World leaders must embrace agricultural biotechnology if they are to cope with the severe food shortages likely to result from global warming in the coming decades, warns a group of scientists. Yields from some of the most important crops for human consumption begin to decline sharply when average temperatures exceed about 30 degrees Celsius, or 86 Fahrenheit, they explain in an article that will appear Friday in the journal Science. “You’re looking at a 20 percent to 30 percent decline in production yields in the next 50 years,” said coauthor David Battisti.