Higher average growing season temperatures(温度) in most major grain-producing countries are beginning to work against the progress made by improving seeds and farming practices, according to the study published in the journal Science.

    "It is better for yields if it gets warmer, but only up to a point," said economist and study co-author Wolfram Schlenker at Columbia University.

    Once temperatures rise above a certain point, which varies for each crop, "yields fall off a cliff," he said.

    Improving technology vs. hotter climateGlobal crop yields have been increasing slowly, but steadily, over the decades as farming technology has improved.

    Schlenker and his colleagues used mathematical models to see if rising temperatures were starting to undo some of the production gains from improving technology.

    They estimate that global maize production is as much as 3.8 percent lower than it would have been without climate change, and wheat production is 5.5 percent lower.

    That may not sound like much, but the study notes that's equal to the annual maize production of Mexico and the wheat production of France.

    Rice and soybeans have not been affected as much on a global scale because yield gains from warmer temperatures in higher latitudes offset the losses in other parts of the globe.

    Here today, more tomorrowSoil scientist Chuck Rice at Kansas State University said much of the previous research on climate change has focused on what the impacts on agriculture(农业) will be in the future. This one gives a sense of what already is happening.

    The U.N. Food and Agriculture Organization estimates that the demands on agriculture will increase by 70 percent by 2050 to feed a growing and increasingly affluent population. Rice said the new study shows the current pace of progress is not enough.

    "Even with the 1 or 1.1 percent [annual] improvement in crop yields, we won't meet those goals," he said. "So if climate change is already holding us back, that means we're going to have to almost redouble our efforts."Schlenker said his study should give policymakers some direction on how to prepare for the accelerated pace of warming that scientists expect in the coming years.

    "If you're worried about rising food prices, it might be good to funnel some research into doing breeding for heat tolerance, and maybe even drought tolerance," he said.

    US spared, for nowWhile average growing season temperatures have been trending higher in most grain-producing nations, a notable exception is the United States, where, surprisingly, the study found those average temperatures have actually declined slightly.

    The authors say that may help explain some of the skepticism about climate change found in the U.S. farm belt.

    But they warn that the cooling trend is not expected to last.(本文由在线英语听力室整理编辑)