From Dr. Jeff Masters blog:
The final numbers are not in yet, but 2015 is virtually certain to beat 2014's record as the planet's warmest year since record keeping began in 1880. Nine of the first eleven months of 2015 set new all-time monthly records for global heat in the NOAA database, and the two most recent months that have been catalogued--October and November 2015--had by far the warmest departures from average of any months on record.
The new record was caused by the long-term warming of the planet due to human-caused emissions of heat-trapping gases like carbon dioxide, combined with a extra bump in temperature due to the strongest El Niño event ever recorded in the Eastern Pacific. Record warm ocean temperatures in the tropics in 2015 led to a global coral bleaching event, which is expected to cause a loss of 10 - 20% of all coral worldwide. The lingering warmth from El Niño makes 2016 a good bet to exceed even 2015's warmth.