Reuters: Scientists must make clear the disastrous effects of climate change so the world takes action now to cut carbon emissions, leading economist Nicholas Stern said on Thursday. “You have to tell people very clearly and strongly just how difficult (a temperature rise of) four, five, six or seven degrees Celsius is,” he said. “Billions of people would have to move and there would be very severe conflict,” said Stern, a professor at the London School of Economics and a former British …