Rubbish collectors check their boats on a lake as a chimney from a nearby coal-burning power station billows smoke behind them in central Beijing March 12, 2010. China's top climate negotiator said on Wednesday that the cause of global warming was still not clear but the problems it is creating are so enormous that the world must anyway act to cut greenhouse gas emissions now. REUTERS/David Gray (CHINA - Tags: ENVIRONMENT POLITICS)
REUTERS/DAVID GRAY