Dozens of world leaders will be meeting at the United Nations Food and Agriculture Organization headquarters in Rome starting Tuesday to address a global food crisis caused in part by water shortages in Africa, Australia and here in southern Spain. Climate change means that creeping deserts may eventually drive 135 million people off their land, the United Nations estimates. Most of them are in the developing world. But Southern Europe is experiencing the problem now, its climate drying to the point that it is becoming more like Africa’s, scientists say.