The 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 something must be done if food shortages are eco-conference this week.Celia Hernandez,our environmental correspondent,joins me.Celia,what is vertical farming? And where did the idea come from?