Average global air temperatures in 2014 were 0.57 degrees Celsius higher than the long-term average of 14 C in a 1961-1990 reference period, the World Meteorological Organization (WMO) said in a statement.
"Fourteen of the fifteen hottest years have all been this century," said WMO Secretary General Michel Jarraud.
"In 2014, record-breaking heat combined with torrential rainfall and floods in many countries and drought in some others, consistent with the expectation of a changing climate," he added.
UN members will meet in Geneva next week for talks on a global climate pact that must be signed in Paris in December for curbing emissions of greenhouse gases.