Guardian: The UN climate summit in Copenhagen may be struggling to break an impasse while the world warms, but Boris Johnson delivered a simple message today to delegates at Copenhagen: cheer up. The London mayor told a gathering of municipal leaders that misery, restraint and self-flagellation would only make matters worse. “We have to stop being so unremittingly negative and gloomy. We need to warn people and be realistic about the peril we face. But we must also mobilise people’s …