The data is officially in, and every credible source has reported that 2015 was the warmest year globally since records began in 1880.
The average temperature across global land and ocean surfaces was 1.62 degrees F (0.9 degree C) above the 20th-century average, according to NASA and the U.S. National Oceanic and Atmospheric Administration (NOAA), which includes the National Weather Service and National Ocean Service among its organizations.
Surpassing 2014’s record by 0.29 degree F (0.16 degree C), this is the widest margin by which the annual global temperature record has been broken. According to NOAA, global temperatures tied or broke existing monthly records for all but two months of the year, which is truly remarkable.
Fifteen of the 16 warmest years on record have occurred since 2001. As you can see from the NASA image (scroll down), the only areas that were colder than normal overall in 2015 were a section of the North Atlantic and the Antarctic region, while much of Eurasia and western North America were more than 3 degrees F warmer than normal for the year as a whole.
In the contiguous United States, 2015 was the second warmest year on record and the third wettest. Temperatures in December were exceptionally warm, as nearly the entire eastern half of the United States experienced temperatures more than 10 degrees above normal, while the coldest places in the West relative to normal were only 1 degree below.
Why Was 2015 So Warm?
The most significant reason that 2015 stood out as so warm was the development of an exceptionally strong El Niño pattern, which is marked by above-normal water temperatures in the central and eastern Pacific Ocean.
The El Niño pattern is one of the most studied and best-known teleconnections, which are particular oceanic or weather patterns that tend to last for weeks to years and influence the weather across the globe. The last strong El Niño occurred in 1998, which, at the time, was the warmest year on record and is the only one of the warmest 16 years that has not occurred in this century.
During El Niño winters, the temperatures, on average, are generally above normal, and snowfalls are less frequent than normal in the northeastern part of the United States. However, many of these winters have also featured one or two large snowstorms that brought exceptionally heavy snow to the region.
The El Niño is expected to gradually weaken through spring and end during late spring or early summer 2016.
El Niño’s Effects in 2015–16
Strong El Niños are a root cause of several phenomena that occurred last year and early this year, including …
the very inactive 2015 Atlantic hurricane season
the very active 2015 Pacific hurricane season
the increased storm and rainfall activity in California and the Pacific states this winter
the heavy rains that ended the drought in Texas and other areas in the Gulf and Southeast regions during 2015
the below-normal snowfall around the Great Lakes this winter
the exceptionally warm temperatures in the eastern states during the first half of winter season
the eastern blizzard in January 2016, which brought record or near-record snowfall from West Virginia to New York City.