Things are definitely heating up. Spurred by global warming and a “super El Niño,” 2015 smashed records, becoming by far Earth’s hottest year since record-keeping began in 1880.
Worldwide surface temperatures were on average 0.90 degrees Celsius warmer than the 20th century average of 13.9°, the National Oceanic and Atmospheric Administration and NASA reported January 20 in a joint announcement. That’s well above the previous record of 0.74 degrees above average set in 2014 (SN Online: 1/16/15). The 0.16-degree difference between the two years is the largest margin by which an annual temperature record has ever been broken.
What’s more, the new record leaves little room for doubt. NOAA reported over 99 percent confidence that 2015 was in fact the hottest year on record, considering gaps in weather data, compared with just 48 percent confidence when 2014 nabbed the title.
“2015 was the warmest year because it was warm throughout the year,” said Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies in New York City. Ten months set all-time records during 2015. December was the biggest record breaker, with temperatures reaching 1.11 degrees higher than the 20th century average for that month. “It was picking up that El Niño assist in the last three months,” Schmidt said. But even without the boost, “this still would have been the warmest year on record,” he said.
Earth has now seen 39 consecutive years of temperatures above the 20th century average, as measured by a global armada of weather stations, buoys and ships. Heat-trapping greenhouse gases in the atmosphere largely contributed to that long-term rise in surface temperatures (SN: 4/4/15, p. 14). Last year, however, got an additional temperature boost from the ongoing El Niño (SN Online: 7/16/15), a naturally occurring worldwide weather disruption caused by unusually warm seawater piling up in the eastern Pacific.
The current El Niño, among the strongest on record, contributed as much as 0.15 degrees to the new record, estimates Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research in Boulder, Colo.
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Similar El Niños contributed to high temperatures observed during previous record-setting years, such as 1998, now tied for the sixth-hottest year on record. During non-El Niño years, heat accumulates under the surface in the Pacific Ocean. Every three to five years or so, changing wind patterns push this vast pool of warm seawater eastward toward the Americas and closer to the sea surface. The warm seawater then heats up the atmosphere.
Strong El Niño events often precede global cooling, however, especially when an event prompts El Niño’s meteorological sibling, La Niña. The rise and fall of temperatures around the 1997–1998 El Niño contributed to a perceived slowdown in global warming (SN: 6/27/15, p. 6).
El Niños usually contribute the most heat during their second years, but that might not be the case this time around. The current El Niño, which kicked off last March, may have done most of its warming early, Trenberth says. That’s partly because the event almost began in 2014 before wavering (SN: 11/1/14, p. 6) and reached its peak strength in November of last year, unlike most events that peak a month later. If the current El Niño is mostly tapped out, 2015’s heat record could stand for a while, Trenberth predicts, though NOAA and NASA gave better-than-even odds of a hotter 2016.