From the US NOAA:
The State of the Climate November 2015 report noted that in order for 2015 to not become the warmest year in the 136-year period of record, the December global temperature would have to be at least 0.81°C (1.46°F) below the 20th century average—or 0.24°C (0.43°F) colder than the current record low December temperature of 1916. In fact, December 2015 was the warmest month of any month in the period of record, at 1.11°C (2.00°F) higher than the monthly average, breaking the previous all-time record set just two months ago in October 2015 by 0.12°C (0.21°F). This is the first time in the NOAA record that a monthly temperature departure from average exceeded 1°C or reached 2°F and the second widest margin by which an all-time monthly global temperature record has been broken. (February 1998 broke the previous record of March 1990 by 0.13°C / 0.23°F.)
With the contribution of such record warmth at year’s end and with 10 months of the year record warm for their respective months, including the last 8 (January was second warmest for January and April was third warmest), the average global temperature across land and ocean surface areas for 2015 was 0.90°C (1.62°F) above the 20th century average of 13.9°C (57.0°F), beating the previous record warmth of 2014 by 0.16°C (0.29°F). This is not only the highest calendar year temperature, but also the highest temperature for any 12-month period on record. The global temperatures in 2015 were strongly influenced by strong El Niño conditions that developed during the year.
The 2015 temperature also marks the largest margin by which an annual temperature record has been broken. Prior to this year, the largest margin occurred in 1998, when the annual temperature surpassed the record set in 1997 by 0.12°C (0.22°F). Incidentally, 1997 and 1998 were the last years in which a similarly strong El Niño was occurring. The annual temperature anomalies for 1997 and 1998 were 0.51°C (0.92°F) and 0.63°C (1.13°F), respectively, above the 20th century average, both well below the 2015 temperature departure.
This marks the fourth time in the 21st century a new record high annual temperature has been set (along with 2005, 2010, and 2014) and also marks the 39th consecutive year (since 1977) that the annual temperature has been above the 20th century average. To date, including 2015, 15 of the 16 warmest years on record have occurred during the 21st century. 1998 is currently tied with 2009 as the sixth warmest year on record.
Overall, the global annual temperature has increased at an average rate of 0.07°C (0.13°F) per decade since 1880 and at an average rate of 0.17°C (0.31°F) per decade since 1970.