Human Flow, the debut feature from the Chinese artist Ai Weiwei, is a bold documentary about the refugee crisis. The film bounds from the cardboard cities of Europe to the burning oilfields of Mosul and from the unmarked graves of Turkey to the Texas-Mexico border. It plays out across 23 different countries. It contains a cast of thousands. In 2010, the artist packed Tate Modern’s Turbine Hall with 100m hand-painted porcelain sunflower seeds that broke up underfoot and filled the air with dust. Here, he crams an entire global tragedy into 140 fraught minutes.