He said the 'lesson' to world leaders should be that 'things can go backwards' and humanity could find itself in 'a convulsion of crisis where things get worse for a long time'. 'Then wham, middle of the 5th century, they hit a complete crisis, uncontrolled immigration, you have the Dark Ages.' The Romans thought they were going to go on forever, they thought that their empire was going to flourish forever. He told ITV News: 'You could not have a more vivid metaphor for the risk that humanity is running. Standing in Rome’s Colosseum ahead of the summit of the G20 leaders, the prime minister likened the impact of unrestricted global warming to the fall of the Roman Empire. Humanity 'can go backwards' if the world fails to urgently tackle climate change, Boris Johnson has warned as he appealed to the world’s leading economies to do more to cut emissions. 'The lesson is that things can go backwards,' the prime minister told ITV News's Political Editor Robert Peston, warning things could 'get worse for a long time'
