Why 2015 was the hottest year on record

Scientists began monitoring worldwide temperatures since 1880. And in the 136 years since then, no year has clocked in hotter than 2015, according to NOAA scientists.

The record-breaking year was fueled in part by abnormally hot areas that included the western Atlantic Ocean, the eastern Pacific Ocean, the Indian Ocean, Europe, central Asia, southern Africa, Central America, northern South America and the eastern United States. A few areas were abnormally cool, including the northern Atlantic Ocean and parts of Northern Africa and the Middle East.

Globally, land and ocean surfaces measured 1.62°F above the 20th century average. It marked the fourth time since 2000 that average global temperature records have been broken.

Rutgers Global Snow Lab analyzed additional NOAA data and found Northern Hemisphere snow cover in 2015 was 9.5 million square miles, the lowest since 2008. And it continues to be a tale of two polar ice caps, with the Arctic seeing its sixth smallest annual value on record, with the Antarctic sea ice extent the third largest on record.

Other climate anomalies of note in 2015:

  • Hurricane activity was 144% above average in the Pacific but 63% below average in the Atlantic.
  • The U.S. saw the second warmest year and third wettest year on record. May 2015 was the wettest month ever recorded.
  • Flooding in China from May to October affected 7 million people.
  • 2012-15 were the four warmest years on record in Argentina.
  • It was the second warmest year on record in Europe (behind 2014).

NASA also did a separate independent analysis of the climate data. The agency reports Earth's average surface temperatures have risen 1.8°F since the late 19th century. Fifteen of the 16 warmest years on record have occurred since 2001.

"Climate change is the challenge of our generation, and NASA's vital work on this important issue affects every person on Earth," says NASA administrator Charles Bolden. "It is a key data point that should make policy makers stand up and take notice - now is the time to act on climate."

El Niño conditions contributed to record 2015 temperatures, notes GISS director Gavin Scmidt, adding that other factors such as carbon dioxide emissions were still significant.

"2015 was remarkable even in the context of the ongoing El Niño," he says. "Last year's temperatures had an assist from El Niño, but it is the cumulative effect of the long-term trend that has resulted in the record warming that we are seeing."

For more information, visit http://go.nasa.gov/2015climate.