The main reasons would be the effects of a weak El Nino and the human-made greenhouse gases.
Last year was the the fourth-warmest recorded, with a global mean temperature of 57 degrees Fahrenheit (14 Celsius), which was about 1.5 degrees warmer than the middle of the century, NASA scientist Drew Shindell said in an interview.The three warmest years until now were 1998, 2002 and 2003 respectively.