* Rice yields threatened by higher temperatures
* Study finds daily minimum temperatures rising faster
* Farmers need to adapt practices to beat the heat impacts
By David Fogarty, Climate Change Correspondent, Asia
SINGAPORE, Aug 10 (Reuters) - Rising temperatures could slow the growth of rice production unless farmers adapt by changing management practices and switch to more heat-tolerant varieties, scientists say.
Rice is among the world’s most important crops and a staple for people in Asia and Africa, with Asia producing and consuming more than 90 percent of the world’s output.
A drop in production could lead to higher prices, fears over food security and more hunger in a world with a rising human population.
A team of researchers led by Jarrod Welch of the University of California, San Diego, found that rice yields drop as night time temperatures rise over time, although the exact reasons why are not perfectly understood.
"With few exceptions, most statistical studies on temperature and rice yield have focused on the impact of daily mean temperature," the team says in the study, published online in the latest issue of the U.S. journal Proceedings of the National Academy of Sciences.
Previous research had found that higher maximum day time temperatures can be beneficial up to a point, beyond which they can be harmful. But higher daily minimum or night-time temperatures can trim or wipe out any gains.
The study is believed to be the first to look at the impact of daily maximum and minimum temperatures on irrigated rice production in farmer-managed rice fields in tropical and subtropical regions of Asia.
The team, which included researchers from the International Rice Research Institute in the Philippines and the U.N.’s Food and Agriculture Organisation, studied data from 227 irrigated rice farms in six Asian countries.
The data covered six years from 1994-1999 based on a major study by IRRI on the productivity of intensively managed irrigated rice farms in China, Vietnam, the Philippines, Indonesia, Thailand and India.
"Our study is unique because it uses data collected in farmers’ fields, under real-world conditions," Welch said.
Using longer-term data, the researchers found that rising temperatures during the past 25 years have already cut the yield growth rate by 10-20 percent in several locations in the study areas.
Temperature increases at night, depending on the quarter of the year and the site, were anywhere from 0.3 degrees Celsius to 0.6 deg C. on an aggregate basis.
"We see some increases in maximum temperatures but we see much more consistently increases in night-time temperature," Welch told Reuters by telephone.
This meant that future yield losses caused by higher night-time temperatures would likely outweigh any gains from slightly higher day time temperatures.
But sharply higher day time temperatures will also lead to lower yields as the plants become too stressed, IRRI and other researchers have found.
IRRI has already been developing heat-tolerant varieties. Welch said farmers in some areas could adapt by shifting the main growing season into the cooler months.
"There’s also the possibility that areas where rice is currently grown becomes too warm and production shifts to cooler areas," Welch said.
But some areas such as Vietnam grow three rice crops a year in the Mekong Delta, with the highest yielding crop during the dry season. Shifting production to cooler areas could be a huge challenge. (Additional reporting by Ho Binh Minh in Hanoi; Editing by Manash Goswami)