A labourer works at a coking factory in Changzhi, Shanxi province March 10, 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/Stringer (CHINA - Tags: ENVIRONMENT BUSINESS) CHINA OUT. NO COMMERCIAL OR EDITORIAL SALES IN CHINA