by the year 2025, world population is expected to rise to 8 billion. in order to grow enough food , it is estimated that 1 billion hectares (almost 2.5 billion acres) of new land will be needed -that is, if the farming methods that are practiced today continue. Environmentalists say say something must be done if food shortages are going to be avoided . that's why the idea of "vertical farming is being discussed at an eco-conference this week. Celia Hernandez, our environmental correspondent , joins me. Celia, what is vertical farming? And where did the idea come from?