The most famous landmarks in the world have been plunged into darkness, to throw the spotlight on climate change. “Earth Hour” began two years ago as a sole gesture in Australia. This year they threw the switch in 4,000 cities worldwide to highlight the threat posed by global warming.After switching off the floodlights of the Colosseum in Rome footballer Francesco Totti said: “It’s very urgent. We only have one world and we absolutely must act now.” Time-zone by time-zone, in 88 countries, tourist attractions fell dark. Organisers hope it is a sign that a sense of urgency is spreading, and that world leaders will hear the message. The Director-General of WWF International, James Leape, said: “For the first time, hundreds of millions of people around of the world have had the chance to speak out to voice their concern about climate change and we’ll send a very powerful signal to leaders everywhere that the time has come to confront this problem and to solve it.” And as the lights were extinguished, America’s president Barack Obama said he has invited 16 world leaders to a forum on global warming next month, hoping to smooth the path towards an international pact on cutting greenhouse gases later in the year.