The data are in: 2015 is the warmest year in the instrumental record. The El Nino phenomenon in the Pacific Ocean on top of the higher average global temperatures that characterize the past decade significantly boosted temperatures in 2015. From the NOAA/NASA press release:
Earth's 2015 surface temperatures were the warmest since modern record keeping began in 1880, according to independent analyses by NASA and the National Oceanic and Atmospheric Administration (NOAA).
Globally-averaged temperatures in 2015 shattered the previous mark set in 2014 by 0.23 degrees Fahrenheit (0.13 Celsius). Only once before, in 1998, has the new record been greater than the old record by this much.
The 2015 temperatures continue a long-term warming trend, according to analyses by scientists at NASA's Goddard Institute for Space Studies (GISS) in New York (GISTEMP). NOAA scientists concur with the finding that 2015 was the warmest year on record based on separate, independent analyses of the data. Because weather station locations and measurements change over time, there is some uncertainty in the individual values in the GISTEMP index. Taking this into account, NASA analysis estimates 2015 was the warmest year with 94 percent certainty.
"Climate change is the challenge of our generation, and NASA's vital work on this important issue affects every person on Earth," said NASA Administrator Charles Bolden. "Today's announcement not only underscores how critical NASA's Earth observation program is, it is a key data point that should make policy makers stand up and take notice—now is the time to act on climate."
The planet's average surface temperature has risen about 1.8 degrees Fahrenheit (1.0 degree Celsius) since the late-19th century, a change largely driven by increased carbon dioxide and other human-made emissions into the atmosphere.
From the UK Met Office:
Provisional full-year figures for global average temperatures reveal that 2015 was the warmest year in a record dating back to 1850
Scientists at the Met Office Hadley Centre and the University of East Anglia's Climatic Research Unit produce the HadCRUT4 dataset, which is used to estimate global temperature. The global temperature series shows that 2015 was 0.75 ±0.1 °C above the long-term (1961-1990) average, a record since at least 1850. When compared with the pre-industrial period, the 2015 average global temperature was around 1 °C above the long-term average from 1850 to 1900.
Peter Stott is head of climate monitoring and attribution at the Met Office Hadley Centre. He said: "2015 was a record-breaking year for our climate. Global mean temperatures reached 1 °C above pre-industrial levels* for the first time and the year's average global temperature was the highest ever recorded."
On the other hand, the researchers at the University of Alabama in Huntsville who use satellite data report that 2015 is the third warmest year in that record which extends back to 1979. From the UAH press release:
2015 finished with an average temperature that was 0.27 C (about 0.49 degrees F) warmer than the 30—year norm. The warmest year on record is 1998, when the annual average temperature was 0.48 C (about 0.86 degrees F) warmer than normal. The five warmest years in the satellite temperature record are:
1998 +0.48 C
2010 +0.34 C
2015 +0.27 C
2002 +0.21 C
2005 +0.20 C
Still looking for the 2015 data from Remote Sensing Systems, the other group that uses satellite measurements.