Earth last year wasn't quite as hot as 2016's record-shattering mark, but it ranked second or third, depending on who was counting. Either way, scientists say it showed a clear signal of man-made global warming because it was the hottest year they've seen without an El Nino boosting temperatures naturally. The National Oceanic and Atmospheric Administration and the United Kingdom's meteorological office on Thursday announced that 2017 was the third hottest year on record. At the same time, NASA and researchers from a nonprofit in Berkeley, California, called it the second. The agencies slightly differ because of how much they count an overheating Arctic, where there are gaps in the data. The global average temperature in 2017 was 58.51 degrees, which is 1.51 degrees above the 20th century average and just behind 2016 and 2015, NOAA said.
During an El Nino year—when a warming of the central Pacific changes weather worldwide—the globe's annual temperature can spike, naturally, by a tenth or two of a degree, scientists said. There was a strong El Nino during 2015 and 2016. But 2017 finished with a La Nina, the cousin of El Nino that lowers temperatures, reports the AP. Had there been no man-made warming, 2017 would have been average or slightly cooler than normal, said National Center for Atmospheric Research climate scientist Ben Sanderson. The observed warming has been predicted within a few tenths of a degree in computer simulations going back to the 1970s and 1980s, several scientists said. It has been 33 years since the last month that the globe was cooler than normal, according to NOAA.