OSLO (Reuters) - Rising temperatures trigger a runaway melt of Greenland's ice sheet, raising sea levels and drowning Pacific islands and cities from New York to Tokyo. In Siberia, the permafrost thaws, releasing vast frozen stores of greenhouse gases that send temperatures even higher. In the tropics, the Amazon rainforest starts to die off because of a warmer, drier climate. Such scenarios may read like the script of a Hollywood disaster movie but many scientists say there are real risks of "tipping points" -- sudden, catastrophic changes triggered by human activities blamed for warming the planet. "Even small risks in...