WMO chief says human activities main cause of climate change Last year tied for the sixth hottest on record, confirming that Earth's climate system is in the grip of warming that will affect generations to come, the UN's weather agency said Wednesday. Michel Jarraud, Secretary General of the World Meteorological Organization (WMO), said the main causes of global warming are "human activities". Images and soundbites from an AFP interview with Michel Jarraud.