the results to what the yields might have been without climate changes. They found that corn production decreased by almost four percent and wheat production decreased by 5.5 percent.

Warming temperatures were reported in almost all of Europe, much of Asia and some of South America and Africa. During the study period most countries had greater temperature changes from year to year than they have had historically. But the study says the United States was an important exception -- at least so far.

Corn and wheat yields in most of North America remained about the same. Russia's wheat yields decreased the most. The largest loss in corn yields were in China and Brazil.

The report is in the journal Science.

One of the researchers was economist Wolfram Schlenker at Columbia University in New York and the National Bureau of Economic Research. He says the report can serve as a planning tool for policy makers.

WOLFRAM SCHLENKER: "If you are worried about rising food prices, it might be good to funnel some research into breeding for heat tolerance and maybe even drought tolerance."

Jeffrey Stark is with a public policy organization called the Foundation for Environmental Security and Stability. This group is supported by the United States Agency for International Development. Mr. Stark recently described the effects of climate change on pastoralists in Uganda. They travel with their cattle to find grasslands and water.

JEFFREY STARK: "We did hear repeatedly that there have been increasing temperatures, drying wetlands, less frequent but more intense rain, hail storms and, most significantly, unpredictable shifts in seasonal patterns. The pastoralists in the cattle corridor have to travel farther in search of pasture and water."

And that search, he says, can bring them into contact and conflict with farmers who have problems of their own.

And that's the VOA Special English Agriculture Report, written by Jerilyn Watson. For more stories, go to voaspecialenglish.com, where you can also find English teaching activities. You can also find captioned videos of our reports on YouTube at the VOA Learning English channel. I'm Bob Doughty.

pastoralist: a sheep or cattle farmer 牧民

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（来源：VOA 编辑：崔旭燕）