Metro Canada: On Saturday, hundreds of millions of people will take part in what could be the biggest global movement ever. Earth Hour — at 8.30 p.m. local time — is a unique opportunity for us all to demand action to combat climate change. From London to Beijing, from Cape Town to New York, in more than a thousand towns and cities, citizens will send a clear signal to the world’s leaders that they want to keep the lid on global warming. Earth Hour is far more than a symbolic gesture. By …