Extreme heat hurts wheat yields as world warms: study SINGAPORE (Reuters) - Extreme heat can cause wheat crops to age faster and reduce yields, a U.S.-led study shows, underscoring the challenge of feeding a rapidly growing population as the world warms. Scientists and farmers have long known that high heat can hurt some crops and the Stanford University-led study, released on Monday, revealed how the damage is done by tracking rates of wheat ageing, or senescence. Depending on the sowing date, the grain losses from rapid senescence could reach up to 20 percent, the scientists found in the study, published in the journal Nature Climate Change. Lead author David Lobell and his colleagues studied nine years of satellite measurements of wheat growth from northern India, tracking the impact of exposure to temperatures greater than 34 degrees Celsius to measure rates of senescence. They detected a significant acceleration of ageing that reduced the grain-filling period. The onset of senescence imposes a limit on the time for the plant to fill the grain head. "What's new here is better understanding of one particular mechanism that causes heat to hurt yields," Lobell told Reuters in an email. He said that while there had been some experiments showing accelerated ageing above 34 degrees Celsius, relatively few studies considered temperatures this high. We decided to see if these senescence effects are actually occurring in farmers' fields, and if so whether they are big enough to matter. On both counts, the answer is yes. Climate scientists say that episodes of extreme heat are becoming more frequent and more prevalent across the globe, presenting huge challenges for growing crops. Wheat is the second most produced crop in the world after corn and the United Nation's Food and Agriculture Organization says global food production must increase by 70 percent by 2050 to feed a larger, more urban and affluent population. Wheat is particularly sensitive to temperature and is typically sown in late autumn or early winter and harvested before the heat of summer. Lobell said his team's findings could help refine steps to adapt crops and growing times as the planet warms. Heat-tolerant varieties will be key. Whether this means faster growing in order to escape extreme heat, more capable of coping with extreme heat, or a combination of both is hard to say," he said. One challenge with sowing earlier is that there is a summer crop, usually rice, which has to be harvested before wheat is sown. That is why in many places wheat is actually sown well after the optimum window climatically. Lobell said extreme heat wasn't the only reason for lower yields. "But in hot places it is important enough to be among the top few reasons for why heat hurts," he added. A 2010 study by scientists in Australia found wheat output fell by up to half during a growing season where temperatures were two degrees Celsius higher than average, with much of the losses caused by temperatures above 34 degrees Celsius. Another study published in the journal Nature Climate Change last week said geo-engineering could help lower temperatures and boost crop yields, although it had drawbacks. Ken Caldeira of the Carnegie Institution for Science, at Stanford University, and his colleagues found that shielding the Earth with aerosol particles lobbed into the stratosphere could dramatically increase yields of corn, wheat and rice. Using two computer models, the researchers estimated corn production would rise by 14 percent, wheat by 21 percent and rice by 8 percent. This contrasts with suggestions in the scientific community that geo-engineering would threaten food and water supplies for billions of people. But Caldeira noted that it could result in localized drops in crop productivity, and the shading effect would not stop ocean acidification, which could affect marine productivity. Additional reporting by Deborah Zabarenko in Washington; Editing by Miral Fahmy