"Here and elsewhere, desertification acts as the trigger; climate change and population growth act as amplifiers; interethnic and tribal conflicts are the political by-product, and WhatsApp provides both an alluring picture of where things might be better--Europe--and a cheap tool for hopping a migration caravan to get there. "In the old days," says Barbut, "we could just give them a Live Aid concert in Europe or America and then forget about them. But that won't work anymore. They won't settle for that. And the problem is now too big." No walls will permanently hold them back."