Wheat scientists say new, higher-yielding varieties are needed to meet rising world demand for food and to offset the impact of climate change. The food grain provides 20 percent of the calories and protein consumed worldwide. In a meeting this week at the International Maize and Wheat Improvement Center, researchers said wheat yields are not increasing as quickly as in the past, and the hotter weather projected in coming years could depress yields even further. The head of CIMMYT’s wheat program says wheat production must rise by 60 percent to keep up with growth in the world population. A public-private project, the International Wheat Yield Partnership has raised half of its goal of $100 million to develop high-yielding varieties.